J C Komorowski

C Dessert

A Michel

O Crispi

G Hammouya

F Beauducel

J B De Chabalier

    Evidence for a new shallow magma intrusion at La Soufrière of Guadeloupe (Lesser Antilles). Insights from long-term geochemical monitoring of halogen-rich hydrothermal fluids B. Villemant,

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A bstract :

More than three decades of geochemical monitoring of hot springs and fumaroles of La Soufrière of Guadeloupe allows the construction of a working model of the shallow hydrothermal system. This system is delimited by the nested caldera structures inherited from the repeated flank collapse events and the present dome built during the last magmatic eruption (1530 AD) and which has been highly fractured by the subsequent phreatic or phreatomagmatic eruptions. Because it is confined into the low volume, highly compartmented and partially sealed upper edifice structure, the hydrothermal system is highly reactive to perturbations in the volcanic activity (input of deep magmatic fluids), the edifice structure (sealing and fracturing) and meteorology (wet tropical regime).

The current unrest, which began with a mild reactivation of fumarolic activity in 1990, increased markedly in 1992 with seismic swarms and an increase of degassing from the summit of the dome. In 1997 seismic activity increased further and was accompanied by a sudden high-flux

HCl-rich gas from summit fumaroles. We focus on the interpretation of the time-series of the chemistry and temperature of fumarolic gases and hot springs as well as the relative behaviours of halogens (F, Cl, Br and I). This extensive geochemical time-series shows that the deep magmatic fluids have undergone large changes in composition due to condensation and chemical interaction with shallow groundwater (scrubbing). It is possible to trace back these processes and the potential contribution of a deep magmatic source using a limited set of geochemical time series: T, CO 2 and total S content in fumaroles, T and Cl -in hot springs and the relative fractionations between F, Cl, Br and I in both fluids.

Coupling 35 years of geochemical data with meteorological rainfall data and models of ion transport in the hydrothermal aquifers has allowed us to identify a series of magmatic gas pulses into the hydrothermal system since the 1976-1977 crisis. The contrasting behaviours of S-and Clbearing species in fumarolic gas and in thermal springs suggests that the current activity is the result of a new magma intrusion which was progressively emplaced at shallow depth since ~1992.

Although it might still be evolving, the characteristics of this new intrusion indicate that it hasalready reached a magnitude similar to the intrusion that was emplaced during the [1976][1977] eruptive crisis. The assessment of potential hazards associated with evolution of the current unrest must consider the implications of recurrent intrusion and further pressurization of the hydrothermal system on the likelihood of renewed phreatic explosive activity. Moreover, the role of hydrothermal pressurization on the basal friction along low-strength layers within the upper part of the edifice must be evaluated with regards to partial flank collapse. At this stage enhanced monitoring, Long term monitoring of geochemical and geophysical parameters of shallow hydrothermal systems has been undertaken at numerous active volcanic areas [START_REF] Newhall | Historical unrest at large calderas of the world[END_REF], such as Flegrean Fields [START_REF] Chiodini | Magma degassing as a trigger of bradyseismic events: The case of Phlegrean Fields (Italy)[END_REF], Long Valley caldera [START_REF] Sorey | Summary of recent research in Long Valley caldera, California[END_REF], Yellowstone (, [START_REF] Lowenstern | Monitoring super-volcanoes: Geophysical and geochemical signals at Yellowstone and other large caldera systems[END_REF][START_REF] Hurwitz | Spatial and temporal geochemical trends in the hydrothermal system of Yellowstone National Park: Inferences from river solute fluxes[END_REF], Mt Baker [START_REF] Crider | Magma at depth: a retrospective analysis of the 1975 unrest at Mount Baker[END_REF] and La Soufrière of Guadeloupe [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF]. In these volcanic areas, unrest events have been interpreted as driven by magmatic processes at depth, involving the transfer of heat and fluids from magma intrusions to the shallow hydrothermal system. However numerous processes not directly related to magmatic activity may lead to similar changes in hydrothermal systems. These complex processes increase the difficulty of interpreting monitoring data (see e.g., [START_REF] Todesco | Hydrothermal fluid circulation and its effects on caldera unrest, in Caldera Volcanoes: Analysis, Modeling and Response[END_REF][START_REF] Todesco | Signals from the Campi Flegrei hydrothermal system: Role of a fluids[END_REF][START_REF] Moretti | Multiple magma degassing sources at an explosive volcano[END_REF]. The composition of magmatic and hydrothermal fluids collected in active volcanic areas is highly sensitive to the magma source composition and degassing processes at high temperature, but also to their lower temperature evolution and interactions with the surrounding hydrothermal systems during their shallow transfer paths (see e.g. [START_REF] Giggenbach | Redox processes governing the chemistry of fumarolic gas discharges from White Island, New Zeland[END_REF][START_REF] Giggenbach | Geothermal solute equilibria. Derivation of Na-K-Mg-Ca geoindicators[END_REF], Symonds et al., 2001 among others). Monitoring the background composition, temperature and fluxes of magmatic and hydrothermal fluids over long periods of time thus constitutes a major tool for short-term hazard evaluation during unrest conditions.

The last major magmatic eruption at La Soufrière of Guadeloupe occurred in 1530 AD [START_REF] Boudon | A new scenario for the last magmatic eruption of La Soufrière de Guadeloupe (Lesser Antilles) in 1530 A.D.: evidence from stratigraphy, radiocarbon dating and magmatic evolution of erupted products[END_REF]Komorowski et al., 2008) and possibly in 1635 [START_REF] Hincks | Retrospective analysis of uncertain eruption precursors at La Soufrière volcano, Guadeloupe, 1975-77: volcanic hazard assessment using a Bayesian Belief Network approach[END_REF]. Six historical phreatic eruptions have occurred since 1635AD, namely in 1690, 1797-98, 1812, 1836-37, 1956[START_REF] Beauducel | Rappel des faits et point de vue personnel sur les événements qui ont marqué la Guadeloupe et la communauté volcanologique[END_REF]-77 (Komorowski et al., 2005). The July 1976 -March 1977 eruption was particularly violent and complex [START_REF] Feuillard | The 1976-1977crisis of la Soufrière de Guadeloupe (F.W.I.): a still-born magmatic eruption[END_REF][START_REF] Komorowski | Guadeloupe, in: Volcanic Hazard Atlas of the Lesser Antilles[END_REF]. The lack of adequate monitoring and knowledge of the eruptive past contributed significantly to scientific uncertainty and the difficulty of reaching a concensual expert scientific judgement on the likely scenarios for the evolution of the crisis [START_REF] Komorowski | Guadeloupe, in: Volcanic Hazard Atlas of the Lesser Antilles[END_REF][START_REF] Feuillard | La Soufrière de Guadeloupe, un volcan et un peuple[END_REF][START_REF] Hincks | Retrospective analysis of uncertain eruption precursors at La Soufrière volcano, Guadeloupe, 1975-77: volcanic hazard assessment using a Bayesian Belief Network approach[END_REF]Beauducel, Soufrière 1976-77 web page). Hence, the study and management of this eruption was particularly difficult for scientists and national authorities. Since this eruption, La Soufrière of Guadeloupe has become one of the best monitored volcanoes in the world, with a network of pluridisciplinary methods that are et Sismologique -IPGP). Permanent networks monitor seismicity and ground deformations. Sampling and analysis of the physico-chemistry of thermal springs, fumaroles and acid ponds on a forthnightly base since 1979 has provided a continuous data record over ~35 years (see [START_REF] Ovsg-Ipgp | Bilan mensuel de l'Activité volcanique et de la sismicité régionale de l'Observatoire Volcanologique de la Soufrière[END_REF][START_REF] Ovsg-Ipgp | Bilan mensuel de l'Activité volcanique et de la sismicité régionale de l'Observatoire Volcanologique de la Soufrière[END_REF]and 1999[START_REF] Beauducel | Rappel des faits et point de vue personnel sur les événements qui ont marqué la Guadeloupe et la communauté volcanologique[END_REF]. This large extended baseline

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dataset is a unique opportunity to analyse in details the long term evolution of the magmatichydrothermal system of a seldomly erupting explosive andesitic volcano.

The 1976-1977 crisis has been interpreted either as a failed magmatic eruption [START_REF] Feuillard | The 1976-1977crisis of la Soufrière de Guadeloupe (F.W.I.): a still-born magmatic eruption[END_REF], or as triggered by the pulsatory behaviour of the surficial hydrothermal system [START_REF] Boudon | The evolution of la Grande Découverte (La Soufrière) volcano, Guadeloupe, F.W.I[END_REF][START_REF] Zlotnicki | The volcanic activity of la Soufrière of Guadeloupe (Lesser Antilles): structural and tectonic implications[END_REF]. The geochemical surveys from 1979 to 2000 evidenced a series of major geochemical anomalies in thermal springs around the dome. These anomalies have recently been interpreted as the consequence of the intrusion of a small volume of andesitic magma at shallow depth which triggered the phreatic explosions in 1976-1977 and then, through a slow crystallisation -degassing process, episodically injected gas pulses into the shallow hydrothermal system [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF][START_REF] Boichu | A Model for Episodic Degassing of an Andesitic Magma Intrusion, Part 1[END_REF][START_REF] Feuillard | La Soufrière de Guadeloupe, un volcan et un peuple[END_REF]. This model is supported by Cl isotopes investigations [START_REF] Li | Isotopic evidence for quick freshening of magmatic chlorine in the Lesser Antilles arc volcanoes[END_REF]. More recently, on the basis of noble gas and C isotopes studies, [START_REF] Ruzié | Noble gas isotopes in hydrothermal volcanic fluids of La Soufrière volcano, Guadeloupe, Lesser Antilles arc[END_REF][START_REF] Ruzié | Carbon and helium isotopes in thermal springs of La Soufrière volcano (Guadeloupe, Lesser Antilles): Implications for volcanological monitoring[END_REF] have proposed a similar model where the gas flux generated by a new fresh magma injection at depth is modulated by the shallow hydrothermal system and the sealing of the edifice.

Since 1976, the geochemical anomalies recorded in hot springs have progressively decreased with time and vanished for most springs between 1992 and 1995. The sustained fumarolic activity that developed on and around the dome during the 1976 crisis vanished initially very rapidly from the dome summit in 1977 and then decreased more progressively at the base of the dome to disappear almost completely in 1981. From 1992, though the geochemical anomalies in hot springs continued to decline, fumaroles at the summit of the dome were progressively reactivated, and a persistent high flux of HCl-rich H 2 O vapour appeared in late 1997 [START_REF] Komorowski | éruptive de la Soufriére de Guadeloupe: problèmes et implications de la phénoménologie et des signaux actuellement enregistrés, Atelier sur les aléas volcaniques Les volcans antillais : des processus aux signaux[END_REF][START_REF] Bernard | Remote and in situ plume measurements acid gas release from La Soufrière volcano, Guadeloupe[END_REF]. Two small intermittent acid ponds were emplaced during that period at the summit of the dome [START_REF] Komorowski | Guadeloupe, in: Volcanic Hazard Atlas of the Lesser Antilles[END_REF]. From 2006, as most springs reached a steady composition, a single spring, Galion, registered a new series of Cl anomalies culminating in 2009.

Heat flux that is mainly driven by the main summit fumaroles increased by a factor of ~3 from 2005 to 2010 (Beauducel Pers. Com. see also Allard et al., 2014). In parallel, since the end of the 1976-1977 crisis, the seismic activity rapidly decreased (<10 earthquakes/month from [1984][1985] to reach a minimum in 1990 (OVSG-IPGP website). No other major anomalous geophysical signals were recorded between the end of 1977 and 1992. Since the end of 1992 the monitoring system has recorded a progressive increase in shallow and low energy seismicity (VT, LP), and a slow rise of temperature of thermal springs close to the dome (OVSG-IPGP and IPGP website). This situation of unrest at La Soufrière of Guadeloupe and its related phenomenology is not clearly understood at the present time. A magmatic origin cannot be excluded. How to identify the possible intrusion of a new magma batch as seems to have occurred during the 1976-1977 volcanic crisis?

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Numerous studies have shown that geochemical monitoring provides valuable insights as to processes in the deep magmatic system that can be responsible for unrest recorded at the surface [START_REF] Symonds | Magmatic gas scrubbing: implications for volcano monitoring[END_REF], Stimac et al., 2003[START_REF] Crider | Magma at depth: a retrospective analysis of the 1975 unrest at Mount Baker[END_REF], Melian et al. 2012[START_REF] Chiodini | Long-term variations of the Campi Flegrei, Italy, volcanic system as revealed by the monitoring of hydrothermal activity[END_REF][START_REF] Moretti | Multiple magma degassing sources at an explosive volcano[END_REF]. Halogens are of particular interest because they display simple behaviours at high temperature during magma degassing at relatively shallow level and gas transfer to the surface [START_REF] Symonds | Calculation of multicomponent chemical equilibria in gassolid-liquid systems: calculation methods, thermochemical data and applications to studies of high temperature volcanic gases with examples from Mount St. Helens[END_REF], Villemant et al., 2003, 2005[START_REF] Balcone Boissard | Behavior of halogens during the degassing of felsic magmas[END_REF].

Halogens (F, Cl, Br and I) display simple behaviours during magma differentiation and degassing because they generally have low mineral/melt partition coefficients and high H 2 O vapour-melt partition coefficients. Experimental data show that halogens are highly soluble in silicate melts in the absence of an exsolved H 2 O vapour phase and that their volatile behaviour is mainly controlled by H 2 O degassing [START_REF] Webster | Chloride and H2Osolubiltiy in basalt and andesitic melts and implications for magmatic degassing[END_REF]Aiuppa et al. 2008 and references therein). Cl, Br and I are extracted as halogen acids (HCl, HBr, HI) from the magma with similar vapour-melt partition coefficients, whereas F is not significantly extracted and remains concentrated in silicate melts (Villemant andBoudon, 1999, Balcone et al., 2010). Halogen contents in magmatic gases are almost not modified by decompression, gas-wall rock interactions or cooling (down to temperatures ~120°C i.e. without interaction with a shallow hydrothermal system or atmospheric gases [START_REF] Symonds | Calculation of multicomponent chemical equilibria in gassolid-liquid systems: calculation methods, thermochemical data and applications to studies of high temperature volcanic gases with examples from Mount St. Helens[END_REF]. If gas interacts with hydrothermal or phreatic waters ( er path halogen acids are completely and transported as conservative ions in low temperature aqueous systems [START_REF] Symonds | Magmatic gas scrubbing: implications for volcano monitoring[END_REF][START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF]. From these properties it is inferred that halogen abundance ratios (Cl/Br/I) of the degassing magmas are preserved in gases from high to intermediate temperatures (T >120°C) when they are transferred through a to the surface as observed, for example, in Mt Etna fumaroles [START_REF] Aiuppa | Emission of bromine and iodine from Mount Etna volcano[END_REF]. These halogen ratios can also be preserved when halogen acids are completely dissolved in phreatic and hydrothermal systems. However, halogens may be highly fractionated during low temperature gas condensation (<120°C), boiling of acid or ion-rich waters or precipitation of halides in highly concentrated brines (se e.g. Berndt and Seyfried, 1997 and references therein). Halogens speciation in the gas plume may also be highly modified by mixing with oxidizing atmospheric gas and photocatalytic reactions [START_REF] Bobrowski | Detection of bromine monoxide in volcanic plumes[END_REF][START_REF] Gerlach | Volcanic sources of tropospheric ozone-depleting trace gases[END_REF][START_REF] Millard | Halogen emissions from a small volcanic eruption: Modeling the peak concentrations, dispersion, and volcanically induced ozone loss in the stratosphere[END_REF][START_REF] Oppenheimer | BrO formation in volcanicplumes[END_REF][START_REF] Villemant | Andesitic Magma Degassing Investigated Through H2O Vapour-Melt Partitioning of Halogens at Soufrière Hills Volcano, Montserrat (Lesser Antilles) Earth Planet[END_REF][START_REF] Aiuppa | Halogens in volcanic systems[END_REF]Von Glasow et al., 2009). Thus halogen systematics in volcanic fluids provide an efficient tool for identifying their origin and transfer histories, including condensation-evaporation and dissolutionprecipitation processes, which is the goal of geochemical surveys of active volcanoes.

In this paper we present and interpret the geochemical time-series data (temperature and chemical compositions) that was measured during about 35 years of sampling of hot springs,

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fumaroles and acid ponds between 1979 and the end of year 2012. It extends the 1979-2003 data series from hot springs only that was discussed by [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF] and focuses more specifically on halogens (F, Cl, Br,I) and temperature records. The interpretation of gas monitoring data remains a specific challenge because these are often marred by large errors caused by numerous difficulties in fumaroles sampling as well as the analytical protocols. The approach used here is to combine all the information (gas and hot spring water geochemistry, phenomenology) to overcome this difficulty and to extract an interpretative scheme that is as consistent as possible with all available geochemical data. On the basis of of the entire monitoring data set we propose a general model for the evolution of the surficial hydrothermal system since 1976 and its relationship with deep magmatic activity.

2. Geological setting and phenomenology of the hydrothermal activity at L a Soufrière of

G uadeloupe

The last magmatic activity at La Soufrière, dated at 1530 AD, produced pyroclastic pumiceous tephra fallout and pyroclastic flows and built an andesite lava dome [START_REF] Boudon | Volano flank instability in the Lesser Antilles Arc: diversity of scale, processes, and temporal recurrence[END_REF][START_REF] Todesco | Hydrothermal fluid circulation and its effects on caldera unrest, in Caldera Volcanoes: Analysis, Modeling and Response[END_REF], Komorowski et al., 2008). This dome is located in a complex horseshoe-shaped structure (Fig. 1) inherited from recurrent flank collapse events (11500 and 3500 B.P.) that affected the summit area of the Carmichael volcano which is part of La Soufrière -Grande Découverte volcanic complex [START_REF] Boudon | The evolution of la Grande Découverte (La Soufrière) volcano, Guadeloupe, F.W.I[END_REF][START_REF] Todesco | Hydrothermal fluid circulation and its effects on caldera unrest, in Caldera Volcanoes: Analysis, Modeling and Response[END_REF], Komorowski et al., 2005). The historic activity since 1635 has been limited to six explosive phreatic eruptions without participation of magma. The most violent eruptions were those of 1797-1798 and 1976-1977, the latter of which has been interpreted as a failed magmatic eruption [START_REF] Feuillard | The 1976-1977crisis of la Soufrière de Guadeloupe (F.W.I.): a still-born magmatic eruption[END_REF], Komorowski et al., 2005;[START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF][START_REF] Boichu | A Model for Episodic Degassing of an Andesitic Magma Intrusion, Part 1[END_REF][START_REF] Feuillard | La Soufrière de Guadeloupe, un volcan et un peuple[END_REF][START_REF] Ruzié | Noble gas isotopes in hydrothermal volcanic fluids of La Soufrière volcano, Guadeloupe, Lesser Antilles arc[END_REF].

The co-existence of an active magma chamber and abundant ground-water fed by a tropical climate regime with abundant rainfall (mean value for 1983-2010: 10 ± 2 m/yr) has led to the occurrence of numerous permanent thermal springs and permanent to intermittent fumarolic degassing on the summit and at the periphery of the dome [START_REF] Bigot | Surveillance hydrogéochimique de la Soufrière de Guadeloupe, 1978-1985: diminution de l'activité ou confinement?[END_REF][START_REF] Zlotnicki | The volcanic activity of la Soufrière of Guadeloupe (Lesser Antilles): structural and tectonic implications[END_REF][START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF], Komorowski et al., 2005, Fig. 1). Interaction of the more active summit fumaroles with perched aquifers favours the formation of intermittent acid ponds (Cratère Sud and Tarissan). Historical observations show that the nature, distribution and intensity of hydrothermal activity have considerably varied over time [START_REF] Komorowski | Guadeloupe, in: Volcanic Hazard Atlas of the Lesser Antilles[END_REF][START_REF] Bernard | Remote and in situ plume measurements acid gas release from La Soufrière volcano, Guadeloupe[END_REF].

The large development of intense and recent fumarolic activity on the volcano is also evidenced by numerous zones of hydrothermally altered volcanic material. These alteration zones generally consist of elongated areas varying in size from tens to hundreds of meters (Fig. 1a). They

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are located at the summit of the dome (Faille du Nord, Tarissan, Cratère Sud, Lacroix) and on its Echelle-Chaudières, La Ty fault) and generally correspond to the main fracture systems. All of the altered zones have been historically active, but at present, only the fumarolic fields of the La Ty fault and the dome summit are active. This fumarolic alteration affects massive dome lavas (Faille du Nord) or pyroclastic material (scoria and ashfall deposits consists of the development of smectite at the expense of the magmatic glass and the precipitation of sulfates and sulfides [START_REF] Salaün | Hydrothermal alteration in andesitic volcanoes: a study of trace element redistribution in active-and paleohydrothermal systems of Guadeloupe (Lesser Antilles)[END_REF]. Frequent partial edifice-collapse events (at least ten in the last 15 000 years) have affected the La Soufrière-Grande Découverte volcanic complex and emplaced debris-avalanche deposits which contain abundant fragments of hydrothermal alteration products [START_REF] Komorowski | Guadeloupe, in: Volcanic Hazard Atlas of the Lesser Antilles[END_REF][START_REF] Salaün | Hydrothermal alteration in andesitic volcanoes: a study of trace element redistribution in active-and paleohydrothermal systems of Guadeloupe (Lesser Antilles)[END_REF]. Finally, the six historical phreatic eruptions emitted large amounts of ash which contain abundant hydrothermally altered material [START_REF] Feuillard | The 1976-1977crisis of la Soufrière de Guadeloupe (F.W.I.): a still-born magmatic eruption[END_REF] and were easily altered after deposition by fumarolic and hydrothermal activity [START_REF] Salaün | Hydrothermal alteration in andesitic volcanoes: a study of trace element redistribution in active-and paleohydrothermal systems of Guadeloupe (Lesser Antilles)[END_REF].

Although the fumarolic fields at the periphery of the dome (Collardeau, Carbet-Echelle, Chaudière-Souffleur, Morne-Mitan) were active before the 1976 -1977 volcanic crisis, their activity increased markedly after the onset of the eruption which began with a violent phreatic explosion in July 1976 [START_REF] Feuillard | The 1976-1977crisis of la Soufrière de Guadeloupe (F.W.I.): a still-born magmatic eruption[END_REF][START_REF] Komorowski | Guadeloupe, in: Volcanic Hazard Atlas of the Lesser Antilles[END_REF]. vapour (up to 1M HCl) that forms plumes visible at large distance from the dome [START_REF] Komorowski | éruptive de la Soufriére de Guadeloupe: problèmes et implications de la phénoménologie et des signaux actuellement enregistrés, Atelier sur les aléas volcaniques Les volcans antillais : des processus aux signaux[END_REF]2005;OVSG-IPGP, 1999[START_REF] Beauducel | Rappel des faits et point de vue personnel sur les événements qui ont marqué la Guadeloupe et la communauté volcanologique[END_REF]. No significant fumarolic activity has been maintained at the base of the dome except persistent weak and non pressurized emanations at Morne Mitan and La Ty fault. Boiling ponds of extremely acid water were emplaced at Cratère Sud and Tarissan in late 1997 and 2001 respectively. The Cratère Sud pond is not easily accessible and likely intermittent, and Tarissan pond is permanent but its level (ca. 87 m below the summit surface: [START_REF] Kuster | Guadeloupe, Canyons, Gouffres, Découverte[END_REF] is highly variable in time (Beauducel pers. com., Allard et al., 2014).

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Thermal springs are concentrated around the base of the dome mainly in the SW, S and NE sectors (Fig. 1a). This distribution is controlled by the structure of the volcanic edifice and the extensive development of argilic hydrothermal alteration along preferential zones (Fig. 1b). The basement of the volcanic structures (caldera and flank collapse craters), to which the recent volcanic activity has been confined, forms preferential zones of shallow ground-water circulation such as the Cratère Amic structure. Most springs are located at high altitudes (950 to 1170m) within or at the exit of the Cratère Amic around the dome (at a maximum distance of 1.2 km from the summit):

Carbet Amic structure within which La Soufrière dome has been built [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF]. CC spring, which displays similar geochemical anomalies, is located on the fault system of the Chutes du Carbet, suggesting that its phreatic system is related to that developed inside Cratère Amic. The 3. Geochemical survey of gas plume, fumaroles, acid ponds and thermal springs

Sampling and analytical methods

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A very complete and systematic survey of the volcanic activity of La Soufrière has been progressively developed after the 1976-1977 volcanic crisis by the Observatoire Volcanologique et Sismologique de Guadeloupe (OVSG-IPGP), which is part of the Institut de Physique du Globe de Paris (Feuillard et al., 1983, Bigot et al., 1994, andOVSG-IPGP website.) [START_REF] Delorme | Composition chimique et isotopique de la phase gazeuse des volcans calcoalcalins: Amérique Centrale et Soufrière de Guadeloupe[END_REF][START_REF] Bernard | Remote and in situ plume measurements acid gas release from La Soufrière volcano, Guadeloupe[END_REF]. Gas compositions are also corrected for air pollution (N 2 or O 2 ) but samples displaying evidences for large air/gas ratios (e.g. N 2 > 5 mol%) are discussed separately. H 2 O and HCl are measured in gas condensates (by gravimetric and ion chromatography methods at the OVSG-IPGP and by IC-ICP-MS at the LGSV-IPGP). Additional measurements of halogens (F, Cl, Br and I) contents are also performed by IC and ICP-MS at theLGSV-IPGP [START_REF] Michel | Determination of halogens (F, Cl, Br,I), sulphur and water in 17 reference geological material[END_REF]. When sampling is possible, acid pond compositions are determined with the same methods as those used for thermal springs. The specific field conditions of La Soufrière fumaroles and particularly their low temperature, which is near the boiling point, lead to significant difficulties for gas sampling and analysis. Sampling and analytical methods are described in details on the OVSG WebObs website [START_REF] Beauducel | WEBOBS : Integrated monitoring system interface for volcano observatories[END_REF][START_REF] Beauducel | Recent advances in the Lesser Antilles observatories Part 2 : WebObs -an integrated web-based system for monitoring and networks management[END_REF] and in [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF]. Mean temperature and major element compositions of springs and acid pond waters and fumaroles condensates collected between 1979 and the present day are reported in Table I. Mean temperature and major gas compositions of fumaroles (dry gas basis) are reported in Table II. Temperature and major elements compositions for more limited periods have also been reported in [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF] and [START_REF] Chen | Zn isotope compositions of the thermal spring waters of La Soufrière volcano, Guadeloupe Island)[END_REF].

Complete data sets (temperatures, springs and gas compositions in major elements) are included in

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the OVSG-IPGP database (WebObs website, [START_REF] Beauducel | WEBOBS : Integrated monitoring system interface for volcano observatories[END_REF][START_REF] Beauducel | Recent advances in the Lesser Antilles observatories Part 2 : WebObs -an integrated web-based system for monitoring and networks management[END_REF]; see also OVSG-IPGP and IPGP -public access-websites). A comprehensive consistent data set for halogens data and CSC gas composition is available as Supplementary Material .

3.2 M ain geochemical characteristics of thermal springs, acid ponds and fumaroles 3.2.1 Thermal springs:

All springs have relatively high flow rates ranging from some Kg/s to 100 Kg/s for BCM. and oxidised H 2 S-bearing magmatic fluids [START_REF] Bigot | Traçage chimique de la circulation des eaux souterraines sur le volcan de la Grande Découverte (la Soufrière), Guadeloupe[END_REF][START_REF] Brombach | Geochemistry of the thermal springs and fumaroles of Basse-Terre Island, Guadeloupe, Lesser Antilles[END_REF]; they are all located within the Cratère Amic structure. CC waters could be the result of mixing between Ca-SO 4 waters and Na-Cl waters found in the deep geothermal system of Bouillante on the eastern coast of Basse Terre [START_REF] Traineau | The Bouillante geothermal field (F.W.I.) revisited: new data on the fractured geothermal reservoir in light of a future stimulation experiment in a low productive well[END_REF]. However, CC waters cannot be considered as a simple mixing between these two components because of the large Cl -enrichment counterbalanced by Ca ++ and Mg ++ (in the 1:1 ratio, [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF]. BCM-EV waters differ from the other Ca-SO 4 waters in having very low Cl contents. HR waters could correspond to shallow groundwaters with compositions close to that of regional cold springs heated by magmatic heat transfer. Large Cl enrichments are characteristic of springs inside or connected to (CC) the Cratère Amic structure. 2). However, SO 4 --and HCO 3 -contents of acid ponds are much lower and even negligible relative to their Cl -contents contrary to most hot springs, which typically have more SO 4 --than Cl -. Acid ponds compositions define the Cl-rich end-member towards which most waters of active springs are trending.

Fumaroles.

The gas temperatures are generally close to the boiling point, inevitably leading to H 2 O condensation along the sampling line which dissolves mainly HCl and to a lesser degree some sulphur species and little CO 2 (see Giggenbach et al., 2001 and references therein). In addition, mixing with air within the gas plume may modify redox conditions (e.g. lowering the H 2 S/SO 2 ratio or to a lesser extent the CO/CO 2 ratio), but these reactions are kinetically limited. For these reasons large amounts of gas and condensates are collected, minimising condensation effects on dry gas measurements. Solid S may also occur as deposits around the summit fumarole vents or as precipitates in sample condensates, leading to underestimation of the total S/ total C ratio. The most robust gas data are mainly total S/total C ratio, minimum gas temperatures, and Cl (and other halogen) contents measured in gas condensates es currently have to be interpreted with caution (see discussion below).

CSC is the most active summit fumarole and the most continuously sampled; it mainly consists of H 2 O vapour containing CO 2 , H 2 S and HCl with only minor SO 2 and H 2 , as well as traces of CH 4 and CO, with temperature varying between 96 and 130 °C (Table II). Low temperature gases (T = 94-95°C) were collected just before the renewal of the fumarolic activity in 1997 at both the summit of the dome and the La Ty fault [START_REF] Brombach | Geochemistry of the thermal springs and fumaroles of Basse-Terre Island, Guadeloupe, Lesser Antilles[END_REF]. They mainly consisted of water vapour (93-97 mol % H 2 O) with minor CO 2 , H 2 S and H 2 (Fig. 3) and no significant amounts of SO 2 and HCl and other reduced species (CO, CH 4 ). These compositions do not directly derive from a pure magmatic gas. Isotopic compositions of He, C, H and O indicate that all these fluids are mixtures between atmospheric, meteoric and magmatic components and likely result from boiling hydrothermal aqueous solutions equilibrated at low temperatures [START_REF] Allard | Dégazage magmatique diffus à la Soufrière de Guadeloupe, Antilles[END_REF][START_REF] Brombach | Geochemistry of the thermal springs and fumaroles of Basse-Terre Island, Guadeloupe, Lesser Antilles[END_REF][START_REF] Ruzié | Noble gas isotopes in hydrothermal volcanic fluids of La Soufrière volcano, Guadeloupe, Lesser Antilles arc[END_REF][START_REF] Beauducel | Rappel des faits et point de vue personnel sur les événements qui ont marqué la Guadeloupe et la communauté volcanologique[END_REF]. The fumarolic field at the base of the dome (LaTy and Route de la Citerne -RC-) has not been significantly modified in composition and temperature since 1997.

A C C E P T E D M A N U S C R I P T

The fumarolic activity at the summit of the dome (Cratère Sud Central and North -CSC and CSN-and Tarissan fumaroles) has increased considerably since 1997. Maximum vapour fluxes were observed between 2000 and 2010. Vapour fluxes at Cratère Sud were estimated on the basis of direct gas speed determination in 2005 and through remote thermal imagery in 2010 (Dupont, 2010, Beauducel Pers. Com): they have increased from ~500 to ~1600 T/day. This represents the major part of present day heat loss at la Soufrière, estimated at around 60 MW (Beauducel Pers. Com.).

Compared to pre-1998 fumaroles, the summit fumaroles have also high H 2 O contents (93-98%) but relatively lower contents of CO 2 and H 2 and are higher in H 2 S, SO 2 and CO. They are also characterised by significant amounts of HCl (up to 0.5 mol% in total gas), which has only been observed in summit fumaroles during the 1976-1977 volcanic crisis (Table II). These compositions suggest an increasing contribution of a S-and HCl-rich component of magmatic origin. Conversely there is no evidence for changing CO 2 input from the deep magmatic systems [START_REF] Ruzié | Carbon and helium isotopes in thermal springs of La Soufrière volcano (Guadeloupe, Lesser Antilles): Implications for volcanological monitoring[END_REF]. [1979][1980][1981][1982][1983][1984][1985][1986][1987][1988][1989][1990][1991][1992][1993][1994][1995] have been interpreted in previous papers [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF][START_REF] Boichu | Degassing at la Soufrière de Guadeloupe Volcano (Lesser Antilles) since the Last Eruptive Crisis in 1976-77 : Result of a Shallow Magma Intrusion[END_REF] and we will here mainly focus on the second period (1995-present day).

Major element composition variations.

Time series of hot spring compositions are reported in Fig. 4 and in Supplementary Material.

They display long term and short term (months to one year) variations. The short term variations have similar frequency characteristics for all ion species in all springs, but with very different amplitudes. The short term variations are of low amplitude in springs outside Amic Crater and of higher amplitude for springs inside the structure and in particular the sources at higher altitude (CE,

A C C E P T E D M A N U S C R I P T

Ta, Ga). Springs outside the Cratère Amic (HR, BCM EV), except CC, do not display significant long term variations in their chemical compositions over the entire sampling period (~35 years).

Cation contents of springs inside Cratère Amic and CC generally display a slight and regular decrease over the whole sampling period(. I). Two springs however, Ta and Ga, display specific variations: Ta spring, since the beginning of sampling in 1995, displays short term oscillatory variations for all ions, whereas Ga spring, since 2001, displays large Cl anomalies lasting up to some months. respectively). CE spring displays a very large temperature decrease between 1979 and 2010. In this spring, from 1979 to 1992, some small temperature spikes (1 to 3°C) lasting ~3-4 months were observed and are clearly synchronous to the largest Cl spikes of this period (Fig. 4c). Later, the temperature smoothly decreases down to the ambient air temperature (~20°C). It is to be noticed that temperatures of the springs located inside the Cratère Amic structure tend, at least since 1995, to increase towards higher values closer to the steady temperatures of springs located outside the structure.

T hermal springs: short term variations

The large chemical and thermal variations at short time scales observed in springs located inside the Cratère Amic structure during the 1979-1995 period (mainly CE spring and to lesser extents BJ and Ga springs) have been described in details by [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF]. These variations have no periodic character and the delay between two Cl (and T) pulses progressively increases with time. They have been interpreted as the result of the pulsatory degassing of a magma intrusion [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF][START_REF] Boichu | A Model for Episodic Degassing of an Andesitic Magma Intrusion, Part 1[END_REF][START_REF] Feuillard | La Soufrière de Guadeloupe, un volcan et un peuple[END_REF]. This regime ended in ~1995. In the recent period most springs evolve towards steady compositions and temperatures, but two springs, Ta and Ga, display large composition and temperature variations at short time scales (some months).

Since the beginning of sampling in 1995, Ta spring displays correlated short period variations in both temperature and bulk major element composition, with a clear periodic character (Fig. 5a).

Fourier transform analysis of T and Cl variations indicates a dominant period of ~12 months, which corresponds to the annual variations in rainfall regime. The T-and Cl-maxima correspond (with a delay varying in time between 1 and 3 months) to rainfall minima at the dome summit, suggesting that runoff and rain water infiltration represent the major control of the temperature and composition variations of Ta spring. The rain-fall regime recorded since 1992 at the dome summit -and major elements-time series of Ta spring in Fig. 5a, and to the temperature and Cl-time series for BJ and Ga springs in . This increase suggests a significant decrease in edifice permeability, which is more compatible with progressive bulk sealing of the host-rock than with any significant meteorological or seismic event occurring at that period. The 21 Nov. 2004 Les Saintes regional earthquake (Mw 6.3;[START_REF] Beauducel | Empirical model for rapid macroseismic intensities prediction in Guadeloupe and Martinique[END_REF][START_REF] Feuillet | The Mw = 6.3, November 21, 2004, Les Saintes earthquake (Guadeloupe): Tectonic setting, slip model and static stress changes[END_REF] might have been expected to enhance fluid circulation and thus shorten the delay between rainfall maxima and T and Cl minima, but this was not observed. In BJ spring, the delay is approximately constant (~2 months) over the whole record period. This delay is consistent with that obtained by hydrologic tracing of rain water transfer from the dome summit to the BJ spring (2-3 months) performed in 1993 by [START_REF] Bigot | Traçage chimique de la circulation des eaux souterraines sur le volcan de la Grande Découverte (la Soufrière), Guadeloupe[END_REF]. Cl-time series of Ta spring is also well correlated with the delayed rainfall regime, but for the other springs (BJ and Ga) the agreement is poor. Runoff and infiltration of the intense rainfalls on the summit area of La Soufrière volcano (~ 10 m/year) are thus major direct contributors to hydrologic and thermal budgets of the springs which are the closest to the dome summit. In addition, EC spring which has a similar position to Ta spring relative to the dome and is thus fed by a small summit aquifer. The large variations in composition observed from 1979 to 1985 could also be explained by the rainfall regime. However, unlike Ta spring, there is no correlation between composition variations and rainfall (Fig. C, Supplementary material) and the main source of these fluctuations in composition must be found in magma or hydrothermal deep sources (see [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF][START_REF] Boichu | Degassing at la Soufrière de Guadeloupe Volcano (Lesser Antilles) since the Last Eruptive Crisis in 1976-77 : Result of a Shallow Magma Intrusion[END_REF]. For all other springs there are no significant correlations between temperature-or composition-time series and rain fall regime over the whole sampling period, especially for springs outside the Cratère Amic structure and far from the dome (BCM-EV, HR, CC).

Since II.

The pH, Cl -and SO 4 --are also measured in some fumaroles condensates from CSC and CSN (Table Ib and Fig. 7). Few accurate and consistent condensates compositions exist for fumaroles active before 1983, in particular for the 1976 crisis (IPGP, Internal reports, 1976-1984). Since 1992 the evolution of the activity of summit fumaroles has shown a characteristic increase in gas flux from ~ 1998 to ~2008, followed by a slight decrease. Simultaneously there has been a large increase in total sulfur content (mainly as H 2 S species) relative to CO 2 content (Fig. 3 and7a). On the basis of the few bulk gas composition measurements (including H 2 O), this is interpreted to result from a significant increase in S-species in the gas, rather than an actual decrease in CO 2 , which has likely remained approximately constant (see also [START_REF] Ruzié | Carbon and helium isotopes in thermal springs of La Soufrière volcano (Guadeloupe, Lesser Antilles): Implications for volcanological monitoring[END_REF]. Since 1992, total S content has peaked in early 2008 before initiating a decrease that is ongoing up to the present time. This evolution is roughly correlated to the bulk increase in CSC fumarole temperature.

Temperature and the HCl contents in fumaroles (Table Ib) Halogen (F, Cl, Br and I) contents in spring waters and fumarole condensates have been determined using methods described in [START_REF] Michel | Determination of halogens (F, Cl, Br,I), sulphur and water in 17 reference geological material[END_REF] and [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF].

Data are reported in Table III and the time-series diagrams of Fig. 8. 1b). In BCM EV and HR spring waters the Cl contents are low (< 40 ppm, ~1 mMol/L) and almost constant over the entire sampling period (Fig. 4a). However in all other springs Cl contents are significantly higher and strongly variable in time. In CE spring Cl contents have strongly decreased with time from a maximum of ~20 mMol/L in 1981 to values close to the detection limit at the end of its activity in 2009. In CC and Ga springs Cl variations with time are large and complex (see major element variations, above).

Trace halogen elements: Br and I.

The range of composition of Br and I in hydrothermal fluids is very large (4 to 5 orders of magnitude, Table Ib, Fig. 8a). For the 1979-1998 period no iodine measurements are available, except for Ga spring. Fumaroles have generally much lower Br and I contents than thermal spring waters: Br contents are lowest in pre-1998 fumaroles (RC, Br < 10 ppb) and they are < 0.1 ppm in most summit fumaroles. Some fumarole condensates however reach Br contents ppm. Br contents of thermal springs range between 10 ppb and 1 ppm but, exceptionally, reach higher values (~2 ppm) as in Ga or CC springs. Like Cl, Br and I contents display large temporal variations in fumaroles, in waters of springs located inside the Cratère Amic structure as well as in CC spring.

Halogens ratios.

Although the range in Br and I composition is large, the range of Br/I of all hydrothermal fluids is relatively narrow (5 -40; mass ratio) compared to analytical errors (~20% relative, [START_REF] Michel | Determination of halogens (F, Cl, Br,I), sulphur and water in 17 reference geological material[END_REF]. Fumarole condensates have similar Br/I ratios as thermal springs. This ratio is in the lower range of the few Br/I ratios reported for Antilles magmas and other andesitic magmas 8b, see also [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF]. Cl/Br ratios are slightly different from one spring to another and range between 350 and 450. These Cl/Br ratios are in the range of the Cl/Br ratios of most andesitic magmas and high temperature magmatic gases in the Antilles [START_REF] Villemant | H 2 O and halogen (F, Cl, Br) behaviour during shallow magma degassing processes[END_REF]. Since 2001, the Cl/Br ratio in Ga spring displays distinctive features (Fig. 8b). The very large increases in The thermal and geochemical features of thermal springs during the first period have been consistently interpreted in previous works as the impact of long-lasting degassing of a shallow magma intrusion emplaced in 1976 or slightly earlier on the hydrothermal system confined inside the Cratère Amic structure [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF][START_REF] Boichu | A Model for Episodic Degassing of an Andesitic Magma Intrusion, Part 1[END_REF][START_REF] Feuillard | La Soufrière de Guadeloupe, un volcan et un peuple[END_REF]. This interpretation is also consistent with Cl isotopes measurements [START_REF] Li | Isotopic evidence for quick freshening of magmatic chlorine in the Lesser Antilles arc volcanoes[END_REF]. The reactivation of seismic activity in mid-1992 and of fumarolic activity at the dome summit beginning in 1997 was relatively rapid.

This raises the issue of interpreting geophysical and geochemical signals in terms of: (1) a possible magmatic reactivation at depth, (2) a disturbance of the shallow hydrothermal system in response to the development of a thermal anomaly (for example related to the assumed 1976-1977 magmatic intrusion), and (3) modification of the properties of the host rock at shallow depth in the volcanic structure (by sealing or fracturing for example, [START_REF] Feuillard | The 1976-1977crisis of la Soufrière de Guadeloupe (F.W.I.): a still-born magmatic eruption[END_REF][START_REF] Zlotnicki | The volcanic activity of la Soufrière of Guadeloupe (Lesser Antilles): structural and tectonic implications[END_REF][START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF][START_REF] Boichu | Degassing at la Soufrière de Guadeloupe Volcano (Lesser Antilles) since the Last Eruptive Crisis in 1976-77 : Result of a Shallow Magma Intrusion[END_REF]. The present unrest period differs from the post-1976-1977 crisis period in that only one spring is affected and also by the nature and characteristics of halogen anomalies. The variations of the geochemistry of Ga spring waters and of summit fumaroles (mainly variations in HCl contents and in Cl/Br and S/C ratios) and the temperatures variations in fumaroles since 1997 have to be combined with phenomenological observations to constrain the possible causes of the current unrest.

6.2 Structural control of the shallow hydrothermal system.

The long term geochemical monitoring at La Soufrière of Guadeloupe shows that only fluids emitted by fumaroles at the dome summit and from springs inside a narrow domain delimited by the Cratère Amic structure and at distances < 1.5 km from the dome summit (Fig. 1) display significant variations in composition (Villemant et al., 2005 and this work). In addition, thermal springs inside the Cratère Amic structure have much more variable temperatures but overall lower background temperatures than springs outside the structure. The only exception is CC spring, outside the Cratère Amic structure, which displayed a large chemical anomaly in conservative ions (halogens), but no variation in temperature with time.

6.2.1

The local thermal anomaly.

The shallow structure of La Soufrière-Grande Découverte volcano is highly controlled by recurrent flank collapse events [START_REF] Boudon | A new scenario for the last magmatic eruption of La Soufrière de Guadeloupe (Lesser Antilles) in 1530 A.D.: evidence from stratigraphy, radiocarbon dating and magmatic evolution of erupted products[END_REF], Komorowski et al., 2005, Fig. 1b) which have created superimposed collapse structures whose basal surfaces constitute zones of preferential fluid circulations. Intense and protracted hydrothermal alteration of the wall rocks leads to progressive sealing of the system of fractures system and confinement of fluid circulation within the collapse structures. This hydrothermal system is confined into a relatively small volume (few km 3 maximum; [START_REF] Komorowski | Guadeloupe, in: Volcanic Hazard Atlas of the Lesser Antilles[END_REF], Komorowski, 2008, Lesparre et al., 2012) of the volcanic edifice that consists of several zones separated by fracture and structural discontinuities. anomalies (Fig. 4b,c). The absence of such thermal variations in other springs inside Cratère Amic during the same period is a clear evidence of the partitioning of the summit volcanic structure. The inverse correlation (with a specific time delay) between temperature variations and rain-fall regime in Ta, Ga and BJ springs over the entire survey duration (Fig. 5) is evidence of the significant yet local influence of surface runoff and infiltration of rainwater from La Soufrière dome area. Time delays, which are a measure of the transit time for meteoric water from the summit zone to the different springs, range between 1 and 5 months for the springs (Ta, Ga and BJ) which are at a distance of ~1 km from the dome and within the Cratère Amic structure. These values are consistent with those determined by hydrologic tracing from Tarissan pond to BJ spring in 1990 [START_REF] Bigot | Traçage chimique de la circulation des eaux souterraines sur le volcan de la Grande Découverte (la Soufrière), Guadeloupe[END_REF]. The Cratère Amic structure also acts as a watershed draining a large part of the rainwater falling on the summit area and thus strongly dilutes and cools the thermal waters. The background temperatures of springs inside the Cratère Amic structure show a systematic increase since 1979 (Fig. 4b) that could result from readjustment of the temperature of the edifice to a local conductive thermal anomaly. Indeed the equilibrium values for thermal springs inside Cratère Amic would be higher than 60°C without the effect of meteoric recharge and circulation (Fig. 9). The increase in mean spring temperatures can be explained either by reduction of regional rain-fall since 1994 and particularly over the last decade, or by progressive sealing of the system that can also lead to a reduction of the cold meteoric input into shallow phreactic systems. Assuming that the temperature gradient as a function of the distance to the dome is defined by springs outside Cratère Amic structure (Fig. 9), one can estimate the mixing mass fraction of thermal waters and rainwater for springs inside the structure. The rainwater temperature is almost constant over the whole year (22 ±2°C) leading to bulk contributions of cold rainwater varying from 50 to 75% for springs inside Cratère Amic. The increased heat input inferred for the current unrest period from the summit gas flux may not yet have affected the system as a whole..

A C C E P T E D M A N U S C R I P T

This inference is consistent with heat flux estimates from aerial infrared thermal imaging (Beauducel et al., . Pers. Com.). anomalies recorded in the different springs during this period, are interpreted as resulting from the injection of a series of HCl-rich magmatic gas pulses which have been dissolved and transported by groundwater over distances varying with springs position (see the advection-dispersion model of [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF]. This input of Cl -anions in spring waters was balanced by cationic species (Mg, Ca, Na) in varying proportions in the different springs, indicating that acid gas pulses followed different pathways to shallow depth where they chemically interacted with different materials. As with Cl -, cations anomalies also vanished in all springs between 1992 and 1995. These results

Geochemical anomalies in

suggest that the large and long-term disturbance of the hydrological system within the Cratère Amic structure that was induced by the 1976-1977 volcanic crisis had ended by about 1994-1995.

Either magma degassing at depth had stopped or the surficial hydrologic system became isolated from the magmatic source [START_REF] Boichu | Degassing at la Soufrière de Guadeloupe Volcano (Lesser Antilles) since the Last Eruptive Crisis in 1976-77 : Result of a Shallow Magma Intrusion[END_REF].

The Cl anomaly in CC spring extends over a much longer period of time (~25 years) and is not accompanied by significant temperature anomalies. This may be explained by a similar model by assuming that CC spring is directly related to the shallow hydrothermal system of Cratère Amic through a fracture system. The temperature of the spring is determined by the main local thermal anomaly (Fig. 9) whereas its composition is determined by the transfer of conservative ions of magmatic origin (mainly Cl -) over long distances (>2 km) within the edifice; the same series of magmatic gas pulses identified in springs inside Cratère Amic is able to generate the large chemical anomaly of CC spring assuming very long transfer times (~12 years, [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF]. The nested edifice collapse structures of La Soufrière-Grande Découverte volcano are cut by several fault systems: La Ty fault system N-S [START_REF] Zlotnicki | The volcanic activity of la Soufrière of Guadeloupe (Lesser Antilles): structural and tectonic implications[END_REF], and arc-transverse i.e. ~E-W normal faults [START_REF] Planches Feuillet | Extension active perpendiculaire à la C[END_REF][START_REF] Feuillet | Differential uplift and tilt of Pleistocene reef platforms and Quaternary slip rate on the Morne-Piton normal fault (Guadeloupe, French West Indies)[END_REF]) which may also act as drains for hydrothermal fluids. The main ~E-W fracturing system is underlined by the series of prominent waterfalls of the Chutes du Carbet.

A C C E P T E D M A N U S C R I P T

The marked increase in fumarolic activity in 1997, culminating in 2008, the bulk increase in temperatures (limited to some degrees) of the springs inside Cratère Amic, and the appearance of complex chemical anomalies restricted to a single spring (Ga), is evidence of a new regime of the surficial hydrothermal system at La Soufriere, with no residual effect originating from the 1976-1977 volcanic crisis. Cratère Amic, the mean composition calculated for the steady period provides a good estimate of their chemical background, which can be used in the future for detecting new anomalies (Table IV).

These background values allow us, for example, to compare the amount of Cl issued from the degassing of the 1975-1976 magma intrusion to that which has been input in spring waters up to ~1994 and the amount of Cl added in Ga spring since 1999. The two considered periods are similar in Ga spring (~2 10 5 Kg) and represent a third of the total estimated amount of Cl injected in all springs from 1979 to 1994 [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF]. The reactivation of the fumarolic activity at the summit of the dome in 1990-1992 has significantly preceded the chemical anomalies observed in Ga spring and is almost coincident with the seismic activity. Finally, it is to be noticed that a significant delay is also observed between the maximum HCl contents recorded in summit fumaroles (2000)(2001) and in the maximum total S contents (2009)(2010).

A C C E P T E D M A N U S C R I P T

6.4 M agmatic and hydrothermal fluids.

The fluids collected in fumaroles and thermal springs are mixtures in variable proportions of end-members: (1) a pure magmatic gas of likely relatively steady composition (because the magmatic emissions at la Soufrière have a highly homogeneous andesitic composition, [START_REF] Boudon | A new scenario for the last magmatic eruption of La Soufrière de Guadeloupe (Lesser Antilles) in 1530 A.D.: evidence from stratigraphy, radiocarbon dating and magmatic evolution of erupted products[END_REF] which however may be more or less strongly modified by decompression, temperature decrease and interaction with conduit walls rocks or aquifers during their transfer to surface (see e.g. [START_REF] Symonds | Calculation of multicomponent chemical equilibria in gassolid-liquid systems: calculation methods, thermochemical data and applications to studies of high temperature volcanic gases with examples from Mount St. Helens[END_REF] and ( 2) pure meteoric water. The compositions of hydrothermal waters confined to the shallow edifice over long periods of time, result from complex interaction and thermo-chemical re-equilibration of the two primary components with the edifice rock material. They may be represented by the steady compositions of Cratère Amic spring waters which, which are very similar at similar mean temperatures (SAS waters, Ca(Mg)-SO 4 type, Table IV). The hottest springs (T ~42°C), Ga and Ta, located at altitudes > 1100 m, have slightly higher ion contents (1.5 to 3 times higher, except SO 4 --

) than colder springs (T~29°C) located at lower altitudes (600-900 m). This suggests that compositions in equilibrium with the surrounding material (of mean andesitic composition) primarily depends on temperature. Temperatures and chemical characteristics of the magmatic and hydrothermal fluids during the 1976-1977 crisis or the current unrest period allow to identify the contributions of these different components.

Very high-flux gaseous emissions through the whole dome fracture system accompanied the seismo-volcanic crisis of 1976-1977.. Gas sampling conditions did not generally ensure reliable analyses, in particular for the ratios SO 2 /H 2 S, CO/CO 2 or H 2 /H 2 O (IPGP Internal Reports, 1976). By the end of the year 1977, improvement of sampling and analytical techniques allowed production of some significant gas analyses (see Javoy et al., 1978[START_REF] Delorme | Composition chimique et isotopique de la phase gazeuse des volcans calcoalcalins: Amérique Centrale et Soufrière de Guadeloupe[END_REF]and IPGP, Internal Reports, 1978 and subsequent years, for discussion). The most accurate analyses are those for the Lacroix Inférieur fumarole (1290 m) on the Lacroix fracture (Fig. 1a). This was the hottest summit fumarole which remained active at the end of the volcanic crisis. Its temperature rapidly decreased from shows variations of ~2 log units for the recent period and up to ~4 log units since 1977). Although these variations could be partly explained by sampling difficulties (see section 3.2), they define trends which are close to those defined by Soufrière Hills fumaroles collected at the onset of the eruption (Montserrat, [START_REF] Chiodini | Fluid geochemistry of Montserrat island, West Indies[END_REF], Hammouya et al., 1998;Figs. 3 and 10). Redox conditions and equilibrium temperatures of fumarolic gases may be estimated using H 2 /H 2 O, CO/CO 2 and CH 4 /CO 2 ratios Marini, 1998) and assuming single fluid phase equilibrium. H 2 and CO being the more rapidly reequilibrated species, their abundances reflect the last equilibration steps during the ascent of fumarolic gases or the sampling (see section 3.2.2). Different diagrams have been proposed to represent the CO-CO 2 -CH 4 -H 2 -H 2 O equilibria and are calibrated in temperature and redox conditions for different buffers (Giggenbach, 1987, Chiodini and[START_REF] Chiodini | Hydrothermal gas equilibria: The H2O-H2-CO2-CO-CH4 system[END_REF]; they also allow estimation of steam condensation paths during gas transfer or sampling. High temperature gases of the 1977-1978 crisis have equilibrated at relatively reducing conditions and at temperatures higher than 400°C; they plot in the pure vapour domains. Fumaroles emitted at the dome summit in the recent period have equilibrated at progressively increasing temperatures from ~150°C at the onset of the reactivation of the fumarolic activity in 1997 [START_REF] Brombach | Geochemistry of the thermal springs and fumaroles of Basse-Terre Island, Guadeloupe, Lesser Antilles[END_REF] to values > 300°C for the 2000-2010 period. Since 1997, fumaroles compositions also display an evolution towards more reducing conditions: CO/CO 2 and CO/CH 4 ratios of gases sampled at temperatures higher than 98°C slightly increase from 1997 to 2009 and then seem to decrease. However equilibrium temperatures and reducing conditions have remained slightly lower than those recorded in the 1977-1978 gases, after the main volcanic crisis.

The imprint of long-lived hydrothermal fluids on compositions of volcanic fluids is characterised by ( 1) H 2 S as the dominant S-bearing species and low or negligible SO 2 , (2) low or negligible HCl contents and (3) CH 4 > CO [START_REF] Giggenbach | Geothermal gas equilibria[END_REF][START_REF] Giggenbach | Redox processes governing the chemistry of fumarolic gas discharges from White Island, New Zeland[END_REF][START_REF] Chiodini | Hydrothermal gas equilibria: The H2O-H2-CO2-CO-CH4 system[END_REF].

The fumaroles at the base of the dome (La Ty and RC fumaroles active since the end of the 1976-1977 crisis) have typical hydrothermal characteristics: very low or undetectable CO, H 2 , HCl and SO 2 contents, and temperatures always very close to that of boiling water at the considered altitude.

They likely correspond to a boiling hydrothermal aquifer heated by the thermal anomaly and with initial composition represented by the SAS water (Table IV shallow phreatic or hydrothermal systems. The efficiency of gas -water interaction depends on the ratio between the mass of magmatic gas and hydrothermal fluids and their bulk solubility in hydrothermal waters, but also on the equilibrium temperature reached by gas before interaction (Fig. 11). At low magmatic fluid/hydrothermal water ratios, almost complete scrubbing of magmatic HCl and SO 2 by surficial hydrothermal aquifers [START_REF] Symonds | Magmatic gas scrubbing: implications for volcano monitoring[END_REF] leads to low temperature fumaroles characterised by large H 2 S content and low or undetectable SO 2 and HCl contents. CO 2 content should be less affected by scrubbing. When the proportion of magmatic fluid increases as a result, for example, of drying of the conduit or an increase in the deep magmatic gas flux, the process of scrubbing becomes less efficient. As the gas/water mass ratio approaches or exceeds 1, models for the gas phase predict a significant increase in temperature, (SO 2 +H 2 S)/CO 2 ratio, HCl content, precipitated sulphur and, later, possibly in SO 2 content as well. Because CO 2 is much less affected by scrubbing, it should experience less intense variations than HCl or SO 2 . In addition, the temperature at which the magmatic gas interacts with the groundwater is critical for S-bearing species, because if the magmatic gas is already significantly cooled (typically T < 700°C) the fraction of soluble SO 2 is negligible and almost all S is in the form of H 2 S which is almost insoluble (Fig. 11).

In 1977-1978 high temperature gases were likely very close in compositions to a pure magmatic component as suggested by the comparison with compositions of juvenile gas collected at the onset of Soufrière Hills volcano eruption in Montserrat (Hammouya et al., 1996;Fig. 10). Even the highest temperature gases (up to ~ 140°C) emitted during the current unrest period since 1997 never reached the compositions, equilibrium temperatures and redox conditions which prevailed during the 1976-1978 crisis. However, the evolution of temperature and composition of summit fumaroles since 1998, is consistent with a new and progressively increasing influx of magmatic gases into the shallow hydrothermal system with a progressive reduction, though still relatively efficient, of the scrubbing effects: increase in temperature, total S/CO 2 , and HCl content, and the developments of S precipitates at the gas vents.

Available temperature and chemical composition data of La Soufrière gases cannot be related unambiguously to a unique gas source and evolution path. Indeed the total S content and the SO 2 /H 2 S ratios may be modified by numerous processes such as influx of deep magmatic gases, gas scrubbing by meteoric water, variations of redox conditions or condensation-evaporation cycles [START_REF] Giggenbach | Geothermal solute equilibria. Derivation of Na-K-Mg-Ca geoindicators[END_REF][START_REF] Symonds | Magmatic gas scrubbing: implications for volcano monitoring[END_REF]. However, the simple re-heating of surficial aquifers is not sufficient to explain the range of temperatures and compositions observed since 1977, but a two endmember mixing model involving the periodic influx of hot magmatic gases into the surficial hydrothermal system is consistent with observations. This model is also consistent with studies that have identified sustained passive degassing of CO 2 and rare gases of deep magmatic origin [START_REF] Allard | Dégazage magmatique diffus à la Soufrière de Guadeloupe, Antilles[END_REF][START_REF] Brombach | Geochemistry of the thermal springs and fumaroles of Basse-Terre Island, Guadeloupe, Lesser Antilles[END_REF][START_REF] Ruzié | Noble gas isotopes in hydrothermal volcanic fluids of La Soufrière volcano, Guadeloupe, Lesser Antilles arc[END_REF][START_REF] Beauducel | Rappel des faits et point de vue personnel sur les événements qui ont marqué la Guadeloupe et la communauté volcanologique[END_REF]. This flux is variable in space and is significantly higher in high-temperature summit fumaroles than in low-temperature fumaroles and thermal springs. The isotopic composition of CO 2 flux has not significantly varied since 1995 [START_REF] Ruzié | Carbon and helium isotopes in thermal springs of La Soufrière volcano (Guadeloupe, Lesser Antilles): Implications for volcanological monitoring[END_REF].

6.5 H alogen behaviour in volcanic fluids: constraints on degassing-condensation-boiling processes 6.5.1 Halogens fractionation, tracer of late evolution processes of volcanic fluids.

The model discussed in section 6.4, defined on the basis of major gas species chemistry, is poorly constrained due to both the complexity of low-temperature chemistry and poor knowledge of compositions and evolution in time and space. The a posteriori model of the evolution of in halogen

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species (F, Cl, Br) of thermal springs of La Soufrière between 1979 and ~1995 has allowed us to identify the source and the impact of the 1976-1977 eruptive unrest on the surficial hydrothermal system [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF]. A consistent interpretative model of the episodic anomalies in Cland Br-contents and temperature of thermal springs allows to constrain some chemical and physical characteristics of the magma source [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF][START_REF] Boichu | A Model for Episodic Degassing of an Andesitic Magma Intrusion, Part 1[END_REF][START_REF] Boichu | Degassing at la Soufrière de Guadeloupe Volcano (Lesser Antilles) since the Last Eruptive Crisis in 1976-77 : Result of a Shallow Magma Intrusion[END_REF].

Temperature variations and halogen (F, Cl, Br and I) fractionation in post-1995 fluids (spring waters and fumaroles) put significant additional constraints on the recent evolution of the magmatic and hydrothermal system.

Halogen fractionation is very sensitive to low temperature evolution of hydrothermal fluids, but not to their high temperature evolution. It is now well established that the heavy halogens Cl, Br and likely I do not highly fractionate relative to each other during degassing of silicic melts (Villemant andBoudon, 1998, Balcone-Boissard et al., 2010) and during gas cooling and decompression at temperatures higher than ~120°C in dry conditions (Symonds et al. 1998). In addition, due to their high solubility in water, halogen acids produced by degassing of H 2 O-rich melts are very efficiently scrubbed by surficial hydrothermal and phreatic systems [START_REF] Symonds | Magmatic gas scrubbing: implications for volcano monitoring[END_REF]. D magmatic gases are thus almost completely depleted in halogens, which are transported as dissolved ions in hydrothermal waters. Halogens are not fractionated relative to each other during their dissolution in water and are transported as conservative ions, which means that they are not affected by the usual chemical evolutions in hydrologic systems (interaction with host-rock leading to mineral precipitation or dissolution, low temperature evaporation, dilution by meteoric water). The relative abundances of Cl, Br and likely I thus should remain constant over large domains of the compositional and thermal evolution of hydrothermal fluids. F has a specific behaviour in magmatic systems because it mainly remains bound to the silicate melt structure during H 2 O degassing and is thus much less efficiently extracted from melts than other halogens; this effect leads to high and variable halogens/F ratios in magmatic gases and in all derivatives [START_REF] Villemant | H 2 O and halogen in volcanic clasts: tracers of degassing processes during plinian and dome-forming eruptions[END_REF]. However, during the subsequent evolution of gases rising to the surface and their interactions with hydrothermal fluids at high temperatures, F behaves as other halogens and is not significantly fractionated.

In contrast, significant fractionations between halogens are expected during evolution at low temperature and pressure (i.e. pressures close to atmospheric pressure and T < 130°C) of the gas phase and particularly during vapour/liquid equilibrium. For example, recent studies have shown that low temperature interaction of magmatic gases with the atmosphere at Soufrière Hills (Montserrat) led to high Cl/Br fractionation in the gas plume through the formation of BrO species [START_REF] Bobrowski | Detection of bromine monoxide in volcanic plumes[END_REF], Villemant et al., 2008). It is also well known that azeotropic behaviour of

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halogen acids induces significant fractionation between the different halogen during evaporation or condensation cycles (see Azeotrope Data Bank, 2001; Fig. 12). Conversely, evaporation of neutral solutions and brines containing halogen ions causes no significant Cl/Br fractionation over a large range of vapour/liquid ratios, unless extreme degrees of evaporation are reached (see e.g. Liebsher et al., 2006, Shimulovich et al., 2004 andreferences therein).

Thus fractionation between halogens is a good tracer of the processes involved during magma degassing and further interaction of gases with hydrothermal fluids and the atmosphere. The halogen composition of thermal fluids collected at La Soufrière display characteristic features which are consistent with the model of episodic mixing between two fluid sources (a pure magmatic source and a shallow hydrothermal source) . The HCl-rich magmatic component supplies the present day high-temperature summit fumaroles but has suffered significant low-temperature condensation that leads to a drastic depletion in Br and I. When this magmatic gas is completely scrubbed by shallow groundwater, such fractionations do not occur. This means that heavy halogen fractionation only occurs in the very late evolution of the gas phase.

F-Cl fractionation.

The very low F contents of all volcanic fluids and specifically of the fumaroles at T> 96 °C is consistent with a major contribution of magmatic gases to the surficial hydrologic system with no significant contribution of a sea-water component: the magmatic component is F depleted because F is not extracted from melt into the H 2 O-vapour during magma degassing and the sea-water has a much higher F/Cl ratio than fumaroles (Fig. 8). The absence of a sea-water component has also been demonstrated for the pre-1995 period using Na/Cl ratios [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF]. Moreover, the Cl isotopic composition of thermal spring waters [START_REF] Li | Isotopic evidence for quick freshening of magmatic chlorine in the Lesser Antilles arc volcanoes[END_REF] is characteristic of a marked magmatic signature. This result excludes any significant direct contribution of sea-water to the shallow hydrothermal system of La Soufrière in contrast to the Na-Cl fluids of the geothermal area of Bouillante on the western coast of Basse Terre [START_REF] Brombach | Geochemistry of the thermal springs and fumaroles of Basse-Terre Island, Guadeloupe, Lesser Antilles[END_REF]. Moreover, the high Cl/F ratios of thermal springs inside the Cratère Amic structure (including CC spring) compared to springs outside the structure (e.g. HR, BCM-EV) can be also simply explained by the imprint of HCl-rich and HF-poor magmatic gases which are channelized and confined to this shallow structure.

Cl-Br and Br-I fractionation.

Though Cl abundances vary over a very large range in thermal springs inside Cratère Amic, the Cl/Br ratio remains generally constant and close to that of andesitic magmas of La Soufrière

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(mass ratio ~400) during the entire survey period (1979-present day), except for Ga spring during a transitional en ~2000 and 2008 (Fig. 8b). The Ga spring during that period displays a constant Br content ~0.2 ppm regardless of the Cl content, leading to extremely high Cl/Br ratios.

Moreover, the large Cl anomaly that culminated in 2009 is again characterized by a Cl/Br ratio in the normal range, which indicates that this enrichment affects Cl and Br similarly. Conversely, all gases from the summit fumaroles since 1998, display a very strong depletion in Br relative to Cl, whereas the Br/I ratio is not modified (Fig. 8).

Mechanisms of halogen fractionation.

Evaporation or ebullition of near neutral halogen-rich solutions generally produces almost H 2 O-pure vapour and all dissolved ions are enriched in the solution (residual brine) by the same factor: no relative fractionation between halogen ions is expected neither in the brine nor in the vapour phase, except as solubility product constants of salts are reached which only occurs at very high degrees of evaporation (Berndt and Seyfried, 1997 and references therein). An exception is the hydrolysis at high temperature of Cl-rich brines or chloride (Na-or Ca-) salts (Bischoff et al. 1995).

In natural systems this occurs at very high temperature by hydrolysis of solid salts or brines, as for example during interaction of basaltic lava flows (T > 1000°C) with sea-water leading to the production of HCl- Edmonds and Gerlach, 2007 and references therein) or through hydrolysis at high temperature (>400°C) and depth of CaCl 2 produced by water-rock interaction as suggested for Vulcano island [START_REF] Liberto | Genesis of chlorine and sulphur in fumarolic emissions at Vulcano Island (Italy) : assessment of pH and redox conditions in the hydrothermal system[END_REF]. Though not documented by in situ measurements, such processes very likely induce large halogen fractionation, as is usually observed during analysis of halogen contents in solids by pyrohydrolysis. In this method, halogens are extracted as halogen acids by high temperature hydrolysis and extraction kinetics are much slower for heavy halogens than for light halogens (see e.g. Michel and Villemant, 2003 and references therein). These processes are unlikely in the surficial hydrothermal system of La Soufrière where temperatures do not exceed 300 -400°C (see Fig. 10b). If they occur at depth they probably require the interaction of a high temperature magmatic source with salts or brines of hydrothermal origin. But the halogen signature of these gases produced at depth would likely be highly fractionated in heavy halogens and should be recovered in both thermal springs and fumarolic gas, which is not the case. Though experimental data on heavy halogen fractionation during hydrolysis do not yet exist, in view of our current knowledge this process is highly unlikely at La Soufrière.

Boiling or condensation of acid solutions may lead to significant halogen fractionation since mixtures between halogen acids and H 2 O have azeotrope solution with a temperature maximum

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(Fig. 12). Mixtures of H 2 O with HI and HBr have similar azeotrope maxima, whereas mixtures of H 2 O with HF and HCl have azeotrope maxima of lower temperature and halogen acid contents. The phase diagrams predict that cooling of high-temperature halogen acid vapours with initial compositions less concentrated than azeotrope maxima will produce highly concentrated liquids and residual vapours which are progressively impoverished in halogen acids. In addition, due to the shapes of the various ebullition and dew curves and the initial halogen contents , less impoverishment in HCl relative to heavy halogen acids (which are much less concentrated) is expected in magmatic gases. Finally, the phase diagrams predict that vapours condensation preserves a Br/I ratio close to that of the initial vapour. This simple process may explain the large heavy halogen depletion in the summit fumaroles.

Conversely, if a model involving acid lake evaporation is considered, the high ionic strength of these solutions has to be taken into account. The phase diagrams of all halogen azeotropes are well known only for pure acid solutions; for solutions of high ionic strength there is a significant shift of the azeotrope maxima towards higher temperatures and more halogen-rich compositions (Fig. 12b). The shift in temperature is likely similar to that of pure H 2 O, andthe increase of the ionic I) are evaporated by a factor ~20, the ebullition temperature will increase by ~ 30°C, consistent with the maximum temperatures observed for summit fumaroles (Table I, Fig. 7). If the relative positions of the azeotrope maxima of halogen acids are preserved, the ratio of halogen content in the vapour phase over that in the liquid phase is much greater for Cl than for Br (Br content in the vapour phase in these conditions is negligible) and the Cl/Br ratio of the vapour should be much higher than the initial ratio (~400). The Br/I ratios should remain similar. Such a mechanism could also explain most observed characteristics of La Soufrière summit fumaroles (CSC and CSN) which have very high HCl contents (up to 1.5 mol/L let ~0.5wt%) with a rough correlation between HCl content and ebullition temperature (up to 130°C, Fig. 7b), very high Cl/Br (and Cl/F) ratios and no significant Br/I fractionation relative to spring waters or low temperature fumaroles (RC, Table III). The production of such HCl-rich vapours has however never been observed at Tarissan , where temperature has remained lower than boiling temperature. The production of such highly HCl-rich vapours also requires extreme HCl contents in solutions (Fig. 12b),far above those observed in Tarissan or other acid ponds. Such a process could however occur below the CSC and CSN vents, where intermittent acids ponds have been observed

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and may reach such extreme compositions when close to total evaporation. Maintaining a high gas flux at the dome summit since 1997 by this mechanism would require the existence of permanent acid lakes of very large volume heated to high temperature. In addition, long term preservation of such acid lakes requires the continued input of new magmatic gas to maintain a low pH, which tends to increase by reaction with the wall-rock material. These conditions are inconsistent with observations and such a process may, if it exists, represent only a small and occasional contribution.

6.6 G a spring contamination.

The special chemical features of Ga spring waters since ~2000 may be explained by contamination by derivatives from magmatic gases of variable composition through time (Fig. 6 and SO 4 --but less acid than their gaseous source. They do not contain HCO 3 -because condensation and dissolution in liquid water mainly affects halogen acids and SO 2 but not CO 2 with its very low solubility. Since 1998, because the dominant low altitude wind direction is E-W, gas plume condensates have destroyed the vegetation on the western flank of the dome and could potentially contaminate all surficial aquifers in that sector, which is however not observed. Thus, the pre-2009 contamination of Ga spring was more likely generated by gas condensation inside the edifice during percolation at shallow depth through the Ga spring aquifer. The main fracture system affecting the southern flank of the volcano (Fig. 1) likely represents the connection between the sealed central feeding system for volcanic fluids and the phreatic system of Ga spring. Note that in order to generate SO 4 --anomalies in a phreatic system the SO 2 content in the gas must be significant, which is strongly in favour of a magmatic origin.

Since the end of 2008 the contamination of Ga spring has grown significantly more intense and the contaminant composition is modified: waters are slightly more acid and rich in Cl -, Ca ++ and Mg ++ and display a magmatic Cl/Br ratio, but no SO 4 --anomaly. A contaminating fluid of magmatic origin should have lower SO 2 contents and scrubbing of acid gas (mainly HCl) should be

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followed by a significant interaction with rock material. This suggests that interaction of the magmatic gas with surficial aquifers occurs at temperatures lower than ~700°C, where conversion of SO 2 into insoluble H 2 S is almost complete, but at sufficiently high temperature to prevent significant gas condensation, which would have led to Cl-Br fractionation. The absence of a positive HCO 3 anomaly also indicates that CO 2 scrubbing was, as expected, inefficient. The dissolved acids are progressively neutralised during transfer to the spring, by reaction with surrounding rocks and exchange of H + ions with Ca ++ and Mg ++ . These characteristics are similar to those observed in CE spring shortly after the 1976-1977 volcanic crisis [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF], but the absence of anomalies in SO 4 --and HCO 3 -confirms that present-day magmatic gases scrubbed in the Ga aquifer have lower CO 2 content and are more depleted in soluble SO 2 contents than those during the 1976-1977 crisis, consistent with gas plume compositions (see section 6.4).

We thus propose that the geochemical characteristics of Ga spring waters since 2000result from the contamination by a fluid the composition of which evolve in time with magmatic gas flux and gas interaction conditions inside the upper feeding conduit. The contamination occurs through a

At the onset of the fumarolic reactivation, progressively increasing amounts of high temperature and HCl-and SO 2 -rich magmatic gases were injected at shallow depth contaminating the Ga spring in both Cl -and SO 4 --. This transitional period was followed by more active magmatic gas production leading to greater contamination of the surficial phreatic system of Ga spring. The temperature of gases interacting with groundwater was low enough (i.e. < 700°C) for almost all SO 2 to be converted into insoluble H 2 S. More intense cooling and condensation of these magmatic gases en route to the surface through led to the summit fumaroles which are thus SO 2 -poor and H 2 S-and HCl-rich and highly fractionated in Br relative to Cl. The initial high temperature magmatic gases are also likely less rich in CO 2 than the fumaroles of the 1976 volcanic crisis. The shape and duration of the Cl anomalies recorded in Ga spring since 2000 suggest that they result from a series of contaminating gas pulses injected within the edifice and transported to Ga spring. Because of these long transfer duration the geochemical anomalies time series recorded in Ga spring are strongly delayed relative to the gas pulse time series recorded in summit fumaroles. Given the efficient conversion of magmatic SO 2 to insoluble H 2 S with decreasing temperature, scrubbing does not significantly affect the variation in total S flux at the dome summit, which represents a good record of the gas pulse time series.

6.7 H C l pulse time series, relative behaviours of halogens, SO 2 and C O 2 and seismicity:

evidences for a new shallow magma intrusion?

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The preceding discussion suggests that the total S flux in summit fumaroles and Cl anomalies in Ga spring are related to the same series of magmatic gas pulses. Transport of these fluids within the shallow edifice of la Soufrière to Ga spring may be described using the advection dispersion model developed by [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF] for the post-1976 volcanic crisis (1979-1992 period).

Time series evolution of the bulk seismic activity (total number of earthquakes/month), the Cl content in three key springs (CE, BJ and Ga) as corrected for advection delays [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF], and variations in total S content of summit fumaroles is shown in Fig. 13a. The clear correlation between the total S content of summit fumaroles and the Cl content in Ga spring correctedfor transfer duration indicate a common origin for SO 2 (+H 2 S) and HCl in the current crisis. Using the same physical characteristics of the transport model of [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF] (in particular transfer time of ~2 years), we have calculated the best fitting dates and relative amplitudes of a series of gas pulses recorded by Cl contents in Ga spring for the 2000-present period (Fig. 14a and Supplementary Material). The fit of the same series of pulses to the Cl-time series in Ga spring and the total-S contents in CSC fumarole (using very low gas transfer durations and large dispersion factors compared to those for Cl-pulses in Ga aquifer) leads to a highly consistent model (residues equal to 0 within analytical errors; Fig. 14b). In addition, this model shows that the flux of SO 2 +H 2 S is significantly less variable than that of HCl, suggesting some disconnectbetween the sources of the two magmatic gases (i.e. mafic vs andesitic magma sources; Fig. 14c).

The gas pulse time series calculated for both crises are compared with the seismic activity in Fig. 13a. HCl, SO 2 and CO 2 degassing after the 1976-1977 volcanic crisis and during the present unrest period are compared in Fig. 13b. In the absence of reliable gas composition data for the 1976-1977 volcanic crisis, we have used the Cl -and HCO 3 -records in CE spring, which is the closest to the dome, as a proxy for HCl and CO 2 degassing some years (4-5) after the crisis. These proxies are the current volcanic crisis. Both records display the same fundamental characteristics: SO 2 and CO 2 display long-term buffered-like variations and HCl displays pulsatory variations which, as discussed above, cannot be solely attributed to surficial effects. Such contrasting behaviours have also been described for example at the active Soufrière Hills volcano (Montserrat) nearby (Edmonds et al., 2002, 2003, Villemant et al., 2008). The behaviour of these different gas species is directly related to their solubilities in andesitic or dacitic magmas and to the degassing regime such that CO 2 and SO 2 (in that order) have much lower melt solubilities than H 2 O and HCl, which exsolve at much lower pressure and from more differentiated melts. It is thus likely that intrusions of new magma at shallow depth are at the origin of both volcanic crises and the simultaneous degassing of SO 2 (+H 2 S) and HCl. Although CO 2 was produced in large amounts in 1976-1977 as testified by gas and SO 2 [START_REF] Boudon | A new scenario for the last magmatic eruption of La Soufrière de Guadeloupe (Lesser Antilles) in 1530 A.D.: evidence from stratigraphy, radiocarbon dating and magmatic evolution of erupted products[END_REF]. CO 2 and SO 2 necessarily originate from deeper and more basic magma sources. Following a scenario frequently proposed [START_REF] Doukas | Sulfur dioxide scrubbing during the 1992 eruption of Crater Peak, Mount Spurr, Alaska[END_REF][START_REF] Crider | Magma at depth: a retrospective analysis of the 1975 unrest at Mount Baker[END_REF][START_REF] Edmonds | Excess volatiles supplied by mingling of mafic magma at an andesite arc volcano[END_REF] as examples) the reactivation of the seismo -volcanic activity in 1976-1977 and since 1992 at La Soufrière of Guadeloupe may be explained by input of new basic magma batches within the deep magma chamber, triggering ascent of small differentiated (andesite or dacite) magma intrusions and that stall at shallow depth. Rejuvenation of the magma feeding conduits facilitates the ascent of SO 2 and possibly CO 2 of deep origin through the edifice, with a degassing regime characterized by relatively long time constants.

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Every emplacement of a new differentiated magma intrusion leads to a two-step degassing regime [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF] mainly involving H 2 O and HCl. The first degassing step is synchronous with emplacement of a new magma batch and characterized by large production of HCl-rich H 2 O vapour.

Subsequently, when an intrusion stalls, a second, long lasting regime may occur which corresponds to pulsatory degassing induced by magma crystallization. This second regime is characterised by regular increase of the time interval between successive degassing pulses and regular decrease of their intensity [START_REF] Boichu | A Model for Episodic Degassing of an Andesitic Magma Intrusion, Part 1[END_REF][START_REF] Boichu | Degassing at la Soufrière de Guadeloupe Volcano (Lesser Antilles) since the Last Eruptive Crisis in 1976-77 : Result of a Shallow Magma Intrusion[END_REF]. From 1979 to 1995, after the 1976 volcanic crisis, the time delays between successive gas pulses increased with time (from ~ 2-4 months in 1979 to ~ 8 months in ~ 1990) and intensities also decreased with time. Since 1997, time delays between HCl degassing pulses have been longer but more stable (mean ~13 ±3.5 months) although their intensities have increased from 1994 to 2005-2006 to a more or less steady state (Fig. 14a). The present unrest conditions may represent progressive feeding of a system of shallow and differentiated magma intrusions since 1994-1995.

However, the degassing of such acid andesitic magma is not able to produce significant amounts of magmatic S-bearing gas, which requires the presence of more basic SO 2 -rich magmas. Thus the volcanic activity recorded in 1976 and the current period may have been triggered by involvement of more basic magmas which produce SO 2 -rich gases emitted at a similar rate, at least during the initial degassing stages. Among the different models proposed to explain the volcanic activity at La Soufrière since the 17 th century, this model best explains both the clear magmatic character of expelled gas pulses and the main physical characteristics of the pulsatory degassing regimes observed after the 1976-1977 crisis and in the present unrest.

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The relative amounts of magmatic gas involved since 1995 may be estimated from the spring flow rates and the Cl contents of contaminated springs, and the gaseous HCl emissions. These estimates indicate that the amounts of magmatic gas scrubbed within the Ga aquifer during the current crisis (1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013) Since the 1976-1977 volcanic crisis seismic activity has been characterised by a regular decrease in intensity reaching a quiescence period from ~ 1983 to 1991 (< 30 earthquakes/month or < 50 VT/year, Fig. 14), followed by renewal of the activity in 1992 that was not immediately accompanied by significant surficial volcanic manifestations. The renewal of fumarolic activity at the dome summit became significant at the end of 1997. During the 1979-1995 period a close correlation between the chemical and thermal anomalies generated in the shallow geothermal system and the seismic activity was observed [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF]. Reactivation of seismic activity from 1992 to 1997was concomitant with progressive migration of fumarolic activity from the basement to the summit of the dome, with no direct evidence of new magmatic gas input and no geochemical imprint in thermal springs until 1995. However, the sealing of the shallow edifice and the scrubbing effects of shallow aquifers can delay components of magmatic origin. A series of magma intrusions likely began in 1994-1995 (Fig. 13). We thus suggest that renewal of the seismic activity in 1992 was related to reactivation of the deep magmatic activity and progressive emplacement of a new small magma intrusion. Ascent and degassing of this intrusion reactivated fracturing of the surficial parts of the edifice which progressively allowed juvenile gases generated by at least two types of magmas (acid andesite and a more basic magma) to reach the surface and to generate the summit fumarolic activity beginning in 1997. 1 During the process of review of the article, Allard and co-workers (Allard et al., in press) have published independent estimates of the gas flux from composition gradients measurements in the plume and wind speed. These data lead to production rates 8 times lower. Comparing these estimation methods is beyond the scope of this paper. This is discussed in a forthcoming paper by Beauducel and co-workers (submitted).

A C C E P T E D M A N U S C R I P T 7. Conclusions: unrest conditions at L a Soufrière of G uadeloupe and implications for volcanic hazard assessment.

The shallow hydrothermal system of La Soufrière of Guadeloupe is confined to the Cratère Amic structure inherited from a series of flank collapse event affecting the upper parts of La Soufrière-Grande Découverte volcano (Figs. 1 and 15;[START_REF] Komorowski | Guadeloupe, in: Volcanic Hazard Atlas of the Lesser Antilles[END_REF], Boudon et al., 2008). The low permeability of the volcanic edifice and the existence of numerous major discontinuities in the dome and the host-rock generates small, discrete hydrothermal reservoirs.

Hence, different parts of this complex hydrothermal system are highly reactive to two major disturbances: the rainfall at the dome summit which is particularly intense (average ~10-12 m/year and currently ~ 6-7 m/year) and episodic input of magmatic gas from deep magma reservoirs or shallow intrusions. Monitoring the composition and temperature of the hydrothermal fluids (hot springs, fumaroles, acid ponds) thus provides a particularly effective technique to track the evolution of magmatic activity and volcanic crises (phreatic, phreato-magmatic or magmatic) and provides constraints for hazard assessment. Geochemical monitoring over ~35 years shows that the major modification of the shallow hydrothermal system related to the seismo-volcanic crisis of 1976-1977 ended in ~1991, about 15 years after the onset of the crisis (Fig. 15). After a period of seismic quiescence (1983)(1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991), renewed seismic unrest (of much lower intensity) occurred in 1992, followed by the reactivation of fumarolic activity at the summit of the dome which then further increased in 1997-98. Both unrest (1976[START_REF]Rapports d'activité des Observatoires Volcanologiques des Antilles (Gadeloupe-Martinique)[END_REF]1992-present) were likely triggered by emplacement of shallow magma intrusions.

We can estimate present day volatile fluxes through the summit fumaroles using recent gas flux and composition measurements (Beauducel et al., pers. com. and this work) to obtain total fluxes of CO 2 ~ 100 T/day, HCl~ 10 T/day and S expressed as SO 2 ~ 40 T/day. These can be compared for example, to present day total S fluxes at Soufrière Hills (Montserrat) of ~ 300-500

T/day (expressed as SO 2 ; MVO Website). On the basis of the composition of summit fumaroles (HCl, H 2 S and SO 2 ), contents of Cl -and SO 4 --in thermal springs, and the Cl/Br/I ratios in all the fluids, the gas flux has a juvenile magmatic origin and cannot be generated by fluctuations of the shallow hydrothermal system in response to a pure thermal disturbance, or by the interplay of shallow sealing/fracturing processes. The apparent low variation in CO 2 flux and the high HCl/total S ratio of the magmatic gas implies that the intrusion generating the major Cl -anomalies consists of a differentiated acid andesite or dacite magma.

A C C E P T E D M A N U S C R I P T

The mass of H 2 O vapour (M H2O ) extracted from the acid andesitic magma since 1997 may be roughly estimated assuming a Cl content of the melt of ~3000 ppm and D Cl vapour/melt ~20 [START_REF] Villemant | H 2 O and halogen (F, Cl, Br) behaviour during shallow magma degassing processes[END_REF]) at ~2x10 8 Kg, which represents ~10 % of the estimated total mass of H 2 O vapour produced since 1997 at the dome summit (i.e. summit fumaroles emit 90% of the H 2 O which however is of meteoric origin). This value is also in the same range as the estimated mass of H 2 O vapour produced at the dome summit during the 1976-1977 crisis (~10 8 Kg; P. Allard, personal communication). The corresponding mass of acid andesitic magma (M m ) able to produce this mass of H 2 O vapour at emplacement may be roughly estimated from the ratio M m /M H2O (~ 200) as proposed in the model of [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF]. This mass (~50x10 9 Kg) is also in the same range as the mass of magma involved in the 1976-1977 volcanic crisis as deduced from the pulsatory cristallisation-degassing model proposed by [START_REF] Boichu | Degassing at la Soufrière de Guadeloupe Volcano (Lesser Antilles) since the Last Eruptive Crisis in 1976-77 : Result of a Shallow Magma Intrusion[END_REF]: 0.05 to 0.1 km 3 equivalent to ~100 -200x10 9 Kg of magma. All these estimates must however be considered with caution due to the large number and possible range of assumed parameters.

In addition, the involvement of more basic magmas (basic andesites?) is likely because of the production of magmatic SO 2 . This contribution should be eventually identifiable through increases in total S fluxes (and possibly -but not necessarily-in CO 2 fluxes), SO 2 /total S ratio, and a decrease in HCl content relative to other major species. The trends observed since 2011 argue in favour of such a contribution, with a decrease in HCl production and a sustained flux of total S (Fig. 13). Such a balance between the contributions from two types of magmas is observed at the ongoing eruption of Soufrière Hills (Montserrat): a pulsatory production of HCl directly related to explosive activity and dome growth period involving a dacitic magma superimposed on a continuous production of SO 2 corresponding to degassing of a deep more basic andesite [START_REF] Villemant | Andesitic Magma Degassing Investigated Through H2O Vapour-Melt Partitioning of Halogens at Soufrière Hills Volcano, Montserrat (Lesser Antilles) Earth Planet[END_REF][START_REF] Edmonds | Excess volatiles supplied by mingling of mafic magma at an andesite arc volcano[END_REF]. Such evolution increases the hazard of a future magmatic or phreato-magmatic eruption.

Continuous determination of these key parameters represents a major challenge for geochemical monitoring of unrest conditions at La Soufrière of Guadeloupe.

The estimated masses of gas and magma involved in the current unrest (from 1997 to the end of 2013) are thus similar to those estimated for the ~2 year long 1976-1977 crisis, but gas emissions and magma input are distributed over a much longer period of time (~17 years) such that overpressures generated in the system are likely lower and the mean flux of magmatic gas is at least one order of magnitude lower than during the 1976-1977 volcanic crisis. However, both the progressive confinement of the fumarolic activity to the dome summit and the restricted distribution of the hydrothermal featuresindicate that the upper volcanic edifice is highly sealed, and has probably been only slightly fractured by the recent seismic activity. This result is consistent with VLF and electrical tomography surveys of La Soufrière dome (Zlotnicki et al., 2006, Nicollin et al., 2006,2007), and with the recent increase in rainfall transfer durations within the edifice 2 . The decreasing flux of cold rain water associated with a sealed and compartmented upper volcanic edifice promotes, the development of fumarolic activity and concentrated acid ponds at the expense of gas scrubbing by the upper phreatic system. This contrasts with after the aftermath of the 1976-1977 crisis, when intense seismic activity and phreatic explosions led to an intense fracturing of the upper edifice and favoured widespread circulation of magmatic gas. Relative to the post 1976 situation, the current unrest situation may last even longer if the system does not become more highly fractured in the future. However, if current unrest conditions are the consequence of a new magma production at depth, similar magma intrusions may occur over a relatively short interval of time, as suggested by [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF] for the period preceding the 1976-1977 crisis.

A C C E P T E D M A N U S C R I P T
Conversely, if emplacement of deep magma intrusions has stopped, we may expect long-lasting (decades) degassing of the current stalled intrusion, according to the model proposed by [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF] and [START_REF] Boichu | A Model for Episodic Degassing of an Andesitic Magma Intrusion, Part 1[END_REF][START_REF] Boichu | Degassing at la Soufrière de Guadeloupe Volcano (Lesser Antilles) since the Last Eruptive Crisis in 1976-77 : Result of a Shallow Magma Intrusion[END_REF]. A direct consequence of the confinement in volcanic and hydrothermal activity and the reduction of rainfall is a local increase of the hydrothermal alteration that likely favours continued sealing and development of zones of mechanical weakness within the upper edifice. The combination of sealing and injection of a new intrusion in a fragile edifice could lead to over-pressurisation of the shallow hydrothermal system, increasing the likelihood of partial edifice collapse and explosive activity at La Soufrière volcano as theoretically modelled for a different setting by [START_REF] Reid | Massive collapse of volcano edifices triggered by hydrothermal pressurization[END_REF] and proposed by Komorowski (2008). Edifice collapse and associated phenomena have been recognized as one of the most frequent hazards of the volcano over the last 10 000 years (LeFriant et al., 2006, Komorowski et al., 2005[START_REF] Todesco | Hydrothermal fluid circulation and its effects on caldera unrest, in Caldera Volcanoes: Analysis, Modeling and Response[END_REF].

Reconstruction of the evolution of the shallow and deep evolution of the volcanic and hydrothermal ) and Cl-bearing species is particularly well illustrated in this system. AlthoughS species 2 In addition, a recent tracing experiment using the injection of KI inside the Tarissan lake performed in April 2010, with a systematic survey of iodine recovery in the different springs around La Soufrière dome (ANR DOMOSCAN, D. Gibert, pers. com.) indicates very low recovery even 2 years after injection. This indicates a significant sealing of the surrounding of volcanic fluids feeding conduits.

(mainly H 2 S) and CO 2 are almost entirely transported in the gas phase from depth to the surface, gaseous HCl (and other halogen acids), because of their very high solubility in water, are almost dissolved in phreatic systems wherever the magmatic gas interacts with them (scrubbing effect of [START_REF] Symonds | Magmatic gas scrubbing: implications for volcano monitoring[END_REF]. Because of these contrasting behaviours it is necessary to combine measurement of S species (and specifically to survey the eventual increase of the SO 2 /total S ratio) in the gas plume and Cl -in the thermal springs to identify a common magmatic origin and reconstruct the time series of magma degassing pulses (Fig. 14).

The systematic and accurate high resolution measurement of halogen ratios in volcanic fluids, used in combination with the measurement of major species, provides a very efficient tool to determine the possible magmatic origin of the fluids and to reconstruct their complex evolution in the shallow volcanic edifice. (2013). See also [START_REF] Brombach | Geochemistry of the thermal springs and fumaroles of Basse-Terre Island, Guadeloupe, Lesser Antilles[END_REF] for comparison. See Fig. 1 for source locations.

Ib: Acid ponds and fumaroles condensates

Acid ponds (Tarissan and Cratère Sud Central) and gas condensates (Cratère Sud Central and Napoléon) are irregularly sampled, depending on accessibility. Gas compositions are recalculated to 100%, on an anhydrous basis by combining compositions of dry gas and condensates (HCl) when available on the same sampling.

T able I I: T emperature and major gas compositions of fumaroles. (1999[START_REF] Ovsg-Ipgp | Bilan mensuel de l'Activité volcanique et de la sismicité régionale de l'Observatoire Volcanologique de la Soufrière[END_REF]( ) and Lacroix (1977[START_REF] Ovsg-Ipgp | Bilan mensuel de l'Activité volcanique et de la sismicité régionale de l'Observatoire Volcanologique de la Soufrière[END_REF] fumarole gas compositions are given in Annex 2.

T able I I I: H alogen contents in spring waters, acid ponds and fumarole.

Maximum, minimum and mean values of halogen contents; Cl/Br and Br/I mass ratios. F, Cl and Br are in ppm, I in ppb. Abbreviations and explanations as in Tables I andII. Ta spring are clearly related to precipitation variations, which is not the case for other springs.

However, the increasing temperature of spring waters inside the Cratère Amic structure since the is directly related to a decrease in precipitation.

F ig. 4c: comparison of Cl and temperature anomalies for CE spring during the first period. T is measured as the difference between a smooth polynomial fit over the whole time series and individual values. There is a close correlation between the main T and Cl anomalies. This evolution is not related to the precipitation regime (see text for explanations). F ig. 7a: Variations of T, total S and CO 2 contents. Notice the anti-correlation between CO 2 and total S contents which likely corresponds to an absolute increase in S content rather than a depletion in CO 2 (see text for explanation). and Boudon, 1998[START_REF] Balcone Boissard | Behavior of halogens during the degassing of felsic magmas[END_REF]. Open blue square and blue arrow: sea-water and low temperature (< magmatic T) vapours.

Fumarole gas is characterised by very low F, Br and I contents at similar Cl contents compared to high temperature volcanic gas. The Cl/Br and Cl/F ratios of fumaroles are extremely high (>10 5 ), whereas the Br/I ratios are in the same range as that of spring waters and high T volcanic gas. Red dashed lines correspond to rock buffered vapour-liquid equilibrium (FeO-FeO 1.5 buffer, [START_REF] Giggenbach | Redox processes governing the chemistry of fumarolic gas discharges from White Island, New Zeland[END_REF], red dotted line are composition paths of vapours separated at different temperatures from liquid and equilibrated at different initial temperatures (see Chiodini and Marini, 1987). Other rock buffers: FMQ: fayalite-magnetite -quartz; NNO: nickel-nickel oxide. La Soufrière fumaroles: light blue domain summit fumaroles (CSC); blue dots refer to gas compositions with air contamination < 10% and sampling T > 98°C; yellow diamonds are 1997 fumaroles (data from [START_REF] Brombach | Geochemistry of the thermal springs and fumaroles of Basse-Terre Island, Guadeloupe, Lesser Antilles[END_REF]; red triangles are highest temperature (T > 130°C) fumaroles of [START_REF] Beauducel | Rappel des faits et point de vue personnel sur les événements qui ont marqué la Guadeloupe et la communauté volcanologique[END_REF][START_REF]Rapports d'activité des Observatoires Volcanologiques des Antilles (Gadeloupe-Martinique)[END_REF](data from compilation of Delorme, 1983)). Soufrière Hills data (Montserrat; [START_REF] Hammouya | Preand syn-eruptive geochemistry of volcanic gases from Soufriere Hills of Montserrat, West Indies[END_REF]: solid square is 720°C fumarole (1996); open triangles are low temperature fumaroles. White

Island fumaroles as a reference [START_REF] Giggenbach | Redox processes governing the chemistry of fumarolic gas discharges from White Island, New Zeland[END_REF] F ig. 11b: Scrubbing: Theoretical evolution of the composition of the gas phase during interaction of a high temperature (900°C) andesitic gas with low temperature (20°C) water. When the gas/liquid (water) ratio is close to or exceeds 1, contents of the gas phase in soluble and reactive species (SO 2 , HCl, CO, H 2 ) rapidly increase, whereas contents of other major species (H 2 O, H 2 S and CO 2 ) do not vary significantly. If scrubbing affects a volcanic gas cooled to T < 700°C, S-bearing species are mainly represented by H 2 S, which is insoluble, and the total S content of the gas phase is not significantly affected by scrubbing. No SO 2 emission can be expected even if the gas/water ratio is very large. ). In addition this signal is altered by the long transportation time for Ga spring water (~2 y delay) within the edifice (see Fig. 14a). The geochemical records display contrasting features for proxies of the poorly soluble (S-species and CO 2 ) and highly soluble (HCl) gas species for both fumaroles and proximal springs). For a given volcanic degassing activity CO 2 and SO 2 proxies display long-term buffered-like variations and the HCl proxy displays pulsatory variations. This reflects the difference in solubility thresholds of SO 2 , CO 2 , HCl (and H 2 O) in melts, the composition of magma sources, and the degassing mechanisms at depth (see text for further explanation).

The evolution of the composition of the summit fumaroles and Ga spring show significant differences for Cl-and S-bearing species: the first period corresponds to influx of HCl-and SO 2rich magmatic gas, followedby a higher flux of magmatic gas less rich in HCl but initially enriched 1979-1994 [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF](Villemant et al., ), 1994[START_REF] Chen | Zn isotope compositions of the thermal spring waters of La Soufrière volcano, Guadeloupe Island)[END_REF]this work, see Supplementary material). Model the same pulse injection dates, but it is assumed that transfer time for gas pulses is negligible compared to those for Cl -within Ga aquifer and that, in contrast, dispersion is much larger (and set arbitrarily 3 times higher). Only relative amplitudes are chosen for the fit of the total-S time series.

Residues are low and within analytical errors. See text for further explanations.

  Evidence for a new shallow magma intrusion at La Soufrière of Guadeloupe (Lesser Antilles). Insights from long-term geochemical monitoring of halogen-rich hydrothermal fluids Benoît Villemant, Jean-Christophe Komorowski, Celine Dessert, Agnès Michel, Olivier Crispi, Gilbert Hammouya, François Beauducel, Jean-Bernard de Chabalier To cite this version: Benoît Villemant, Jean-Christophe Komorowski, Celine Dessert, Agnès Michel, Olivier Crispi, et al.. Evidence for a new shallow magma intrusion at La Soufrière of Guadeloupe (Lesser Antilles). Insights from long-term geochemical monitoring of halogen-rich hydrothermal fluids. Journal of Volcanology and Geothermal Research, 2014, pp.1-89. 10.1016/j.jvolgeores.2014.08.002. hal-01064401

  Echelle (CE), Tarade (Ta), Bains Jaunes (BJ), Pas du Roy (PR), Galion (Ga) and Ravine Marchand (RM) and Ravine Roche (RR). The flow rate and the temperature at CE has strongly decreased over the past years, to reach ambient temperature and eventually to cease at the beginning of 2010. Three other springs are located outside the Cratère Amic structure, either on the eastern flank of the Grande Découverte volcano: at Chute du Carbet (CC) or on the western flank at Bains Chauds du Matouba-Eaux Vives (BCM-EV) and Habitation Revel (HR). CC is located at much lower altitude (~ 600 m) at a distance of around 2.5 km east of the summit. HR is located at ~ 3.5 km east of the dome at an altitude of ~ 600 m and BCM is located at ~ 1km north-west and at an altitude of ~ 1050 m. Eaux-Vives (EV) is in fact the catchment of BCM spring water transported through a pipe: EV waters have a chemical composition identical to those of BCM but the temperatures are systematically shifted by ~ -10°C. The geochemical survey from 1979 documented temporary compositional anomalies for the springs located in the sector delimited by the Cratère

  absence of geochemical anomalies in waters collected in the last ~35 years in the other springs located outside the Cratère Amic (HR and BCEM-EV) indicate that they are isolated from the direct influence of volcanic activity. The major chlorine and temperature anomalies evidenced after the 1976-1977 volcanic crisis for hot springs within the Cratère Amic have progressively decreased with time at a rate decreasing with distance to the dome: CE spring, which is the closest to the dome (~100 m), has reached a steady composition in 1992, whereas CC spring, which is the furthest one (~2.5 km), reached its steady composition in 2004. However since 2006, a single spring, Galion (Ga) has registered a new series of Cl anomalies which culminated in 2009.

  The acid ponds are emplaced inside dome summit fractures at Cratère Sud and Tarissan at depths of ~ 20 m and ~100 m respectively and are not easily accessible. These acid ponds are almost permanent since 1997 and 2001 respectively, but their depth may rapidly vary with time (±50 m at Tarissan). Physical and chemical characteristics of Tarissan acid pond are permanently monitored by OVSG-IPGP. Its temperature varies slightly around 97°C (

  4. T emperature and composition time series of hot springs and fumaroles4.1 T hermal springs: long term variationsMajor element composition evolution of springs define three distinct groups (

Fig

  Fig. 4b). It varies generally at constant SO 4 --/HCO 3 -ratios (linear arrays trending towards the Cl - apex of Fig. 2). The variations in Cl contents are the largest in CE spring during the 1979-1995 period and in Ga spring during the 2002-present day period. CC spring also displays a very large Cl anomaly during the period 1980-2005. Charge balance in Cl -rich waters is maintained by correlative increases in the bulk cation content, with no dominant variation of a specific cation as already observed by Villemant et al. (2005). F -does not display variations. SO 4 --and HCO 3 -display variable, smooth and of low amplitude (generally less than 1 mMol/l) changes, except for HCO 3 -in CE during the period 1979-1995 (no data are however available for SO 4 --during this period). Thermal springs located inside the Cratère Amic and close to the dome (mainly CE, BJ and Ga, sampled since 1979, and likely PR, RM and Ta springs which, however, were not sampled before 1995) display two distinct evolution regimes over the 1979-1994 and 1995-present day periods. The first regime is characterised by series of large Cl positive anomalies, the intensities and extents of which are more pronounced for the springs closest to the dome (as CE located ~100 m from the dome). In CE spring the pre-1995 Cl anomalies are also correlated to increases in SO 4 --,

4. 1 . 2

 12 Temperature variations. Time series of hot springs are reported in Fig. 4b, c. All hot springs located outside the Cratère Amic structure (including CC) display steady or slightly decreasing temperatures ( T< 5°C) since 1979. BCM remains at a remarkably high and constant temperature (58.5 ±0.5 °C). Hot springs located inside the structure, except CE, are characterised by almost constant or slightly variable A C C E P T E D M A N U S C R I P T temperatures before 1995 and a slow increase in temperature since 1995 ( T ~ 5°C in average over ~15 years). Ta and Ga springs display the largest temperature increases since 1995 (~7°C and ~6°C

Fig. 5b .

 5b Fig. 5b. To match the main characteristics of both rainfall-and T-time series, different time delays characteristic of each spring have to be applied. These time delays must also vary in a stepwise fashion. The delays are calculated using a simple best fit of the rainfall temperature correlations over some time windows. The delays increase with time from ~1.2 to 3months for Ta spring and from 3.8 to 4.5 months in Ga spring. The shifts in delay values are relatively sharp and occur for

-

  2001, the evolution of Ga spring composition is characterised by a series of significant Cl anomalies which are a-periodic and of increasing intensity with time: Feb. 01, Nov. 02, Oct. 03, Nov. 04, Aug. 05, June 07 and the largest one culminating in October 2009 (Fig. 4a). Cl enrichments are large (from ~7 10 -3 Mol/L in 2001 and 2003 up to 20 10 -3 Mol/L in October 2009, relative to a background of ~5 10 -3 Mol/L). They last approximately one month to 18 months (Fig. 6). The increases in Cl contents are diversely correlated with other major element variations and pH. The largest Cl anomaly (2009-2010) is also associated with slight increases in other major cations (Mg ++ , Ca ++ and Na+) and very slight decreases in HCO 3 pH. The other Cl anomalies are lower in intensity and characterised by slight increases in Ca ++ and SO 4 --and slight (or no) decreases in pH. None of these anomalies are correlated to temperature variations which remain low ( T = ± 2°C around the mean trend) or to the rainfall regime, even in 2010 during which dry and wet periods on La Soufrière dome were particularly extreme. There is also no apparent correlation with seismic A C C E P T E D M A N U S C R I P T activity and the main anomalies (2001 and 2009) occur during almost aseismic periods. Such anomalies have not been observed in any other spring since 1995. 4.3 F umaroles: temperature and composition variations. Temperatures and the of fumaroles at the base of the dome (Route de la Citerne, RC, which almost completely vanished in 2001) and at the summit of the dome (Cratère Sud Central -CSC-since 1992 and Cratère Sud Nord -CSN-since 1995) are reported in Table

  display three distinct periods of variation, the first one from ~1997 to 2002, the second one from 2003 to the end of 2008, and the third one since the end of 2009 (Fig. 7b). During the first two periods there is a large increase in fumaroles temperatures with maximum values that show transient peaks at ~ 108°C in mid-1999 and ~ 125°C in mid-2000, an overall increasing trend until mid-2001 (~115°C) and the highest value at the end of 2007 (~140°C recorded in CSN). Because of infrequent sampling difficulties and frequency (~1/month), these spikes are indicative of periods of high gas temperatures of unknown duration. Since 2009, fumaroles temperatures and vapour flux slightly decrease. HCl contents in fumarole condensates also reached a maximum (~2.5 Mol%) in 2001, but have remained relatively low (< 0.5 Mol%) since 2003. Because H 2 O contents are rarely determined and compositions of other major species (H 2, CO, CH 4, N 2, H 2 S, CO 2, SO 2, O 2 ) are measured and normalised to 100%, the absolute variations in HCl contents which are measured in condensates cannot be compared to those of other gas species. However the periods of maximum HCl (2001-2002) and total S contents (2009-2010) do not coincide. In Fig. 7b, variations of temperatures and Cl contents of CSC fumaroles and Ta spring are compared for the 1998-2008 period. When available, pH measurements of CSC acid ponds are also A C C E P T E D M A N U S C R I P T reported. Short term variations of Cl and T of both fluids are very closely correlated. They are also correlated to CSC pond pH values. This result clearly indicates that, as for Tarade spring, variations in composition and temperature of Cratère Sud fumaroles are mainly controlled by the rainfall regime. This control occurs through runoff and infiltration in the upper part of the dome. The fumaroles and Ta spring exhibit a similar time delay of ~ 3.5 months (see § 4.2).

  5. H alogens (F, C l, B r,I) in hydrothermal fluids.

5. 1

 1 Major halogen elements: F and Cl. All fluids (spring waters, fumarolic condensates) have systematically low F contents. Given that F contents in fumarolic condensates are very low compared to usual detection limits of Ion Chromatography (~0.1 ppm), we have implemented a protocol of pre-concentration by evaporation at low temperature to reduce these detection limits to ~ 0.05 ppm (~2.5 Mol/L). F contents of all condensates are now near this new detection limit. In springs outside the Cratère Amic, F contents are almost constant with ~ 2 ppm (0.1 mMol/L) in BCM-EV and about 0.2 ppm (0.01 mMol/L) in HR. F contents in springs inside the Amic structure, are slightly more variable and range between the detection limit and ~1 ppm (0.005 to 0.05 mMol/L). Contrary to F, Cl contents are elevated and highly variable in most sampled fluids. The pre-1998 fumaroles (RC) have very low Cl contents (~ 2 ppm ~ 50 Mol/L, Fig. 7b), whereas summit fumaroles since 1998 are characterised by very large and highly variable Cl contents (ranging between 20 and 50 10 3 ppm, Table

(

  ~20-100, Balcone-Boissard et al., 2010). Despite the very large temporal Cl variations observed in fumaroles or spring waters, the Br/I ratio remains almost constant (within analytical precision) for each spring. Cl/Br ratios of all thermal springs, with the notable exception of Ga spring for the period 2001-2009, also remain almost constant despite the occurrence of large Cl anomalies (Fig.

-

  Cl contents (up to 700 ppm Cl, l~20 mMol/L) that have been recorded since 2001 are also accompanied by Br and I enrichments, although they occur with a significant time delay since 2009.The Cl/Br and Br/I ranges in Ga spring before 2000 and after 2009 are ~350-400 and ~5-50 respectively, i.e. in the usual ranges. Conversely, from 2001 to 2008, the few low-amplitude Cl anomalies occurred at low and almost constant Br and I contents (~0.25 ppm and 15 ppb respectively) and lead to very high Cl/Br ratios (up to 3500) and Cl/I ratios (up to 150 000).Compared to thermal springs, summit fumaroles have much larger Cl contents but are highly depleted in both Br and I. The Cl/Br ratios of summit fumaroles range between 10 5 and 10 6 , whereas those of the pre-1998 fumaroles are much lower (RC, Cl/Br~750) and close to those of thermal springs. Thus, in comparison to the periods immediately after the 1976 volcanic crisis and since 2009, the 1998 to 2008 period, is characterised by the production of fluids which are strongly fractionated in Br and I relative to Cl and that have fed the summit fumaroles and specifically contaminated only the thermal spring (Ga) located south of the dome and closest to the path of gases that feed fumaroles in the center of the dome (Fig.1).A C C E P T E DM A N U S C R I P T 6 Discussion 6.1 Monitoring volcanic unrest at L a Soufrière of G uadeloupe.Geochemical monitoring of volcanic hydrothermal fluids can yield valuable insights, providing that the numerous processes that can affect magmatic gases during their transfer to the surface can be identified. By definition, during unrest periods (pre-or post-eruptive) the interactions of magmatic gas with surrounding wall rock material and pre-existing hydrothermal systems are very large and greatly modify the compositions of both residual gas (if not completely assimilated en route to the surface) and hydrothermal fluids. Scrubbing of soluble magmatic volatile species by hydrothermal systems is likely the most effective process[START_REF] Symonds | Magmatic gas scrubbing: implications for volcano monitoring[END_REF]. The less soluble major gas-species (CO 2 ) and the most soluble-gas species (halogen acids) have the simplest behaviors and are the better preserved tracers for geochemical monitoring of magma degassing. At La Soufrière, the evolution over the last 35 years, of the main gaseous species in fumaroles (CO 2 , H 2 S, SO 2 , HCl with the exclusion of H 2 O given its complex origins) and of the main ions of magmatic origin in thermal spring waters (HCO 3 ) clearly shows two or three distinct periods, depending on the location of the spring or the fumarole. For most springs a first period lasted from 1979 (the beginning of the geochemical monitoring) to ~1993-1994, during which time water compositions were significantly affected by the 1976-1977 eruptive crisis; this is followed by a second period which is still going on and during which spring waters reach steady and relatively ion-poor compositions. The only exceptions are the CC and Ga springs. For CC spring the influence of the 1976-1977 crisis lasted until very recently (2004). Ga spring had only a relatively short steady period (~1995 to ~2000) followed by a third (current) period characterised by a complex series of geochemical anomalies. Summit fumaroles were very active in 1976-1977 and then rapidly vanished. They were reactivated in 1991-1992 after a long respose time (>10 years) and are currently characcterised by an intense activity with highly variable gas compositions and relatively high temperatures (up to 140°C).

  The hydrothermal system is thus highly reactive to any disturbance by deep magmatic fluids and surficial input of cold meteoric water. Most short term thermal variations in thermal springs are directly related to rainfall (as in Ta spring for example), but some of them, as for CE spring, are clearly induced by convective transport of hot magmatic fluids. The local thermal anomaly related to conductive transport of heat from deep magmatic sources may be estimated from temperatures of springs outside Cratère Amic (BCM-EV, HR and CC) that do not show large variations over the last 35 years (Fig.4b). The localA C C E P T E D M A N U S C R I P Tthermal anomaly extends over a large zone in the Soufrière-Grande Découverte volcano. Assuming that it is axisymmetric and centred on La Soufrière dome, its extent may be estimated through the almost stable temperatures of thermal springs outside the Cratère Amic : BCM (~60°C, ~1 km), CC (~45°C, ~1.5 km), HR (~35°C, ~3 km; Fig.9). Inside Cratère Amic, the local thermal anomaly may be highly modified by convective heat transfer through the infiltration and confinement of most of the cold meteoric water falling on the dome summit, and, on the other hand, by fluids in direct transit from the magma source to the dome during periods of volcanic crises. The evolution of thermal anomalies in CE spring (the closest to the dome summit) since the 1976-1977 crisis is an evidence for such confinement effects: a large decrease in temperature, from T> 65°C to air temperature in less than 20 years and

  spring waters. Springs inside the Cratère Amic structure are slightly acidic, sulfate-rich and have experienced large variations in their Cl contents since the beginning of the geochemical survey in 1979. In contrast, springs outside the Cratère Amic structure (except CC, see below) are neutral, ranging between sulfate-rich (BCM-EV) and carbonate-rich (HR) end-members but never displaying significant geochemical variations: it is an evidence of the partitioning of the upper structure of the volcano and the confinement of the magmatic fluids within the uppermost central parts. For most springs the Cl anomalies observed since 1979 almost completely vanished by 1994-1995. The Cl -

6. 3

 3 Diversity of chemical and thermal variations in time.The 1979-1992 and post-1997 fluids inside the Cratère Amic structure exhibit a large diversity of evolution modes and intensities between the different fluid sources (springs and fumaroles) since 1997. From 1979 to 1994 composition and temperature variations of springs were large but consistent with each other if transfer time within each aquifer is taken into account: a unique series of chemical anomalies of magmatic origin is transferred within the different aquifers with time delays that are directly related to the distance between the spring and the dome summit[START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF]. The influence of this magmatic source on the shallow hydrothermal system has dominated other possible sources of chemical variations such as dilution by run-off and infiltration of cold meteoric waters prior its disappearance in ~1994-95. Since that period all springs inside Cratère Amic structure (except Ga) affected by periodic dilution by low T meteoric recharge or by a slow and progressive temperature readjustment to the local thermal anomaly. Since 1999-2000 Ga spring displays very specific compositional variations which are not correlated to any thermal anomaly, characteristics that differ from those observed in CE or Ga springs just after the 1976-1977 crisis. For every spring inside

  ~160°C at the beginning of 1977 to ~96°C at the end of 1978. Temperatures of other fumaroles had already decreased to ~96°C at the end of 1977, which is the ebullition temperature of pure water at the elevation of the dome area (1200 -1467 m). By 1981, almost all fumaroles had completely vanished, and only residual fumarolic activity (mainly the RC and LaTy fumaroles) was still observed at the base of the dome until mid-1997 at the same temperature ~96°C (Fig. 7). From 1990, the fumarolic activity reactivated and migrated to the top of the dome with the first evidences at the Cratère Sud (CS). In 1997, flux increased markedly and new fumaroles started to reactivate on the summit plateau (e.g. Napoléon fumarole, Tarissan). In late 1997 and early 1998, the CS gas emissions became markedly acid (pH mean 1.0 ± 0.6 between Mars 1998 and June 2001 for A C C E P T E D M A N U S C R I P T

  strength and shifts in azeotrope composition are likely similar for all halogens. Classical thermodynamic laws show that the boiling-point elevation of aqueous solutions is directly proportional to the molal concentration of non-volatile solutes (C) according to the equation T ~ 0.512 x C. If the most concentrated hydrothermal solutions measured at la Soufrière of Guadeloupe (i.e. the Tarissan pond waters, C max ~3 mol/L; Table

  and 8). During a transitional period from 2000 to 2008, Cl -anomalies were relatively low with very high Cl/Br ratios and accompanied by SO 4 --anomalies of similar amplitude, with only minor variations in other anions or cations and an occasional slight decrease of pH . The Cl -anomaly developed in Ga spring waters since 2009 is large and has a characteristic magmatic Cl/Br ratio and is accompanied by large increases in Ca ++ and Mg ++ but only slight decreases in SO the transitional period, the Cl -and SO 4 --anomalies suggest a contamination by gas of magmatic origin and containing significant amounts of soluble HCl and SO 2 . The large fractionation in Cl and Br indicates that significant condensation has affected the HCl-rich component. These fluids may represent high temperature gas condensates (equivalent to aerosols) that are rich in Cl -

  system was made possible by the remarkably long term high resolution monitoring of a set of geochemical and phenomenological data. Measurements of time series of the same parameters (T, SO 2 , CO 2, HCl and their main derivatives H 2 ) in all of the different hydrothermal fluids provides the data necessary to reconstruct the extremely complex history of volcanic fluids from the magma source to the surface. The complex succession of condensation and scrubbing processes affecting magmatic fluids in the shallow edifice leads to fumarolic activity which is highly variable in temperature and composition, and whose thermal and chemical history cannot be unambiguously reconstructed. The utility of the contrasted behaviours of S-bearing species (

A

  Fig.2and Fig.5). n: number of measurements (fortnight based sampling for most springs). n.d.: not determined.Ia: T hermal springsFor springs outside Cratère Amic structure (HR, BCM-EV) temperature and composition variations are low during the whole survey. Other springs inside the Cratère Amic structure display at least two different variation regimes, generally corresponding to 1979-1995 and 1996 -present day. Ga spring displays three distinct regimes. CC spring (outside Cratere Amic) displays two regimes with time periods different from other springs (see text for explanations). For data sources see OVSG-IPGP website and[START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF] and[START_REF] Chen | Zn isotope compositions of the thermal spring waters of La Soufrière volcano, Guadeloupe Island)[END_REF]. See also[START_REF] Brombach | Geochemistry of the thermal springs and fumaroles of Basse-Terre Island, Guadeloupe, Lesser Antilles[END_REF]

I

  I I a: Thermal springs. Values are calculated for the different regimes observed in the temperatures and Cl contents time series (see also Table I). (*) For Galion spring, two different Cl-Br correlations are observed from 1996. I I I b: Acid ponds and fumaroles. Halogen compositions in fumaroles are determined on condensates. T able I V : Steady compositions of A mic C rater spring waters (SASW). Comparison with pre-1976 compositions and estimates of the C l contamination of phreatic systems. I V a: SASW are calculated as the mean steady compositions of spring waters inside Cratère Amic structure with similar T and altitudes. SASW1: BJ, CE and PR (alt. ~600 m, T ~28°C); SASW2: Ga and Ta (alt. ~1100 m, T ~41°C). BJ (1968) and CE (1968) data from Feuillard, 2011). I V b: estimates of Cl contaminations of spring waters (CE, CC, BJ, Ga) after the 1976-1977 crisis and of Ga spring since the beginning of the current crisis. These are calculated as the difference Cl -time series and multiplied by the flow rate (expressed in Kg). This shows, for example, that the current Cl contamination by volcanic gases of Ga spring (since 2002) is similar to that estimated for Ga spring from 1979 to 1996 (~2 10 5 Kg, Villemant et al., 2005, Table2), and one third of the total estimated for all springs (see text for further explanations).A C C E P T ED M A N U S C R I P T F ig. 4b: Temperatures time series; comparison with precipitations. Symbols as in Fig. 4a. Black line: precipitations at the dome summit (reverse scale). All springs outside Cratère Amic structure (BCM, HR, CC) display only smooth and very low amplitude variations with time, with very low variations at long time scale (~35 y). Springs inside the structure display either long term variations or both long term and short term variations. Short term temperature (and Cl -, Fig. 4a) variations in

F ig. 5 :

 5 Composition and temperatures time series for T a, G a and BJ springs; comparison with precipitation regime. F ig. 5a : (top) Co-variations of major element compositions of Tarade waters and (bottom) close anti-correlation between temperature, Cl content and precipitation regime (dashed line). The dilution effect of rain-water affects both temperature and all major elements. Close adjustment between composition and temperature data variations and rainfall regime black dashed line requires delays in rain-water transfer times varying by step from 1.1 to 3.5 months (thick black line: rainfall corrected from transfer time delays).F ig. 5b: Anti-correlation between temperature variations since ~1992 in Galion (Ga) and Bain Jaune (BJ) springs and rainfall regime. T Anti-correlations between Cl and rainfall variations (not shown) are much less clear for Tarade spring : composition variations in these springs are likely dominated by other factors (see e.g. Fig.6). No relationships between composition or temperature and rainfall regime exist for the other main springs, which are at greater distance from the dome summit. F ig. 6: Composition and temperature anomalies in G a spring since ~2000. Ga spring displays a series of chemical anomalies since 2001 (top), the main one occurred from ~2009 to 2012. These anomalies (vertical dashed lines) are not correlated with rainfall regime (black continuous line) unlike the temperature variations. The main anomaly affects Cl -, Ca ++ , Mg ++ but also weakly pH, SO 4 --or HCO 3 -. The lower amplitude anomalies (arrows) between 2000 and 2006 affect both Cl -and SO 4 --. See also Fig. 8b. A C C E P T E D M A N U S C R I P T F ig. 7: T emperature and composition variations in fumaroles.

F

  ig. 7b: Comparison between variations in Cl contents and temperatures of summit fumaroles and Ta spring and the precipitation regime (not to scale, inverted scale). The clear inverse correlation indicates that precipitations directly control (by dilution and with no time delay) both the composition and the ebullition temperature of hydrothermal fluids at the fumaroles. Dome summit fumaroles: CSN, Cratère Sud Napoléon; CSC, Cratère Sud Central. Fumaroles at the base of the dome: RC, route de la Citerne. F ig. 8 H alogens compositions of L a Soufrière hydrothermal fluids F ig. 8a: Correlations between F,Cl,Br and I. Concentrations in ppm for F, Cl and Br and in ppb for I; log scales. F contents in summit fumaroles are close to or lower than detection limits. Domains: blue: spring waters (notice the relatively high F content of BCM-EV spring); yellow: low temperature fumaroles (RC, since 1983); light red: fumaroles (T> 96°C) at the dome summit, CSC, CSN); dark red: high temperature volcanic gas of Lesser Antilles magmas (model, from Villemant

F

  Fig 10 a, b: Thermodynamic equilibrium grids are from Giggenbach (1987, Fig. 10a) and Chiodini and Marini (1998, Fig. 10 b). Fig. 10 c: evolution of CO/CO 2 and CO/CH 4 of fumaroles for the 1998-2013 period.

  : black dots in Fig. 10 a; ellipses of increasing intensity of grey indicate increasing sampling temperatures (< 120°C, 120-500°C; > 500°C). See text for explanations. F ig. 11: Schematic evolution of the composition of a high temperature andesitic (H C l-rich) gas with cooling and scrubbing.F ig. 11a: Cooling: Theoretical evolution of a volcanic gas of andesitic composition from 900°C to 100°C is characterised by the absence of significant variations in H 2 O, CO 2 and HCl contents and a conversion of SO 2 to H 2 S below 700°C.

  of magma degassing history: comparison with seismic activity and phenomenology. Fluid compositions (gas and thermal waters) are plotted dimensionless. Time series of springs (BJ, GA, CE) are delayed by a factor depending on their distance to dome summit (see TableIVand text for explanations). Time series for summit fumaroles compositions are real time measurements: mean value for total S and maximum value for HCl for time intervals of 2 months. Seismicity (grey lines) is the total number of recorded seisms per month. Phenomenology: red circles are reactivation dates of summit fumaroles ; CS: Cratère Sud, TA: Tarissan, NA: Napoléon, G: Gouffre 1956, LS: Lacroix Supérieur, NB: Nord Breislak. Blue circles: acid ponds.Gas flux trend is deduced from Beauducel (Pers. Com.). The gas pulse (black bars) injection models(1971-1995 and 1997-present day) are both calculated to fit the Cl anomalies observed in thermal springs(CE, BJ, Ga and CC for the 1971-199 period -Villemant et al., 2005-and Ga spring since 1997 Fig.14and Supplementary Material). The advection-dispersion model and the input parameters used are described in the text and in more detail in[START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF]. The dashed black bars correspond to poorly constrained calculations(pre-1974). Unfortunately no highresolution geochemical data exist for the pre-1976 period and the 1976-1977 volcanic crisis. Red bars: S-bearing gas pulse time series calculated from the previous model (see text for explanations).

  13b: C hemical signatures of magmatic gaseous species (C O 2 , SO 2 , H C l) in fumaroles and thermal springs for the 1976-1977 and cur rent crises. Fumaroles: total S (dry composition) and HCl. Springs: Cl -, HCO 3 -and SO 4 --(as proxies of HCl, CO 2 and SO 2 scrubbing respectively). For the 1976-1977 crisis complete composition time series are available only for the nearest spring to the dome (CE). For the current crisis total S and HCl time series are measured in the summit fumaroles; imprints of gas species in aquifers (Ga spring) are significant only for the most soluble gas species (HCl and sometimes SO 2 expressed as Cl -and SO 4 --

  this gas however has suffered enough cooling to convert most SO 2 to insoluble H 2 S. This evolution suggests an increasing contribution from deep basic andesite magma and complete degassing of the acid andesitic magma source (through freezing or nearly complete crystallisation). This situation may be compared to the present day situation at Soufrière Hills where gas flux of deep origin (SO 2 bearing gas from basic magmas) is more or less steady since the beginning of the eruption and likely controlled by the fracturing induced by the magmatic activity, whereas HCl productions is related to a dacitic magma F ig. 14: Modeling gas pulses based on C l-time series in G a spring and total S time series in CSC summit fumaroles. 14a: Gas pulses (dates and relative amplitudes) contaminating Ga spring between 1979 and 2013 are fitted to Cl-time series using the advection dispersion model developed by[START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF]

  parameters: time delay (transfer duration): ~27 months; flow: 140 l/mn; indicative path length (flow distance) within the aquifer: ~650 m (estimation using a normalised flow rate of 10 -5 ms -1 ). Residues are within analytical errors for most measurement points. See Villemant et al. (2005) and Boichu et al. (2011) for further explanations of the model. 14b: Gas pulse (total S content) time-series in CSC fumarole since 1994 calculated from the previous model. The model uses the same advection-dispersion formalism as for Cl time-series and

14c:

  Model time-series of degassing pulses: comparison of calculated relative intensities in the production of HCl (from Cl measurements in Ga spring) and of total S in CSC fumarole. F ig. 15: E volution of the shallow magmatic and hydrothermal systems of L a Soufrière of G uadeloupe, since the seismo-volcanic crisis of 1976-1977.La Soufrière Dome (brown) was emplaced during the 1530 AD eruption in a nested structure of flank collapse (3100 BP, 1530 AD) defining the Cratère Amic, which contains a low volume hydrothermal system. The rainfall regime strongly affects the composition and temperature of this system, which is only episodically modified by the volcanic activity. However, a more or less permanent CO 2 flux is generated by the deep magmatic system

Table Ib

 Ib

	are highly acid, turbid and lightly coloured yellow indicating large ion contents due to interaction
	with host andesite. Unlike temperatures, compositions of acid ponds strongly vary with time. Ion
	contents in Tarissan acid pond reached their highest values during the 2007-2009 period with
	maximum contents as high as 10 wt% Cl -(2.9 mol/L) and 3500 ppm in Ca ++ . Mg ++ and Na + reached
	concentration values of ~1200 ppm. Their normalised Mg-Ca-Na compositions plot within the M A N U S C R I P T composition field of the active hot springs (Fig.
	A C C E P T E D
	) which is the ebullition
	temperature of ion poor aqueous solutions at this altitude (~1400 m). Water boiling is sufficient to
	explain the vapour flux at the Tarissan vent (D. Gibert and S. Vergniolle., pers. com.). Ponds waters

Table II

 II

	). Most
	springs outside the Cratère Amic structure (HR, BCM-EV) display no significant chemical
	variations over 35 years. Most springs inside the Cratère Amic structure display large composition
	variations for the period 1979-1994 and then reach an almost steady composition. Two springs, Ga
	and CC, respectively inside and outside the Cratère Amic structure, display distinctive time series
	features. For CC spring chemical data are reported for the periods 1979-2005 and 2005-present day.
	Ga spring displays a 3-fold time series evolution, the first two being similar to those of other springs
	located inside the Cratère Amic. Since 2006, a series of Cl anomalies are observed in this spring,
	culminating in 2009 with an intense and long lasting anomaly. The composition time series of the
	first regime (

  example) while intermittent acid boiling greenish ponds (85-109°C, pH mean -0.1 ± 0.5, between April 1998 and June 2001 for example) formed in early 1998 at the CS's southern vent and in 2000 in the Tarissan crater, displaying vigorous geyser-like activity[START_REF] Komorowski | Guadeloupe, in: Volcanic Hazard Atlas of the Lesser Antilles[END_REF] Beauducel Pers. Com.). The summit fumaroles temperatures have strongly increased from that date (from

	~96°C in 1997 with spikes of 126°C in March 2000, 118 °C in June 2001, and maximum values of
	~140°C in February 1999 and June 2000, Table II). This activity is still going on. Residual
	fumarolic field at the base of the dome was not affected by the summit reactivation and has even
	slightly diminished since 1997.
	Compositions of all low temperature gases (~ 96°C) are characterised by low (SO 2 +H 2 S)/CO 2
	and very low or undetectable amounts of HCl, SO 2 and CO (Figs. 3 and 10). Compositions of gases
	of higher temperature collected in 1976-1978 (Lacroix Inférieur; Delorme, 1983) and since 1997 at

the summit of the dome (CSC) are similar but have higher HCl and total S contents (Table

II

, Fig.

3

). Though the total S and C contents are similar for both activity periods, the 1977-78 Lacroix Inférieure fumaroles have higher SO 2 /H 2 S, CO/CO 2 and CH 4 /CO 2 ratios than present-day summit fumaroles which indicates that more reducing conditions prevailed during gas emissions in

1977- 1978 (i.

e. closer to pure magmatic gas conditions at depth) than for present activity. The composition domains of fumaroles of La Soufrière are relatively large (especially in CO/CO 2 which

  R I P T compositions and HCO 3 -anomalies in CE spring, this is likely not the case during the present unrest (i.e. other than that recorded in diffuse degassing around the dome since the 19 1998, Ruzié et al., 2013). Rhyolitic melts of acid andesites, which are dominant in La Soufrière-Grande Découverte volcano since 200 ka, contain large amounts of H 2 O and HCl but almost no CO 2

  represents ~25% of that scrubbed in all springs inside Crater Amic from ~1979 to ~1994. The amounts of gaseous HCl emitted at the dome summit since 1997 is much more difficult to evaluate due to the large errors in the estimates of gas fluxes and HCl contents. Using the H 2 O flux data of 2005 and 2010(Beauducel et al., pers. com. 1 ) and assuming a linear increase in H 2 O flux from 1997 to 2010 followed by a steady state to present day, and a mean HCl content in fumaroles of ~ 5-10 mol% (dry gas, TableII) the total amount of gaseous HCl emitted at the dome summit since 1997 may be estimated at ~20x10 6 Kg. Thus the amount of HCl scrubbed in the Ga spring aquifer over the same period of time represents only ~ 1% of the total HCl emitted by summit fumaroles. Similar calculations (involving more basic magmas) may be performed for total S and CO 2 emissionsand indicates production of ~ 100 x10 6 Kg of SO 2 and ~ 200 x10 6 Kg of CO 2 since 1997.

Table III b Cratère Sud: fumaroles condensates and acid pond

 III

			T (°C)	pH	F	Cl	Br	I	Cl/Br	Br/I
					ppm	ppm	ppm	ppb
	Fumaroles condensates
	RC	m	96.4	4.1	0.16	1.74	0.001 0.11	~ 750	10
	(1983-2010)		0.8	0.4	0.08	2.42	0.002 0.05		21
		Max	100.4	5.1	0.34	9.87	0.01 0.22
		min	90.8	2.7	0.08	0.02	b.d.l. 0.03
		Nb values	739	124	18	27	18	13
	CSC	m	102.3	1.3	0.05	3604	0.13	9 ~ 200 000 23
	(1997-2010)		6.7	0.7	0.01	4538	0.40	25		24
		Max	130.3	4.3	0.08	32700	1.67	104
		min	90.0	0.5	0.03	17	0.003 0.1
		Nb values	285	123	9	85	17	17
	CSN	m	99.6	1.0	0.04	9410	0.23	28 ~ 600 000 10
	(1997-2009)		6.3	0.6	0.01	9381	0.52	73		5
		Max	126.3	3.6	0.05	47830	2.37	345
		min	88.1	0.1	0.02	100	0.002	0
		Nb values	211	162	4	145	30	30
	Acid Pond		1998-2004	1998-2004 (1 measurement 14/08/2007)
	CSC	m	88.8	-0.08	n.d.	8745	n.d.	n.d.	-	-
			8.6	0.50
		Max	109.2	1.61
		min	72.0	-0.84
		Nb values	33	88

0.54

A C C E P T E D

M A N U S C R I P T

F igure captions F ig. 1: T he active hydrothermal system at L a Soufriere of G uadeloupe 1a: M ain volcano-tectonic structures and distribution of thermal springs and fumaroles: F ig 1b (inset): Interpretative cross section of the dome structure and hydrothermal circulations.

(Redrawn from [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF], Komorowski, 2008[START_REF] Salaün | Hydrothermal alteration in andesitic volcanoes: a study of trace element redistribution in active-and paleohydrothermal systems of Guadeloupe (Lesser Antilles)[END_REF] F ig. 2: Compositions of spring waters and acid ponds. (1979)(1980)(1981)(1982)(1983)(1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995) have been described and interpreted by [START_REF] Villemant | The memory of volcanic waters: shallow depth magma degassing revealed by long-term monitoring of hydrothermal springs at la Soufrière volcano (Guadeloupe, Lesser Antilles) Earth Planet[END_REF]. [START_REF] Hammouya | Preand syn-eruptive geochemistry of volcanic gases from Soufriere Hills of Montserrat, West Indies[END_REF] and 2008 (eruptive hiatus, estimate from [START_REF] Edmonds | Excess volatiles supplied by mingling of mafic magma at an andesite arc volcano[END_REF]. This evolution trend at Soufrière Hills from 1996 to 2008 is interpreted as the result of increasing contribution of unerupted mafic magmas [START_REF] Edmonds | Excess volatiles supplied by mingling of mafic magma at an andesite arc volcano[END_REF]. Low temperature fumaroles collected at the dome summit in 1997 (red solid diamond, [START_REF] Brombach | Geochemistry of the thermal springs and fumaroles of Basse-Terre Island, Guadeloupe, Lesser Antilles[END_REF], just before the production of a highly acid summit gas plume, have compositions similar to those of RC. CSC gas and condensates compositions are represented by both individual measurements and mean values calculated for different temperature ranges (see Table II). Compositions of fumarolic gases collected in 1997 at La Ty (RC) and at the dome summit are from [START_REF] Brombach | Geochemistry of the thermal springs and fumaroles of Basse-Terre Island, Guadeloupe, Lesser Antilles[END_REF].

A C C E P T E D M A N U S C R I P T

Adapted from Symonds et al., 1998[START_REF] Symonds | Magmatic gas scrubbing: implications for volcano monitoring[END_REF]. Compositions in mol fractions (log units). The initial gas composition is only indicative. intrusion was emplaced at shallow depth (~3 km?) and the volcanic feeder system was strongly fractured. The crisis produced strong gas emissions and a few phreatic explosions which ended in 1977.

1978-1992: The fumarolic activity generated by the volcanic crisis rapidly decreased and almost completely vanished in ~1981. Further degassing of the shallow magma intrusion produced a series of volcanic gas pulses over a long time period. These gas pulses were almost completely scrubbed by the hydrothermal system. This activity also led to sealing of the feeding conduit by hydrothermal alteration.

1993 -present day: At the end of 1992 a new seismic crisis occurred, likely corresponding to emplacement of a new magma intrusion (smaller than in 1976-1977). Degassing through a highly confined system led to a highly evolved gas plume and contamination of the shallow hydrothermal system observed at a single spring (Ga). This unrest may lead to phreatic explosions and flank collapse hazards, as a consequence of over pressurisation of a strongly sealed system by active hydrothermal alteration.