Clara Solaro

Caroline Martel
email: caroline.martel@cnrs-orleans.fr

Rémi Champallier

Georges Boudon

Hélène Balcone-Boissard

Michel Pichavant

Petrological and experimental constraints on magma storage for large pumiceous eruptions in Dominica island (Lesser Antilles)

Keywords: Dominica, Ignimbrite, Dacite, Phase equilibria, Storage conditions

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Island arc subduction magmatism is known to mostly produce andesitic to dacitic eruptions of small volumes (< 1 km 3 dense rock equivalent (DRE) per eruption), as also reported for the Lesser Antilles arc (Lindsay et al. 2005a). Nevertheless, some systems can produce large-volume silicic eruptions (5 to several 10 km 3 DRE/eruption), mostly associated with caldera formation, with important implications in terms of hazard assessment and risk management [START_REF] Self | The 1883 eruption of Krakatau[END_REF][START_REF] Self | Volcanological study of the great Tambora eruption of 1815[END_REF][START_REF] Heiken | Caldera development during the Minoan eruption, Thira, Cyclades, Greece[END_REF][START_REF] Druitt | Explosive volcanism on Santorini, Greece[END_REF][START_REF] Allen | Reconstruction of a major caldera-forming eruption from pyroclastic deposit characteristics: Kos Plateau Tuff, eastern Aegean Sea[END_REF][START_REF] Wilson | The 26.5 ka Oruanui eruption, New Zealand: an introduction and overview[END_REF][START_REF] Reubi | Structure and dynamics of a silicic magmatic system associated with caldera-forming eruptions at Batur volcanic field, Bali, Indonesia[END_REF][START_REF] Wilson | The 26.5 ka Oruanui eruption, Taupo volcano, New Zealand: development, characteristics and evacuation of a large rhyolitic magma body[END_REF]). The genesis, storage and eruption of large-volume magmas in arc settings are thus key issues to evaluate the impact and mitigate the risk on human society.

Editorial responsibility: M. Portnyagin Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00445-019-1313-x) contains supplementary material, which is available to authorized users.

Using thermo-mechanical modelling of heat transfer, [START_REF] Annen | Effects of repetitive emplacement of basaltic intrusions on thermal evolution and melt generation in the crust[END_REF] calculated that intrusion rates higher than 5.10 -4 m/year are necessary to generate large-scale magma bodies in the crust by a process of sill accretion. Such intrusion rates imply periods of high magma production rates (flare-up; [START_REF] Silva | Episodic construction of batholiths: insights from the spatiotemporal development of an ignimbrite flareup[END_REF], which could result from high magmatic fluxes from the mantle (at least 10 -2 km 3 /year; Annen 2009), as evidenced for large plutonic batholiths [START_REF] Lipman | Incremental assembly and prolonged consolidation of cordilleran magma chambers: evidence from the southern Rocky Mountain volcanic field[END_REF][START_REF] Silva | Episodic construction of batholiths: insights from the spatiotemporal development of an ignimbrite flareup[END_REF][START_REF] Silva | Arc magmatism, calderas, and supervolcanoes[END_REF].

Nevertheless, such high magma fluxes can also be detrimental to the growth of large-volume reservoirs. To prevent dike propagation and eruption, recharge rate must be comparable to the viscous relaxation time of the surrounding crust [START_REF] Jellinek | A model for the origin of large silicic magma chambers: precursors of caldera-forming eruptions[END_REF][START_REF] Degruyter | A model for eruption frequency of upper crustal silicic magma chambers[END_REF][START_REF] Degruyter | Magma reservoir response to transient recharge events: the case of Santorini volcano (Greece)[END_REF]). Thus, [START_REF] Jellinek | A model for the origin of large silicic magma chambers: precursors of caldera-forming eruptions[END_REF] suggested that the production of large magma volumes linked with caldera-forming eruptions derives from long periods (10 5 -10 6 years) of magma supply in large magma chambers (> 10 2 km 3 ). Warm viscoelastic walls would allow them to deform, thus preventing large overpressures to be generated and promoting growth of the reservoir. Such a storage regime would be favoured in extensional tectonic contexts. To trigger an eruption from these large magma volumes, a critical overpressure must be reached, either by magma inputs (10 -2 to 10 -1 km 3 /year; [START_REF] Silva | Arc magmatism, calderas, and supervolcanoes[END_REF] that largely exceed the longterm magma supply rates, and/or a chamber vertically flattened favouring dike generation, magma propagation and eruption [START_REF] Jellinek | A model for the origin of large silicic magma chambers: precursors of caldera-forming eruptions[END_REF].

Whether the storage conditions of such large magma volumes are different, especially in terms of pressure/depth and temperature, from those of small-volume magma chambers, is a key issue in understanding magma generation, growth and eruption conditions at crustal levels. In particular, pressure, temperature and volatile content are essential parameters in controlling magma storage. Several phaseequilibrium studies have been conducted on small-volume magma chambers from subduction zones, suggesting temperatures (T) from 820 to 900 °C and pressures (P) from 100 to 200 MPa for andesitic to dacitic magmas (e.g. [START_REF] Luhr | Experimental phase relations of water and sulfur saturated arc magmas and the 1982 eruptions of El Chichon volcano[END_REF][START_REF] Gardner | Experimental constraints on pre-eruptive water contents and changing magma storage prior to explosive eruptions of Mount St. Helens volcano[END_REF][START_REF] Barclay | Experimental phase equilibria constraints on pre-eruptive storage conditions of the Soufrière Hills magma[END_REF][START_REF] Martel | Magma storage conditions and control of eruption regime in silicic volcanoes: experimental evidence from Mt Pelée[END_REF][START_REF] Sato | Groundmass pargasite in the 1991-1995 dacite of Unzen volcano: phase stability experiments and volcanological implications[END_REF][START_REF] Scaillet | The 15 June 1991 eruption of Mount Pinatubo. I. Phase equilibria and pre-eruption P-T-fO2-f H2O conditions of the dacite magma[END_REF][START_REF] Hammer | Magma storage prior to the 1912 eruption at Novarupta, Alaska[END_REF][START_REF] Costa | Petrological and experimental constraints on the pre-eruption conditions of Holocene Dacite from Volcan San Pedro (36 S, Chilean Andes) and the importance of Sulphur in silicic subduction-related magmas[END_REF][START_REF] Holtz | Experimental petrology of the 1991-1995 Unzen dacite, Japan. Part I: phase relations, phase composition and pre-eruptive conditions[END_REF]. In contrast, only a few studies suggest deep storage pressures ~400 MPa for large-volume andesitic reservoirs [START_REF] Parat | Pre-eruptive conditions of the Huerto andesite (fish canyon system, San Juan volcanic field, Colorado): influence of volatiles (C-O-H-S) on phase equilibria and mineral composition[END_REF][START_REF] Andújar | Generation conditions of Dacite and Rhyodacite via the crystallization of an andesitic magma. Implications for the plumbing system at Santorini (Greece) and the origin of tholeiitic or Calc-alkaline differentiation trends in arc magmas[END_REF]). On Dominica island, in the Lesser Antilles arc, where large-volume eruptions are known, petrological studies of dome-forming products and pumiceous ignimbrites (Howe et al. ) have suggested crystallization temperatures of 800-990 °C and oxygen fugacities (f O2 ) of NNO -0.25 to + 0.35 (up to NNO + 0.7 in [START_REF] Howe | Reevaluation of the Roseau tuff eruptive sequence and other ignimbrites in Dominica, Lesser Antilles[END_REF]), but pressures of magma storage have not been determined.

We present hereafter a study combining petrological and experimental approaches to constrain the storage conditions of Dominica magmas. Several important issues are addressed such as (i) the depth, temperature and the volatile content of these large silicic pumiceous eruptions; (ii) the possible differences in storage conditions between small-and large-volume eruptions; and (iii) the factors that control the volumes of erupted magma at the scale of the Lesser Antilles arc, from the comparison between Dominica and the neighbouring islands (Martinique, Guadeloupe, Montserrat).

Geological setting, volcanic activity and petrological background

The Lesser Antilles volcanic arc results from the subduction of the Atlantic oceanic lithosphere below the Caribbean oceanic plate at an average velocity of ~2 cm/year [START_REF] Wadge | Comparison of volcanic production rates and subduction rates in the Lesser Antilles and Central America[END_REF]. Built up by 11 main islands, the arc is 800 km long, with a particular convexity toward the East (Fig. 1). Dominica Island represents a central point of the arc, from which the southern islands (Dominica to Grenada) form a unique active segment, whereas the northern islands (Dominica to Anguilla) separate into a recent active western limb and an older inactive eastern segment [START_REF] Bouysse | Subduction of Atlantic aseismic ridges and Late Cenozoic evolution of the Lesser Antilles island arc[END_REF]. The active arc hosts 12 volcanic islands. Dominica Island (15°25 N, 61°20 W) is located at the centre of the Lesser Antilles arc between Martinique and Guadeloupe. From North to South, four main active volcanic centres are present: Morne aux Diables, Morne Diablotins, Morne Trois-Pitons-Micotrin and Plat Pays and few minor centres such as Foundland, Morne Watt and Morne Anglais (Fig. 1). The dominant volcanic activity is characterized by dome-forming eruptions but also by large ignimbritic eruptions [START_REF] Sigurdsson | Partly-welded pyroclast flow deposits in Dominica, Lesser Antilles[END_REF][START_REF] Carey | The Roseau ash: deep-sea tephra deposits from a major eruption on Dominica, Lesser Antilles arc[END_REF][START_REF] Smith | The volcanic geology of the mid-arc island of Dominica[END_REF].

Even if the recent activity (1980)(1981)(1982)(1983)(1984)(1985)(1986)(1987)(1988)(1989)(1990) of Dominica has been characterized by phreatic activity originated from the Boiling Lake and the Valley of Desolation [START_REF] Fournier | Boiling lake of Dominica West Indies: high-temperature volcanic crater lake dynamics[END_REF][START_REF] Napoli | First multi-gas based characterization of the boiling Lake volcanic gas (Dominica, Lesser Antilles)[END_REF][START_REF] Mayer | Phreatic activity and hydrothermal alteration in the valley of desolation, Dominica, Lesser Antilles[END_REF]), work has also been focused on the ignimbrite deposits that extensively outcrop in the central part of the island. In particular, the Pleistocene Roseau Tuff has been fully described [START_REF] Sigurdsson | Partly-welded pyroclast flow deposits in Dominica, Lesser Antilles[END_REF][START_REF] Carey | The Roseau ash: deep-sea tephra deposits from a major eruption on Dominica, Lesser Antilles arc[END_REF]. Recent on-land stratigraphic and chronological studies have evidenced that the Roseau eruption was not a single event (as previously suggested by [START_REF] Carey | The Roseau ash: deep-sea tephra deposits from a major eruption on Dominica, Lesser Antilles arc[END_REF], but is made of several individual eruptions [START_REF] Smith | The volcanic geology of the mid-arc island of Dominica[END_REF][START_REF] Howe | Reevaluation of the Roseau tuff eruptive sequence and other ignimbrites in Dominica, Lesser Antilles[END_REF][START_REF] Boudon | Revised chronostratigraphy of recurrent ignimbritic eruptions in Dominica (Lesser Antilles Arc): implications on the behavior of the magma plumbing system[END_REF]. In particular, [START_REF] Boudon | Revised chronostratigraphy of recurrent ignimbritic eruptions in Dominica (Lesser Antilles Arc): implications on the behavior of the magma plumbing system[END_REF] recognized at least three major pumiceous explosive eruptions (volume ~5 km 3 DRE/eruption) in the last 50 ky of volcanic activity: Layou (51 ka), Roseau (33 ka) and Grand Fond (24 ka). These eruptions started with a plinian phase producing a relatively thin basal pumice fallout deposit (20 to 70 cm, < 10% of total erupted volume), followed by voluminous pyroclastic density currents (PDC) producing thick pumiceous deposits filling the valleys (detailed stratigraphy of these three eruptions in [START_REF] Boudon | Revised chronostratigraphy of recurrent ignimbritic eruptions in Dominica (Lesser Antilles Arc): implications on the behavior of the magma plumbing system[END_REF]. These authors proposed that the Layou ignimbrites were emitted from Morne Diablotins volcanic centre, and did not originate from Morne Trois Pitons-Micotrin as previously proposed [START_REF] Roobol | Geological map of Dominica[END_REF]Lindsay et al. 2005b). Roseau and Grand Fond ignimbrites were emitted from Morne Trois Pitons-Micotrin volcanic centre [START_REF] Roobol | Geological map of Dominica[END_REF]Lindsay et al. 2005b). Two other pumiceous PDC deposits have been recognized on the island: Grande Savane and Grand Bay. The Grande Savane outcrops on the West coast, near the village of Salisbury, and has been described as an ignimbritic sequence [START_REF] Sparks | The entrance of pyroclastic flows into the sea I. oceanographic and geologic evidence from Dominica, Lesser Antilles[END_REF]. The volume, the stratigraphic extension and the age of this deposit are still not constrained, but the eruptive centre at the origin of this ignimbritic eruption has been proposed to be the Morne Diablotins volcanic complex [START_REF] Smith | The volcanic geology of the mid-arc island of Dominica[END_REF]. The Grand Bay ignimbrite outcrops in the South coast, near the village of Berekua, and has been described by [START_REF] Lindsay | Geological history and potential hazards of the late-Pleistocene to recent plat pays volcanic complex, Dominica, Lesser Antilles[END_REF] and [START_REF] Howe | Reevaluation of the Roseau tuff eruptive sequence and other ignimbrites in Dominica, Lesser Antilles[END_REF] as a massive, pumice-and lithic-rich PDC deposit. The centre of this ignimbritic eruption, previously assumed to be Plat Pays Volcanic centre [START_REF] Lindsay | Geological history and potential hazards of the late-Pleistocene to recent plat pays volcanic complex, Dominica, Lesser Antilles[END_REF] or the Wotten-Waven caldera (Lindsay et al. 2005a;[START_REF] Smith | The volcanic geology of the mid-arc island of Dominica[END_REF], has recently been proposed to be the complex of Morne Trois Pitons-Micotrin [START_REF] Boudon | Revised chronostratigraphy of recurrent ignimbritic eruptions in Dominica (Lesser Antilles Arc): implications on the behavior of the magma plumbing system[END_REF].

In light of these data, Dominica appears as an anomaly in the general eruptive behaviour of the Lesser Antilles arc. Indeed, whereas the neighbouring islands of Martinique and Guadeloupe have only one active volcanic centre producing small volume (< 1 km 3 DRE/eruption) plinian and lava domeforming eruptions [START_REF] Carazzo | The recent Plinian explosive activity of Mt. Pelée volcano (Lesser Antilles): the P1 AD 1300 eruption[END_REF], Dominica is characterized by several active volcanic centres and the eruption of five major large-volume ignimbrites (Lindsay et al. 2005a;[START_REF] Boudon | Revised chronostratigraphy of recurrent ignimbritic eruptions in Dominica (Lesser Antilles Arc): implications on the behavior of the magma plumbing system[END_REF].

Geochemical data for pumice samples from ignimbritic eruptions across the island (Grand Bay, Roseau, Layou, Grand Fond, Grande Savane) are available in [START_REF] Howe | Reevaluation of the Roseau tuff eruptive sequence and other ignimbrites in Dominica, Lesser Antilles[END_REF] and [START_REF] Boudon | Revised chronostratigraphy of recurrent ignimbritic eruptions in Dominica (Lesser Antilles Arc): implications on the behavior of the magma plumbing system[END_REF]. The samples have andesitic to dacitic whole-rock compositions (61-66 wt% SiO 2 , 3.6-6.0 wt% Na 2 O + K 2 O, 5.1-6.5 wt% FeO; recalculated volatile-free with total iron as FeO). Layou and Grande Savane (64-66 wt% SiO 2 , ~5.0 wt% Na 2 O + K 2 O) and Roseau, Grand Bay and Grand Fond (62-63 wt% SiO 2 , 4.5-5.0 wt% Na 2 O + K 2 O) are the most and least evolved samples, respectively (Table 1).

Pumice samples in ignimbrites [START_REF] Howe | Reevaluation of the Roseau tuff eruptive sequence and other ignimbrites in Dominica, Lesser Antilles[END_REF]) contain 25-30 vol% phenocrysts, including plagioclase (15-24 vol%, An 40-95; An: mol% anorthite), orthopyroxene (0-5 vol%, En 43-61; En: mol% enstatite), clinopyroxene (0-3 vol%, Wo 41-44; En 36-40; Wo: mol% wollastonite) and Fe-Ti oxides (0-4 vol%, Mag 65-75 ; Mag: mol% magnetite). Amphiboles are only found in Layou and Grande Savane deposits and few euhedral quartz (1-2%) are only present in Layou deposits [START_REF] Boudon | Revised chronostratigraphy of recurrent ignimbritic eruptions in Dominica (Lesser Antilles Arc): implications on the behavior of the magma plumbing system[END_REF]). The groundmass glass, which is free of microlites, is rhyolitic (76-78 wt% SiO 2 , 5-7 wt% Na 2 O + K 2 O) and very homogeneous [START_REF] Howe | Reevaluation of the Roseau tuff eruptive sequence and other ignimbrites in Dominica, Lesser Antilles[END_REF][START_REF] Boudon | Revised chronostratigraphy of recurrent ignimbritic eruptions in Dominica (Lesser Antilles Arc): implications on the behavior of the magma plumbing system[END_REF]. [START_REF] Howe | Evolution of young andesiticdacitic magmatic systems beneath Dominica, Lesser Antilles[END_REF] performed a petrologic study on products from lava dome-forming eruptions (block-andash flows and lavas) across the island. These lava dome samples, andesitic to dacitic in composition, have a mineral assemblage similar to the pumice samples in ignimbrites, but show higher crystallinity (about 40-55 vol%). The lava dome samples also differ from the pumice samples in that they contain basalt to basaltic-andesite enclaves.

Methodology

A brief review of the chosen samples, as well as the analytical and experimental methodology, is provided below. Further details are given in Supplementary Material SM1.

Choice of the natural samples

The Layou, Roseau and Grand Fond pumice samples used in this study come from the basal plinian fallout deposit of each eruption, which presents three advantages for the determination of pre-eruptive conditions. First, the base of the sequence represents the earliest erupted products, therefore the top part of the reservoir that is likely the less affected by potential magma recharge and magma mixing processes. Second, pumice samples ascended rapidly in the conduit and were rapidly quenched in air, so that syn-and post-eruptive processes that could modify magma chemistry, such as microlite crystallization, are minimized. Third, fallout pumice samples are not transported across long distances, thus post-eruptive chemical and mechanical contamination (entrapment of non-juvenile material, welding, deformation) is also minimized. Thus, the plinian pumice samples likely best represent the magma from the top part of the reservoir prior to eruption. Yet, where the plinian basal deposit was absent from the outcrop, the lowest part of the ignimbritic deposit was sampled, i.e. Grande Savane and Grand Bay.

The pumice samples were collected from the basal plinian fallout deposit of the eruptive sequence of Layou (DOM41a3), Roseau (DOM60d1a) and Grand Fond (DOM43b), and from the main pumice flow deposit of Grande Savane (DOM40b) and Grand Bay (DOM31) (sample numbers as in [START_REF] Boudon | Revised chronostratigraphy of recurrent ignimbritic eruptions in Dominica (Lesser Antilles Arc): implications on the behavior of the magma plumbing system[END_REF]; sampling locations in Fig. 1; whole-rock compositions in Table 1). For each eruptive sequence, the studied samples consisted of 6 to 10 pumices (2 to 6 cm in diameter) chosen from the single mode of the density distributions performed on 30 to 40 andesitic-dacitic pumices by [START_REF] Boudon | Revised chronostratigraphy of recurrent ignimbritic eruptions in Dominica (Lesser Antilles Arc): implications on the behavior of the magma plumbing system[END_REF].

Sample analysis

Both, natural and experimental, samples were analysed for phase identification using the backscattered-electron (BSE) signal of scanning electron microscopes (SEM; ZEISS-Supra 55, ISTeP-UPMC, Paris; EDAX-Pegasus, ISTO-BRGM, Orléans). Chemical analyses of the minerals and glasses were provided using electron microprobe (EMP; CAMECA-SX Five and CAMECA-SX100, CAMPARIS, Paris; CAMECA-SX Five, BRGM-ISTO, Orléans). The phase compositions for the natural samples are given in Table 2. The water contents of the experimental residual glasses were determined following the by-difference method using the EMP analyses and the glass standards in [START_REF] Balcone-Boissard | Deep pre-eruptive storage of silicic magmas feeding Plinian and dome-forming eruptions of central and northern Dominica (Lesser Antilles) inferred from volatile contents of melt inclusions[END_REF]. Phase proportions of the experimental charges were obtained by mass-balance calculations and compared to those obtained on natural samples by [START_REF] Boudon | Revised chronostratigraphy of recurrent ignimbritic eruptions in Dominica (Lesser Antilles Arc): implications on the behavior of the magma plumbing system[END_REF]. France). The experiments lasted 7 to 10 days and the investigated conditions spanned temperatures (T) from 800 to 900 °C, pressures (P) from 150 to 400 MPa, oxygen fugacity (f O2 ) around NNO + 1 (one run at NNO + 2) and H 2 O content from about 4.5 to 10.0 wt%, as reported in Table 3.

Global strategy to determine magma storage conditions

The first step towards estimation of magma storage conditions is to look at the phase compositions of the natural samples and use thermo-baro-oxy-metric formulations to retrieve T-P-f O2 conditions of crystallization. Because these formulations have been calibrated against compositions or conditions that may be different from those of Dominica magmas, their suitability has been tested on experimental phases for which crystallization P-T-f O2 were known. Results are provided in Supplementary material SM2. Yet, the pre-eruptive conditions retrieved from the natural products may only represent first-order estimations, because of uncertainties related to complex crystallization histories and to compositions potentially little sensitive to the intensive parameters. Alternatively, phase-equilibrium experiments may help refining these constraints by providing equilibrium phase assemblages, modal proportions and compositions, under controlled conditions of P, T, f O2 and volatile content. Our goal is to retrieve a set of conditions, hereafter referred as possible magma storage conditions, for which the whole set of criteria compare at best between the experimental and natural samples.

Petrology and geochemistry of the dacitic ignimbrite deposits

Modal proportions

Dominica ignimbritic samples have phenocryst modal proportions of 30 ± 5%, whatever the eruptive deposit, including 2 2 % p l a g i o c l a s e , ~1 -5 % o r t h o p y r o x e n e , ~1 % clinopyroxene, 1-3% Fe-Ti oxides and 0-5% amphibole (as traces in Grand Fond and Grand Bay, < 1% in Roseau and up to 5% in Layou and Grande Savane samples). Layou samples also show < 2% quartz as accessory mineral. Average compositions of the phenocrysts and glasses from the pumice samples are reported in Table 2.

Plagioclase

Core to rim compositional profiles were obtained for about 40 crystals of the different eruptions. Plagioclase often exhibits evidence for core resorption and marked oscillatory zoning (Fig. 2). Statistically speaking, three [START_REF] Newman | VolatileCalc: a silicate melt-H2O-CO2 solution model written in visual basic for excel[END_REF] peaks can be identified in frequency histograms (Fig. 3).

First, An 53-58 dominates the plagioclase population, these compositions being found in both crystal cores and rims.

Instead, An 70-75 forms a secondary maximum corresponding to the middle part of oscillatory zoned crystals.

Finally, An 80-90 are mostly present as cores in rare crystals. For the purpose of constraining storage conditions just prior to eruption, the plagioclase rim composition in contact with the residual glass is of particular interest. A closer look at several profiles indicates An 50-54 rim compositions for Layou and Grande Savane, and An 53-58 for Roseau plagioclases, both ranges overlapping with the main peak of frequency histograms (Fig. 3).

Amphibole

The rare amphiboles in the Roseau samples are anhedral and rimmed by thick resorption boundaries suggesting mineral-melt disequilibrium. In contrast, the amphiboles in Layou and Grande Savane samples are euhedral, large in size (> 300 μm, up to millimetres in Grande Savane pumices), and free of any reaction rim. Layou amphibole BSE images reveal crystal growth zonation associated with compositional variations: Al 2 O 3 content varies from 6.0-7.5 wt% at crystal rims to ~8-9 wt% in cores (Fig. 4a) and Mg# (=Mg/(Mg + Fe tot ), in moles) varies from 0.48 to 0.61 (Fig. 4b). Although all compositions overlap, Layou and Grande Savane amphiboles show the highest and lowest alumina contents, respectively (Fig. 5a). Roseau and Grande Savane amphiboles seem to lack the alumina-rich and low-Mg# cores observed in Layou phenocrysts; note that the anti-correlation between Al IV and Mg# (Fig. 5b) is uncommon in the Lesser Antilles context, since most of natural (as well as experimental) amphiboles show the opposite trend [START_REF] Pichavant | Physical conditions, structure, and dynamics of a zoned magma chamber: Mount Pelée (Martinique, Lesser Antilles arc)[END_REF][START_REF] Martel | Phase relations in trachytes and implication for magma storage conditions in the Chaîne des Puys (French Massif Central)[END_REF]). This indicates contrasted crystallization conditions for the alumina-rich Mg#-poor amphiboles and the alumina-poor Mg#-rich ones.

Orthopyroxene

Orthopyroxenes are hypersthenes with a large compositional range between En 47 and En 63 [START_REF] Howe | Reevaluation of the Roseau tuff eruptive sequence and other ignimbrites in Dominica, Lesser Antilles[END_REF][START_REF] Solaro | Storage conditions and dynamics of magma reservoirs feeding the major pumiceous eruptions of Dominica (Lesser Antilles Arc)[END_REF]. Whereas ~15-20% of the orthopyroxenes show compositional zonings (either reverse, normal or multiple), most of them (~80-85%) are unzoned, so that their compositions may be considered as representative of the pre-eruptive conditions in the reservoir [START_REF] Solaro | Storage conditions and dynamics of magma reservoirs feeding the major pumiceous eruptions of Dominica (Lesser Antilles Arc)[END_REF]. Unzoned crystals have compositions that slightly differ among eruptions: En 49-50 for Grande Savane, En 50-52 for Layou, En 52-53 for Roseau and Grand Bay and En 54-56 for Grand Fond samples (Table 2).

Clinopyroxene

In Roseau, Layou and Grand Bay samples, clinopyroxenes are diopsides, with Wo 44-45 and En 37-38 whatever the eruption considered (Table 2). Samples show Mg# between 0.61 and 0.71, with a broad peak at 0.68. Most of the Roseau, Layou and Grand Bay clinopyroxenes, and all of those (rare) clinopyroxenes present in Grand Fond and Grande Savane samples show strong evidence of resorption (marked by orthopyroxene overgrowths) suggesting that they are likely antecrysts.

Fe-Ti oxides

Residual glasses and glass inclusions

The major and trace element compositions and the speciation and content of volatiles dissolved in both, residual glasses and glass inclusions, have been determined by Balcone-Boissard et al. ( 2018) using secondary ion mass spectroscopy (SIMS).

The results are summarized below. Post-entrapment crystallization has been calculated to be negligible (1-2%) for the glass inclusions trapped in orthopyroxene and up to 19% for those trapped in plagioclase. For the latter, only glass inclusions with a calculated postentrapment crystallization < 8% have been considered for their major-element composition. Residual glasses and glass inclusions from Roseau, Layou, Grand Fond, Grand Bay and Grande Savane pumice are all rhyolitic, showing 75-80 wt% SiO 2 , 11-14 wt% Al 2 O 3 , 1.0-2.5 wt% CaO, 0-3 wt% FeO and 4.9-6.5 wt% Na 2 O + K 2 O. In detail, however, there are significant compositional differences. Firstly, the composition of the glass inclusions slightly varies with the host mineral, often being enriched in components from the host (i.e. plagioclase, orthopyroxene, clinopyroxene, amphibole or ilmenite; Table 2). Secondly, Roseau and Layou residual glasses are slightly less differentiated than glass inclusions, with a narrower compositional range (75-78 wt% SiO 2 ; Fig. 6), which reflects either post-entrapment modifications of inclusions or/and input of a more mafic melt prior to eruption. In contrast, Grande Savane residual glasses and glass inclusions compare well and are less differentiated (76-78 wt% SiO 2 ) than those of Layou samples (> 77.5 wt% SiO 2 ; Fig. 6).

The orthopyroxene-hosted glass inclusions display H 2 O contents between 6.5 and 7.6 wt% in Layou pumice, between 5.5 and 7.5 wt% in Roseau pumice and from 4.5 to 6.3 wt% in Grand Fond pumice. The plagioclase-hosted glass inclusions contain 4.5 to 6.5 wt% H 2 O, i.e. about 1-2 wt% less than in orthopyroxene-hosted ones. The glass inclusions also contain up to 440 ppm CO 2 .

Crystallization conditions retrieved from the natural samples

Temperature and redox state

The presence of both titanomagnetite and ilmenite phenocrysts in pumice samples can be used to constrain the T-f O2 conditions of crystallization, using the thermo-oxymeter formulation of [START_REF] Ghiorso | Thermodynamics of rhombohedral oxide solid solutions and a revision of the Fe-Ti two-oxide geothermometer and oxygen-barometer[END_REF]. The calculation is validated against the experimental charge at 875 °C and 400 MPa (run #9) that gives a f O2 of NNO + 1.1 in agreement CaO (wt%) with the redox sensor, although the calculated T is underestimated by about 60 °C (SM2A). In Grand Fond and Grand Bay (Roseau-like) samples, neighbouring titanomagnetites and ilmenites that passed the chemical equilibrium test of [START_REF] Bacon | Mg/Mn partitioning as a test for equilibrium between coexisting Fe-Ti oxides[END_REF] give T of ~855-860 °C and a f O2 of NNO + 0.6 to + 0.7. Layou Fe-Ti oxides suggest a slightly lower T of ~820 °C for a f O2 of NNO + 0.6 (SM2A), in agreement with the ~720-820 °C range determined in Layou ignimbritic deposits [START_REF] Howe | Reevaluation of the Roseau tuff eruptive sequence and other ignimbrites in Dominica, Lesser Antilles[END_REF]).

The crystallization T may be calculated from amphibole composition using the formulation of [START_REF] Ridolfi | Calcic amphiboles in calc-alkaline and alkaline magmas: thermobarometric and chemometric empirical equations valid up to 1,130°C and 2.2 GPa[END_REF] which, as validated by [START_REF] Erdmann | Amphibole as an archivist of magmatic crystallization conditions: problems, potential, and implications for inferring magma storage prior to the paroxysmal 2010 eruption of Mount Merapi, Indonesia[END_REF], mostly returns reliable T values. When applied to the natural amphiboles with 6-7 wt% Al 2 O 3 , the calculation suggests T from 774 to 809 °C and a f O2 of ~NNO + 1 (SM2B). For the Layou amphiboles with ~9 wt% Al 2 O 3 , the calculated T is ~827 °C and the f O2 is ~NNO + 0.7 (SM2B), which is about 20-50 °C higher and slightly more reduced than calculated for the 6-7 wt% Al 2 O 3 -bearing amphibole.

Another means of estimating crystallization T is to use the orthopyroxene-melt geothermometer of [START_REF] Putirka | Thermometers and barometers for volcanic systems[END_REF], based on the Fe-Mg partitioning between orthopyroxene and melt. Couples of equilibrated orthopyroxenes and glass inclusions (partition coefficient K D = 0.29 ± 0.06; Putirka 2008; K D confirmed by the present experiments) return 850-860 °C for Layou samples, in agreement with T retrieved from Fe-Ti oxides, and 870-890 °C for Roseau and Grand Fond samples (SM2C). These values fall in the range of 800-940 °C determined on (low Mg#) orthopyroxene-melt couples in ignimbritic samples from Dominica Island by [START_REF] Howe | Evolution of young andesiticdacitic magmatic systems beneath Dominica, Lesser Antilles[END_REF].

Crystallization T can be calculated from plagioclase-melt equilibrium, which strongly depends on melt H 2 O content [START_REF] Putirka | Thermometers and barometers for volcanic systems[END_REF][START_REF] Waters | An updated calibration of the plagioclaseliquid hygrometer-thermometer applicable to basalts through rhyolites[END_REF]. The model of [START_REF] Waters | An updated calibration of the plagioclaseliquid hygrometer-thermometer applicable to basalts through rhyolites[END_REF] is applicable to conditions of H 2 Osaturation in fluids and H 2 O contents < 8.3 wt%. Results roughly agree with the run T, but tend to overestimate T by at most 30 °C for H 2 O contents ~5.0 wt%, whereas underestimating T by up to 45 °C for H 2 O contents 7.5 wt% (SM2D). Calculated T for Layou or Roseau samples (plagioclase rim and melt inclusions or residual glass) suggest T of ~780 °C for melt H 2 O content of 7.6-7.8 wt% (maximum H 2 O contents of orthopyroxene-hosted glass inclusions) and T of ~820 °C for melt H 2 O content of 6.5 wt% (maximum H 2 O contents of plagioclase-hosted glass inclusions).

Pressure

Although P may be retrieved from amphibole composition [START_REF] Ridolfi | Calcic amphiboles in calc-alkaline and alkaline magmas: thermobarometric and chemometric empirical equations valid up to 1,130°C and 2.2 GPa[END_REF][START_REF] Erdmann | Amphibole as an archivist of magmatic crystallization conditions: problems, potential, and implications for inferring magma storage prior to the paroxysmal 2010 eruption of Mount Merapi, Indonesia[END_REF], tests using the experimental amphiboles yielded poor agreement between run and calculated pressures (with calculated pressures underestimating experimental ones by ~100 MPa; SM2B).

P may be retrieved directly from the volatile contents of the glass inclusions, assuming that the melt was trapped in the magma chamber and did not suffer significant postentrapment crystallization or volatile loss. From the glass inclusions trapped in orthopyroxenes and using the H 2 O-CO 2 solubility model of [START_REF] Newman | VolatileCalc: a silicate melt-H2O-CO2 solution model written in visual basic for excel[END_REF] for rhyolitic liquids at 850 °C, Balcone-Boissard et al. ( 2018) suggest P from 250 to 350 MPa for the Layou samples, 200 to 300 MPa for the Roseau samples (with a single value suggesting a entrapment pressure of 150 MPa) and 160 to 270 MPa for the Grand Fond samples (with a single inclusion suggesting an entrapment pressure of 750 MPa). Glass inclusions trapped in plagioclase phenocrysts systematically suggest lower entrapment pressures. The high pressures deduced from the H 2 O-CO 2 contents of the glass inclusions in orthopyroxenes and plagioclases are in agreement with those deduced from the H 2 O-chlorine data, i.e. > 200 MPa [START_REF] Balcone-Boissard | Deep pre-eruptive storage of silicic magmas feeding Plinian and dome-forming eruptions of central and northern Dominica (Lesser Antilles) inferred from volatile contents of melt inclusions[END_REF]. Such a pressure range is also in good agreement with the range of 200-350 MPa calculated from multiple-reaction geobarometry on Dominica crustal xenoliths [START_REF] Ziberna | Multiple-reaction geobarometry for olivine-bearing igneous rocks[END_REF]).

Summary

The pre-eruptive storage conditions deduced from the petrological study of the natural pumice samples suggest for Roseau magma: T from 780 to 890 °C, f O2 from NNO + 0.6 to + 1.0, up to 7.5 wt% H 2 O dissolved in melt and P of 350 MPa. For Layou magma, we have T from ~775 to 860 °C, f O2 ~NNO + 0.6, up to 7.6 wt% H 2 O dissolved in melt and P of ~350 MPa. In order to confirm such exceptional magma storage conditions in the Antilles arc and contribute to the general issue of how large magma bodies survive cooling and become eruptible, a set of phase-equilibrium experiments is presented below.

Phase equilibria

The aim of the phase equilibria was to define crystallization conditions that first reproduced the Layou samples. That is, 30 ± 5 wt% phenocrysts consisting of An 50-54 , En 50-52 , amphibole with 6-7 wt% of Al 2 O 3 (hereafter referred as to Am 6- 7 ), Mag 72-74 , Ilm 85-86 and minor quartz, coexisting with 77-80 wt% SiO 2 residual liquid (Liq 77-80 ). The experiments were also run to define the Roseau sample conditions, i.e. 30 ± 5 wt% phenocrysts consisting of An 53-58 , En 52-53 , Mag 73-75 and Ilm 84-85 , coexisting with Liq 76-77 . Amphibole (strongly resorbed) and clinopyroxene are assumed to be unstable in the top of the Roseau reservoir.

Phase relationships and proportions

In the H 2 O-saturated charges (Fig. 7a,c), the crystallization sequence for decreasing P-T conditions is magnetite (liquidus phase), plagioclase and orthopyroxene, the last two appearing at slightly higher P-T conditions in ROS than in LAY. Amphibole is stable in a field above 150 MPa/800 °C and 300 MPa/875 °C. Ilmenite was not found.

In the H 2 O-undersaturated charges (Fig. 7b,d), titanomagnetite, ilmenite, plagioclase and orthopyroxene are ubiquitous for both compositions for the investigated P-T conditions. For X H2Oin ~0.8, the major difference between LAY and ROS comes from amphibole that is stable in LAY at P > 300 MPa (Fig. 7b), whereas it is absent in ROS (Fig. 7d). Clinopyroxene may be present in ROS at 400 MPa and 875 °C. Quartz did not crystallize.

Total crystallinities range from ~10 wt% at high H 2 O contents and high T to ~50 wt% at low H 2 O contents and low T. Crystallinities deduced from mass balance calculations and image analysis are in broad agreement, differing on average by less than ± 5%, exceptionally 10-12% in a few charges (Table 3).

Phase compositions

The experimental phase compositions are given in Supplementary material SM3. Magnetites are Mag 76-83 in the H 2 O-saturated charges and Mag 65-76 in the H 2 O-undersaturated charges. Ilmenites from the H 2 O-undersaturated charges are Ilm 81-89 (SM3A).

Plagioclase covers a wide compositional range from An 87 to An 52-54 (Fig. 7; SM3B). This confirms previous conclusions on the strong influence of melt H 2 O content and T on its composition [START_REF] Martel | Effect of fO 2 and H 2 O on andesite phase relations between 2 and 4 kbar[END_REF]Fig. 8).

Orthopyroxene is hypersthene ranging from En 43 to En 68 (Fig. 7; SM3C). Its composition is known to be strongly dependent on f O2 [START_REF] Martel | Effect of fO 2 and H 2 O on andesite phase relations between 2 and 4 kbar[END_REF]), but since f O2 was kept approximately constant in our experiments, its compositional variation is mainly attributed to changes in P-T-H 2 O conditions. Indeed, a T increase from 825 to 875 °C at constant H 2 O content and P (300 or 400 MPa) increases the En content by 10-15 mol% (Fig. 8). The effect of total P at constant H 2 O content is also remarkable: for ~6 wt% H 2 O at 850 °C, En in orthopyroxene decreases from En 66 , En 57 to En 52 with increasing P from 200, 250, to 300 MPa (Fig. 8a). In contrast, at 850 °C and 250 MPa, En increases from En 54 , En 57 to En 62 with melt H 2 O contents increasing from 5.5, 6.0 to 6.8 wt% (Fig. 8a).

Amphibole is a hornblende with 44-49 wt% SiO 2 , 8-12 wt% Al 2 O 3 , 9-11 wt% CaO, 11-16 wt% FeO and 11-15 wt% MgO (SM3D). Fe-Mg and Al-Si amphibole-melt K D are 0.13 ± 0.04 (calculated with total Fe as FeO) and 0.95 ± 0.06, respectively, in agreement with previous studies on similar magma compositions [START_REF] Martel | Effect of fO 2 and H 2 O on andesite phase relations between 2 and 4 kbar[END_REF][START_REF] Pichavant | Physical conditions, structure, and dynamics of a zoned magma chamber: Mount Pelée (Martinique, Lesser Antilles arc)[END_REF]. For given P-T conditions, amphibole in H 2 Osaturated samples has higher Mg# than in the H 2 O-undersaturated samples, consistent with their slightly higher f O2 (Fig. 5b).

Experimental glass is dacitic to rhyolitic, with 69-79 wt% SiO 2 , 12-18 wt% Al 2 O 3 , 2-7 wt% CaO and 1-3 wt% K 2 O (Fig. 6; SM3E). The glass H 2 O content ranges from 5.7 wt% at 200 MPa to 9.8 wt% at 375 MPa (estimated with the bydifference method; SM1), mostly in agreement (within 1.5 wt%) with H 2 O contents calculated for a rhyolitic melt using the formulation of [START_REF] Newman | VolatileCalc: a silicate melt-H2O-CO2 solution model written in visual basic for excel[END_REF].

Discussion

Pre-eruptive storage conditions of Dominica magmas

Roseau-like compositional group

The Roseau Plinian pumice consists of 30 ± 5% phenocrysts with 22% An 53-55 plagioclase, 1-5% En 52-53 orthopyroxene, 1-3% Mag 72-75 and Ilm 84 . This phase assemblage was experimentally reproduced below ~875 °C at 250 MPa (Fig. 7c) for H 2 O-saturated conditions and below 875 °C at 400 MPa for H 2 O-undersaturated conditions (Fig. 7d). H 2 O-saturated conditions can be ruled out since neither experimental plagioclase (~An 75 at 850 °C, 375 MPa) nor orthopyroxene (> En 57 at 850 °C, ~200 MPa) match natural phenocryst rim compositions. The experiments that best reproduce Roseau pumice phenocrysts are those performed around 400 MPa, 850 °C and for X H2Oin ~0.8 (Fig. 7b). These conditions also allow the natural residual glass compositions (75.5-78.0 wt% SiO 2 ) to be broadly reproduced despite Al 2 O 3 and CaO being slightly overestimated (by 1 wt%; Fig. 6) and plagioclase modes being higher in the experiment (30% at 850 °C, 375 MPa) than in pumice (22%). Therefore, best estimates for the pre-eruptive conditions of the Roseau magma include a pressure of ~400 MPa (equivalent to a depth of ~16 km considering a rock density of 2.45 g/cm 3 ), a temperature of 850 °C, a f O2 of NNO + 1 and a melt H 2 O content of ~7-8 wt%. These results agree with the T-f O2 data calculated from Fe-Ti oxides pairs in Grand Fond samples (Roseau-like compositional group) and with the highest H 2 O contents measured in the orthopyroxene-hosted glass inclusions [START_REF] Balcone-Boissard | Deep pre-eruptive storage of silicic magmas feeding Plinian and dome-forming eruptions of central and northern Dominica (Lesser Antilles) inferred from volatile contents of melt inclusions[END_REF].

The mismatch noted above for Al 2 O 3 and CaO between experimental and natural residual glasses is most probably due to a small compositional difference between the starting ROS sample and the reactive magma (i.e. the subsystem at chemical equilibrium under the pre-eruptive conditions, [START_REF] Pichavant | Equilibration scales in silicic to intermediate magmas -Implications for experimental studies[END_REF] 3 and Qtz for quartz). For each run, black filled squares represent the experimental phase assemblage, with the green label giving crystallinity (in brackets) and phase compositions. The continuous curves delimit the phase stability fields (dashed line for unconstrained). The blue shaded areas correspond to the P-T conditions for which the natural and the experimental phase assemblage, proportions and compositions are comparable, thus defining possible pre-eruptive conditions since it hosts a population of plagioclases more calcic than rim compositions (see Fig. 3). This leads to experimental residual melts that are slightly too rich in calcic plagioclase components (Al 2 O 3 and CaO) and also to charges slightly too crystalrich because experimental plagioclases are not zoned. However, if comparison is made with compositions of glass inclusions, the disagreement between experimental and natural glasses worsens. Glass inclusions in Roseau can reach SiO 2 contents up to 80 wt% (Fig. 6). As developed below for Layou, liquids with SiO 2 contents in this range must be in equilibrium with quartz. Therefore, the lack of quartz in the Roseau group indicates that the major-element composition of such liquids is controlled locally, most probably through posttrapping crystallization and re-equilibration of the inclusions.

The Roseau, Grand Fond and Grand Bay ignimbritic deposits come from the same eruptive centre (Morne Trois Pitons-Micotrin; [START_REF] Boudon | Revised chronostratigraphy of recurrent ignimbritic eruptions in Dominica (Lesser Antilles Arc): implications on the behavior of the magma plumbing system[END_REF]. All show comparable bulk-rock compositions in major (i.e. 63-64 wt% SiO 2 , ~4.5 wt% Na 2 O + K 2 O; Table 1 and Fig. 6) and trace elements [START_REF] Boudon | Revised chronostratigraphy of recurrent ignimbritic eruptions in Dominica (Lesser Antilles Arc): implications on the behavior of the magma plumbing system[END_REF], as well as similar phenocryst assemblages (including unstable amphibole) and compositions (Table 2; [START_REF] Boudon | Revised chronostratigraphy of recurrent ignimbritic eruptions in Dominica (Lesser Antilles Arc): implications on the behavior of the magma plumbing system[END_REF]. Therefore, the experimental results on the ROS sample are considered equally applicable to the Grand Fond and Grand Bay magmas, suggesting identical pre-eruptive conditions (~850 °C, ~400 MPa, ΔNNO + 1 and ~7 wt% H 2 O) for the three ignimbritic eruptions.

Layou-like compositional group

The Layou Plinian pumice consists of 30 ± 5% phenocrysts, with 22% An 50-54 , 5% Am 9-10 rimmed by Am 6-7 , 1-5% En 50-52 , 1-3% Mag 72-74 and Ilm 85-86 and minor quartz. The main part of this assemblage was experimentally reproduced between 150 and 250 MPa at T ~800-850 °C under H 2 Osaturated conditions (Fig. 7a) and for P > 300 MPa at 825-875 °C under H 2 O-undersaturated conditions (Fig. 7b).

Fig. 8 Compositional trends as a function of melt H 2 O content (calculated after [START_REF] Newman | VolatileCalc: a silicate melt-H2O-CO2 solution model written in visual basic for excel[END_REF] for the experimental glasses; Table 3) showing anorthite mol% in plagioclase, enstatite mol% in orthopyroxene, SiO 2 wt% in the residual liquid and crystal content (from left to right) in a LAY and b ROS experiments at ~200 MPa (in blue), 250 MPa (in purple), ~300 MPa (in green) and ~400 MPa (in red).

The vertical grey bands represent the natural data as given in Fig. 7 and the horizontal blue bands give the maximum H 2 O content measured in the glass inclusions (measured by SIMS, Balcone-Boissard et al. 2018)

However, H 2 O-saturated conditions are not appropriate for the Layou pre-eruptive magma. At 800 °C and 150 MPa H 2 Osaturated, the natural plagioclase composition is reproduced but the crystallinity (~50%) is higher than expected (~30%) and the amphibole is too high in Al 2 O 3 (12 wt%). There is also a mismatch concerning magnetite (too TiO 2 -poor), a consequence of a highly oxidizing experimental f O2 (~NNO + 2; Fig. 7a). At 850 °C and 225 MPa H 2 O-saturated, the correct crystallinity is obtained but both plagioclase and orthopyroxene have An and En contents higher by > 10 mol% than the corresponding phenocrysts. In comparison, the H 2 O-undersaturated run at 850 °C and 375 MPa is very close to reproduce the natural samples. It crystallizes 25 wt% An 53 , 7 wt% Am 10 , 5 wt% En 50 and < 1 wt% Mag 74 (Fig. 7b; Table 3). Clinopyroxene is absent, in agreement with the natural phase assemblage. Therefore, the main part of the Layou magma likely crystallized at 350-400 MPa (equivalent to a depth of ~14-16 km) and ~850 °C, under a f O2 of NNO + 0.6, and with a melt H 2 O content of ~7.5-8.0 wt%. These conditions agree with the T-f O2 deduced from the Am 9-10 compositions and the orthopyroxene-liquid thermometer (SM2B-C) and with the ~7.5 wt% H 2 O contents of the orthopyroxene-hosted glass inclusions [START_REF] Balcone-Boissard | Deep pre-eruptive storage of silicic magmas feeding Plinian and dome-forming eruptions of central and northern Dominica (Lesser Antilles) inferred from volatile contents of melt inclusions[END_REF].

Nevertheless, the Layou experimental charges and natural products differ in important three main aspects. First, amphibole is a minor but characteristic phase in Layou deposits, stable in a much larger domain in the LAY than in the ROS composition (Fig. 7). Yet, our experimental amphiboles do not reproduce well the compositions of amphibole phenocrysts (Fig. 5). Amphiboles with 9-10 wt% Al 2 O 3 (Am 9-10 ; corresponding to cores) have been experimentally synthesized (e.g. at 400 MPa, 825 and 875 °C) but they have Mg# significantly higher than the natural crystals, as most experimental amphiboles, and compositions corresponding to Am 6-7 rims (Fig. 4a) have not been reproduced. Experimental amphiboles plot along a broadly positive Al 2 O 3 vs. Mg# trend that contrasts with the well-defined negative correlation shown by the phenocrysts (Fig. 5). Only one experimental amphibole (at 375 MPa, 850 °C) plots in the natural amphibole field (Fig. 5). To complement our experiments and refine the crystallization conditions for Layou amphiboles, additional experimental constraints from the literature have been used. The main conclusions are twofold. First, that Am 6-7 can crystallize from a rhyolitic/granitic magma with ~72 wt% SiO 2 [START_REF] Dall'agnol | An experimental study of a lower proterozoic A-type granite from the Eastern Amazonian Craton, Brazil[END_REF] and Am 9-10 from a dacitic magma with 65-68 wt% SiO 2 (e.g. sample# 875-400-L1 in SM3D; [START_REF] Scaillet | The 15 June 1991 eruption of Mount Pinatubo. I. Phase equilibria and pre-eruption P-T-fO2-f H2O conditions of the dacite magma[END_REF][START_REF] Cadoux | Magma storage conditions of large Plinian eruptions of Santorini volcano (Greece)[END_REF]. Second, the high-Al 2 O 3 low-Mg# end of the Layou natural amphibole field requires more reduced conditions than the f O2 range imposed in this study, being well reproduced by < NNO experiments on Santorini rhyodacites [START_REF] Cadoux | Magma storage conditions of large Plinian eruptions of Santorini volcano (Greece)[END_REF]. In comparison, the low-Al 2 O 3 high-Mg# end of the trend is compatible with redox conditions between NNO and NNO + 1, suggesting that additional LAY experiments at < 825 °C could produce amphiboles in this part of the array. To summarize, the Layou natural amphibole array reflects mixing between two amphibole end-member populations present in the plumbing system, one (Am 9-10 ) corresponding to a phase crystallizing from dacitic magmas under relatively reduced conditions and the other (Am 6-7 ) to a crystallization product of rhyolitic magmas under "average" f O2 conditions. Independent evidence for parts of the Layou magma storage zone being relatively reduced comes from the T-f O2 data which include a group of data points clustered at NNO to NNO-0.2 [START_REF] Howe | Reevaluation of the Roseau tuff eruptive sequence and other ignimbrites in Dominica, Lesser Antilles[END_REF].

Second, quartz occurs as euhedral crystals in Layou deposits, whereas it has not been found in our experiments. Yet, quartz is known to saturate during the crystallization of intermediate bulk compositions such as the Taupo dacite [START_REF] Conrad | Water-saturated and -Undersaturated melting of Metaluminous and Peraluminous crustal compositions at 10 kb: evidence for the origin of silicic magmas in the Taupo volcanic zone, New Zealand, and other occurrences[END_REF], the Pinatubo dacite [START_REF] Scaillet | The 15 June 1991 eruption of Mount Pinatubo. I. Phase equilibria and pre-eruption P-T-fO2-f H2O conditions of the dacite magma[END_REF][START_REF] Prouteau | Experimental constraints on the origin of the 1991 Pinatubo Dacite[END_REF] and the Unzen dacite [START_REF] Holtz | Experimental petrology of the 1991-1995 Unzen dacite, Japan. Part I: phase relations, phase composition and pre-eruptive conditions[END_REF]. For a constant (dacitic) bulk composition, quartz saturation depends on three variables (which also control the SiO 2 content of the melt at saturation): temperature, melt H 2 O content and pressure. Between 200 and 400 MPa, quartz saturates at temperatures from ~775 °C to up to 950 °C, for < 4 to ~8 wt% H 2 O in the melt; melt SiO 2 contents at saturation range from 71 to a maximum of 76-77 wt%, being positively correlated with H 2 O and inversely correlated with temperature [START_REF] Scaillet | The 15 June 1991 eruption of Mount Pinatubo. I. Phase equilibria and pre-eruption P-T-fO2-f H2O conditions of the dacite magma[END_REF][START_REF] Prouteau | Experimental constraints on the origin of the 1991 Pinatubo Dacite[END_REF][START_REF] Holtz | Experimental petrology of the 1991-1995 Unzen dacite, Japan. Part I: phase relations, phase composition and pre-eruptive conditions[END_REF]. For melt H 2 O concentrations as in Dominica magmas (6-8 wt%), quartz saturation would be expected for 76-77 wt% SiO 2 in the melt, i.e. in the same range as our most evolved experimental liquids (Fig. 6). Therefore, the most SiO 2 -rich melts generated from the crystallization of ROS and LAY are quartz-undersaturated (since quartz is lacking in experimental charges) but evolve near quartz saturation. It is most likely that additional experiments at temperatures lower than investigated (< 800 °C?) would lead to the appearance of quartz in experimental phase assemblages. We therefore interpret the presence of quartz crystals in the Layou deposit as evidence for parts of the magma plumbing system being slightly colder (< 800 °C?) than the main magma body (8 50 °C). This conclusion is consistent with the dispersion in the T-f O2 data for Layou and the presence of a group of points at ~780 °C [START_REF] Howe | Reevaluation of the Roseau tuff eruptive sequence and other ignimbrites in Dominica, Lesser Antilles[END_REF].

Third, Layou residual glasses are slightly more SiO 2 -rich than , all being less silicic than glass inclusions which, for both Roseau and Layou, reach up to 80 wt% SiO 2 (Fig. 6). In both systems, compositions of inclusions are outside the range of experimental preeruptive melts. The slightly higher silica content in Layou than in Roseau residual glasses is consistent with the presence of quartz crystals which imposes quartz saturation in Layou residual melts. Glass inclusions have SiO 2 concentrations that extend to beyond quartz saturation, whether defined from experiments (76-77 wt% SiO 2 , see above) or by the Layou quartz-saturated residual glasses (77.5-78.5 wt% SiO 2 ). Liquids above the quartz saturation threshold must be considered as metastable (they should crystallize quartz to equilibrate their SiO 2 concentration) if brought under conditions defined above for pre-eruptive magma storage. Alternatively, these liquids could reflect equilibration at pressures much lower than the pre-eruptive range (350-400 MPa) because decreasing pressure shifts quartz saturation towards higher melt SiO 2 contents. Last, the fact that glass inclusion compositions vary with the host crystal indicates local compositional control, most probably by crystallization and dissolution reactions involving the trapped liquid and the host crystal. We conclude that no direct relation can be established between glass inclusion major-element compositions and pre-eruptive liquids.

The prime implication of the complexities recognized above for the Layou magma is that some parts of the storage zone evolved under conditions significantly different from the main magma body (~850 °C, 350-400 MPa, ~NNO + 0.6 and ~7.5-8.0 wt% H 2 O in the melt). In particular, the Am 6- 7 rims around Am 9-10 cores, the presence of a few An 40-50 rims around plagioclases and the occurrence of quartz suggest that some portions of the magma body underwent further cooling and differentiation. Although data are lacking to constrain the crystallization conditions of those magma batches, cooling down to ~780 °C would be consistent with the presence of quartz, the Fe-Ti oxide T-f O2 data [START_REF] Howe | Reevaluation of the Roseau tuff eruptive sequence and other ignimbrites in Dominica, Lesser Antilles[END_REF] and amphibole thermometry (lower T for Am 6-7 than for Am 9-10 ; SM2A-B), whereas a f O2 of ~NNO + 1 is suggested by Mg# in Am 6-7 and Am 6-7 oximetry (Fig. 5b andSM2B). Cooling-induced differentiation of the Layou dacitic magma could have led to the segregation of rhyolitic (~72 wt% SiO 2 ) magma batches from which the Am 6-7 and < An 50 co-crystallized. There is no clear evidence to constrain the H 2 O content and crystallization P for these magma pockets, but coolinginduced differentiation is more likely to have occurred above the top of the main reservoir (i.e. ≤ 350-400 MPa). Furthermore, the destabilized Am 6-7 in the Roseau Plinian pumice could be inherited from similar differentiated magma parts. Another interesting aspect of the Layou data is the evidence from amphibole compositions (Am 9-10 ) of the presence of a reduced component in the magma storage region. Reduced volcanic rocks are very rare in the Lesser Antilles but 8.5 Ma-old fayalite-ferroaugite dacites, rhyodacites and rhyolites are known on St Lucia (Le Guen de [START_REF] Guen De Kerneizon | Rhyolites with fayalite and ferroaugite in Saint-Lucia, Lesser Antilles island arc[END_REF]. They evolved under a f O2 significantly < NNO as estimated from compositions of titanomagnetite phenocrysts and using the Ti vs. Mg# plot of [START_REF] Martel | Effect of fO 2 and H 2 O on andesite phase relations between 2 and 4 kbar[END_REF].

The recent results of [START_REF] Boudon | Revised chronostratigraphy of recurrent ignimbritic eruptions in Dominica (Lesser Antilles Arc): implications on the behavior of the magma plumbing system[END_REF] suggest that the Grande Savane and Layou ignimbrites were emitted from the same eruptive centre (Morne Diablotins; Fig. 1). Indeed, Grande Savane and Layou rocks share similar major (Table 1; Fig. 6) and trace-element bulk-rock compositions [START_REF] Boudon | Revised chronostratigraphy of recurrent ignimbritic eruptions in Dominica (Lesser Antilles Arc): implications on the behavior of the magma plumbing system[END_REF], and they carry the same phenocryst assemblage with significant amounts of amphibole. The Grande Savane glasses (either glass inclusions or residual glasses) and the Layou residual glasses overlap, although the former are slightly less differentiated (76-78 wt% SiO 2 ) than the latter (Fig. 6). Yet, in terms of phenocryst compositions, the Grande Savane and Layou pumice samples are different. As for amphibole, the Grande Savane pumice samples contain the largest amounts (~5-10% vs. 1-5% in Layou samples) and the largest crystals (up to 1 mm vs. ~200 μm in Layou samples). Compositions are the most depleted in Al 2 O 3 (5.5 wt%) and they lack the Am 9-10 group found in Layou. Plagioclases in Grande Savane have ~An 40 rims which could have crystallized at equilibrium with Am 6 amphiboles [START_REF] Dall'agnol | An experimental study of a lower proterozoic A-type granite from the Eastern Amazonian Craton, Brazil[END_REF] and En 49-50 orthopyroxenes (n.b., En 52-53 in Layou; Table 2). Although H 2 O contents are lacking for the Grande Savane magma, the glass inclusions (six in orthopyroxenes, one in ilmenite and one in amphibole; Table 2) have electron microprobe major-element totals of 94 wt%, which precludes volatile contents higher than 6 wt% (i.e. higher than the H 2 O solubility at 150-200 MPa). Therefore, in comparison with the Layou main magma, the Grande Savane magma is interpreted to have crystallized at shallow pressure, possibly ~150-200 MPa as suggested by the glass inclusions and consistent with the lack of Am 9-10 . The phenocryst assemblage and in particular the presence of abundant Am 5-6 in the Grande Savane samples suggests crystallization from a more evolved and colder batch than the main Layou magma.

Structure of the magmatic system beneath Dominica

The results above suggest a magmatic system located as deep as ~16 km (~400 MPa) beneath Dominica and active over the last 62 ky (Fig. 9b). Although two different magma compositions (amphibole-free andesitic-dacitic magma for the Roseau-like group and amphibole-bearing dacitic magma for the Layou-like group) were involved, the respective magma chemistries are consistent with the Layou-like group being a differentiation product of the Roseau-like group, thus favouring the hypothesis of common parental magmas [START_REF] Gavrilenko | Abrupt transition from fractional crystallization to magma mixing at Gorely volcano (Kamchatka) after caldera collapse[END_REF]). The plumbing system could have the form of either a single magma storage region or independent magma bodies. More differentiated individual magma batches have been identified as indicated by the results for Layou and Grande Savane. These can be viewed as coming from cooler margins of the same large body (Layou) or as different parts of a plumbing system with a significant vertical extension (Grande Savane). In the case of Grande Savane, it is possible to consider an upper segregated magma pocket that ponded at the mid-crustal discontinuity. [START_REF] Kopp | Deep structure of the central Lesser Antilles Island arc: relevance for the formation of continental crust[END_REF] acquired seismic data across the volcanic arc North of Dominica (where the Tiburon Ridge subducts obliquely beneath the fore-arc) and, by a tomographic inversion, proposed a structural model of the arc crust consisting in three distinct layers: a 3-km-thick upper crust of volcanogenic sedimentary rocks and volcanoclastics, underlain by an intermediate to felsic middle crust and a plutonic mafic (gabbros) lower crust. These results are confirmed and extended to the whole active arc by a recent reconstruction of the crustal structure using an inversion approach combining petrology of magmatic crustal xenoliths and seismic receiver functions (Fig. 9a; [START_REF] Melekhova | Lateral variation in crustal structure along the Lesser Antilles arc from petrology of crustal xenoliths and seismic receiver functions[END_REF]. Both studies agree that the mid-crustal discontinuity beneath Dominica is located at ~13-16 km, i.e. in the same range as the magma reservoir depth determined from our experimental study. The mid-crustal discontinuity likely corresponds to a structural, mechanical and physical (density) discontinuity that could favour magma ponding.

Ignimbritic eruptions of very large volumes (> 100 km 3 ) are associated with collapse-caldera structures (e.g. Kos Plateau Tuff, Greece, Allen 2001; Taupo, New Zealand, [START_REF] Houghton | Chronology and dynamics of a large silicic magmatic system: Central Taupo volcanic zone[END_REF]. So far, no caldera has been identified in Dominica [START_REF] Boudon | Revised chronostratigraphy of recurrent ignimbritic eruptions in Dominica (Lesser Antilles Arc): implications on the behavior of the magma plumbing system[END_REF]), but the erupted volumes are < 10 km 3 . Yet, erupted volumes of this order of magnitude often lead to caldera-forming eruptions, such as Krakatau 1883 (Indonesia; 12.5 km 3 ) or Pinatubo 1991 (Philippines; ~2.2 km 3 ). The difference between Dominica and these calderaforming eruptions is the depth of magma storage. Indeed, the caldera-forming eruptions of Krakatau or Pinatubo involved magmas stored in the upper crust (≤ 200 MPa; [START_REF] Mandeville | Magma mixing, fractional crystallization and volatile degassing during the 1883 eruption of Krakatau volcano, Indonesia[END_REF][START_REF] Scaillet | The 15 June 1991 eruption of Mount Pinatubo. I. Phase equilibria and pre-eruption P-T-fO2-f H2O conditions of the dacite magma[END_REF], whereas we find a middle-lower crust storage level (~400 MPa) below Dominica. Collapse-caldera development is likely controlled by the relationships between the chamber aspect ratio, defined as the ratio between magma chamber depth and width, and the volume fraction of erupted magma [START_REF] Geyer | Relationship between caldera collapse and magma chamber withdrawal: an experimental approach[END_REF]). The reservoirs in Dominica are about 16 km deep and 5 km 3 , so that the chamber aspect ratio ranges between ~15 for a spherical chamber and ~3 for sill-type reservoirs. Assuming partial emptying and reservoir volumes two or three times larger than the erupted products would keep aspect ratios greater than one. The combination of magma volumes < 10 km 3 and chamber aspect ratios > 1 has been demonstrated to prevent collapse-caldera forming [START_REF] Geyer | Relationship between caldera collapse and magma chamber withdrawal: an experimental approach[END_REF][START_REF] Marti | A review on collapse caldera modelling[END_REF]). Comparison with magma storage conditions on the neighbouring islands

The other islands along the Lesser Antilles arc (Martinique, Guadeloupe, Montserrat) are characterized by a single active eruptive centre (at least during the recent period of activity) frequently emitting deposits of small volumes, generally < 0.5 km 3 and exceptionally ~1 km 3 . Magma storage conditions for some recent eruptions on these islands have been determined experimentally via phase-equilibrium studies. In Montserrat, the andesitic magma (~59 wt% SiO 2 ) of the ongoing eruption of Soufriere Hills was found to be stored at 820-840 °C, ~130 MPa (< 5 km depth) and ~ΔNNO + 1 with 4.3 ± 0.5 wt% H 2 O in the melt [START_REF] Barclay | Experimental phase equilibria constraints on pre-eruptive storage conditions of the Soufrière Hills magma[END_REF]. In Guadeloupe, the andesitic magma (~60 wt% SiO 2 ) from the 1530 AD sub-plinian eruption of La Soufrière was stored at 875 °C, ~175 MPa and ~ΔNNO + 0.5 to + 1, with ~5.5 wt% H 2 O in the melt [START_REF] Pichavant | Experimental parametrization of magma mixing: application to the AD 1530 eruption of La Soufrière, Guadeloupe (Lesser Antilles)[END_REF]. In Martinique, historical eruptions of Montagne Pelée, either plinian fallout or lava dome-forming, were fed by an andesitic magma (~61 wt% SiO 2 ) stored at 875-900 °C, ~200 MPa (~6 km depth) and ~ΔNNO + 0.4 to + 0.8, with ~6 wt% H 2 O in the melt [START_REF] Martel | Magma storage conditions and control of eruption regime in silicic volcanoes: experimental evidence from Mt Pelée[END_REF].

These studies show that the small-volume andesitic eruptions considered were fed by magmas stored in a similar range of conditions in terms of temperature (850-900 °C), pressure (175-200 MPa), f O2 (~ΔNNO + 1) and melt H 2 O contents (close to saturation). In comparison, Dominica constitutes an anomaly in the Lesser Antilles arc for four reasons. First, larger volumes were erupted (~5 km 3 DRE/eruption; [START_REF] Boudon | Revised chronostratigraphy of recurrent ignimbritic eruptions in Dominica (Lesser Antilles Arc): implications on the behavior of the magma plumbing system[END_REF]. Second, magmas were dominantly evolved (dacites with 63-66 wt% SiO 2 ). Third, magmas were stored at higher pressures (~400 MPa) with higher melt H 2 O contents (7-8 wt%). Fourth, eruptions are much less frequent so that the magma repose times are longer than for the volcanic systems from the other islands.

Larger volumes of erupted magma in Dominica with respect to the neighbouring islands may result from particularities in the subduction geometry [START_REF] Wadge | Segmentation of the Lesser Antilles subduction zone[END_REF][START_REF] Macdonald | The Lesser Antilles volcanic chain: a study in arc magmatism[END_REF], from the presence of oceanic fractures zones near subduction favouring water supply to the mantle wedge [START_REF] Manea | Subduction of fracture zones controls mantle melting and geochemical signature above slabs[END_REF][START_REF] Schlaphorst | Water, oceanic fracture zones and the lubrication of subducting plate boundaries-insights from seismicity[END_REF] or from a fault system that potentially create a structural context with a strong extensional component beneath Dominica [START_REF] Feuillet | Active faulting induced by slip partitioning in Montserrat and link with volcanic activity: New insights from the 2009 GWADASEIS marine cruise data[END_REF]) thus fostering lateral expansion of the reservoirs and making possible the storage of large magma volumes. These hypotheses could explain the accumulation of large magma volumes but not the unusual depths of the reservoirs beneath Dominica. Large-volume eruptions do not necessarily require deep magma storage, as found at the Aegean Arc or the Taupo Volcanic Zone where magmas are stored within the upper crust (≤ 200 MPa; [START_REF] Bachmann | The melt inclusion record from the rhyolitic Kos Plateau Tuff (Aegean arc)[END_REF][START_REF] Bégué | Phase-equilibrium geobarometers for silicic rocks based on rhyolite-MELTS. Part 2: application to Taupo Volcanic Zone rhyolites[END_REF]. At the Antilles arc, one explanation for the voluminous storage region in Dominica being deeper than the ones on the neighbouring volcanic islands could be an along-arc fluctuation of the crustal structure. Indeed, previous seismic refraction studies suggested significant differences in the crust thickness, with the thickest one being under Dominica (up to 37 km; [START_REF] Melekhova | Lateral variation in crustal structure along the Lesser Antilles arc from petrology of crustal xenoliths and seismic receiver functions[END_REF] and in the depth of the middlelower crust interface, with variations from 2 to 20 km [START_REF] Boynton | A seismic refraction investigation of crustal structure beneath the Lesser Antilles island arc[END_REF][START_REF] Macdonald | The Lesser Antilles volcanic chain: a study in arc magmatism[END_REF][START_REF] Melekhova | Lateral variation in crustal structure along the Lesser Antilles arc from petrology of crustal xenoliths and seismic receiver functions[END_REF]. However, this mid-crustal discontinuity could be at similar depth below Martinique, Montserrat and Dominica (Fig. 9a; [START_REF] Melekhova | Lateral variation in crustal structure along the Lesser Antilles arc from petrology of crustal xenoliths and seismic receiver functions[END_REF]. As a result, the storage depth below Dominica must be attributed to other reasons, such as slab dehydration related to the fracture zone near Dominica or peculiar magma productivity where the crust is the thickest.

As an alternative to this regional model, it is worth remembering that the upper crustal reservoirs identified below Martinique, Guadeloupe and Montserrat are shallow (uppercrustal interface) expressions of complex magmatic systems that extend deeper in the crust. For Montagne Pelée, there is petrological evidence for a deep (~400 MPa) basaltic andesite reservoir that differentiates through fractional crystallization to feed the upper crustal andesitic reservoir [START_REF] Pichavant | Physical conditions, structure, and dynamics of a zoned magma chamber: Mount Pelée (Martinique, Lesser Antilles arc)[END_REF]. For La Soufrière of Guadeloupe, the 1530 AD eruption was triggered by the recharge of a deep basaltic magma into a shallow resident andesitic body [START_REF] Pichavant | Experimental parametrization of magma mixing: application to the AD 1530 eruption of La Soufrière, Guadeloupe (Lesser Antilles)[END_REF]. At Montserrat, the involvement of mafic magmas of deep origin is attested by the presence of enclaves in the present-day eruption products [START_REF] Plail | Characterization of mafic enclaves in the erupted products of Soufrière Hills volcano, Montserrat, 2009 to 2010[END_REF]. Therefore, the presence of igneous bodies at mid-crustal levels seems well established in the Lesser Antilles arc, so that the interesting point in Dominica is not so the presence of deep magma storage zones but the scarcity of upper-crustal reservoirs during the period of large-volume eruptions (24-51 ka; except maybe for the Grande Savane ignimbrite). The rarity of shallow reservoirs prevents frequent drainage of the deep igneous bodies (contrary to evidence on the neighbouring islands), thus favours deep and long-lasting accumulation of large magma volumes that can differentiate to dacitic compositions. Thus, magma accumulation rates below Dominica would neither be particularly high nor different from the other islands, but would reflect longer timescales of magma storage.

Conclusions

Combining a petrological study of basal plinian deposits of the main ignimbritic eruptions in Dominica with crystallization experiments suggests that the dacitic magmas were mainly stored at ~400 MPa (~16 km depth), ~850 °C and contained 7-8 wt% H 2 O dissolved in melt. This storage depth is consistent with magma ponding at the mid-crustal discontinuity beneath Dominica. More differentiated magma batches have been identified, possibly reflecting a plumbing system with a significant vertical extension.

With respect to the volcanic centres on the neighbouring islands (Martinique, Guadeloupe, Montserrat), the larger volumes of Dominica eruptions may result from a geodynamic context that locally favours extensional components and magma accumulation. Over the time period of the large eruptions in Dominica, upper-crustal reservoirs seem to be rare enough to prevent frequent drainage of the deep (mid-crust) main magma body. This gives time for growth of large magma chambers and differentiation at Dominica. Therefore, high magma fluxes (i.e. flare-up) may not be a necessary condition for large-volume ignimbritic eruptions in Dominica.

We conclude that the primary magma reservoirs feeding Dominica ignimbritic eruptions are deeper than those feeding other volcanic systems along the arc, and that reservoir depth can explain both the absence of calderas on the island, as well as the unusually large volumes of these events. Linking depths of magma storage to crust structure and regional tectonic setting may be a key point to the general understanding of the generation of large-volume pumice eruptions of arc volcanoes.

Fig. 1

 1 Fig. 1 Dominica Island. Simplified version of Dominica geological map showing the deposit extent of the major pyroclastic density currents. The orange triangles indicate the major eruptive centres and the green stars show the sampling sites (modified after Boudon et al. 2017)

  Selected profiles of plagioclase composition for pumice from a Layou, b Roseau and c Grand Fond. The position of the analysed profile is indicated by the red line-arrow in each BSE images

Fig. 3 Fig. 4

 34 Fig. 3 Natural plagioclase compositions. Frequency histogram of chemical profiles in An mol% from natural plagioclases in Layou, Roseau, Grand Fond, Grand Bay and Grande Savane deposits. Roseau (in green), Grand Fond (in orange), Grande Savane (in purple) and Grand Bay (in blue) crystals show a unimodal distribution (peak at An 53-55 ), whereas Layou (in red) crystals have a bimodal distribution (peaks at An 50-55 and An 68-70 )

Fig. 5

 5 Fig. 5 Natural and experimental amphibole compositions. a Al IV pfu vs. number of cations in the A-site and b Al IV vs. Mg# = Mg/(Mg + Fe tot ); recalculated with iron as FeO and on a H 2 O-free 23 oxygen basis; note that Al IV is anti-correlated to Mg#. Labels on experimental data

Fig. 7

 7 Fig. 7 Experimental phase relationships for the a, b Layou (LAY) and c, d Roseau (ROS) dacitic glasses for a, c H 2 O-saturated and b, d undersaturated conditions. All experiments are performed at ~ΔNNO + 1. The central red box gives the natural phase assemblage (shaded square for phase present) with the label giving crystallinity (in brackets; ± 5 vol% determined by image analysis), and phase compositions (range of plagioclase anorthite mol%, amphibole giving Al 2 O 3 wt% content as subscript, orthopyroxene enstatite mol%, Fe-Ti oxides magnetite and ilmenite

Fig. 9

 9 Fig.9Schematic section of the crust below Dominica. a P wave velocity (Vp) structure beneath the Lesser Antilles arc, modified from[START_REF] Melekhova | Lateral variation in crustal structure along the Lesser Antilles arc from petrology of crustal xenoliths and seismic receiver functions[END_REF]. The Vp of 6.0 km/s outline the upper crust, 6.8 km/s delimit the middle crust and 7.8 km/s outline the lower crust, the basis of which marks the Moho discontinuity with the upper mantle. Below Dominica, the mid-crust discontinuity is located at ~16-18 km in depth, where the ignimbritic reservoirs have been experimentally located (red box). b Storage conditions of Dominica dacitic magmas determined in this study; the main magma ponds at the mid-crust discontinuity (~16 km deep), either as a common or individual reservoirs, with vertical extensions of more differentiated magma batches; the subscript linked to Am gives the Al 2 O 3 wt% of the amphibole

Table 1

 1 Bulk-rock composition of the main ignimbrites from Dominica

	Oxides (wt%)	Layou-like group			Roseau-like group
		Layou		Grande Savane	Roseau		Grand Bay		Grand Fond
		Exp	Nat	Exp	Nat	Exp	Nat	Exp	Nat	Nat
		LAY	Dom41a1	GS	DOM40b	ROS	DOM60d1a	GB	DOM31	DOM43b1
	SiO 2	65.43	66.20	65.65	64.83	64.08	63.50	63.02	63.31	62.76
	TiO 2	0.46	0.38	0.37	0.45	0.51	0.54	0.63	0.51	0.54
	Al 2 O 3	16.60	16.43	17.51	16.45	17.05	16.89	16.92	16.80	17.05
	FeO tot	5.32	4.79	4.18	5.26	5.63	6.11	6.15	5.86	6.11
	MnO	0.18	0.14	0.14	0.14	0.16	0.15	0.15	0.15	0.15
	MgO	1.90	1.62	1.35	2.00	2.25	2.35	2.12	2.25	2.43
	CaO	5.48	5.19	5.54	5.60	6.01	5.92	6.26	6.07	5.99
	Na 2 O	3.41	3.48	3.51	3.54	2.98	2.93	3.21	3.39	3.33
	K 2 O	1.65	1.67	1.76	1.62	1.57	1.50	1.52	1.56	1.53
	P 2 O 5	0.16	0.10	-	0.11	0.21	0.11	-	0.11	0.11
	Total	100	100	100	100	100	100	100	100	100
	Na 2 O + K 2 O	5.05	5.15	5.27	5.15	4.55	4.42	4.73	4.95	4.86
	FeO + MgO	7.22	6.41	5.53	7.26	7.88	8.46	8.27	8.10	8.54
	Total iron recalculated as FeO tot . Exp for experimental starting glass and Nat for natural composition (wet chemistry analysis and sample number after
	Boudon et al. 2017

Table 2

 2 (continued)

	TiO Al O FeO MnO MgO CaO Na O K O T o t a l C a l c	0.47 12.34 1.73 0.12 0.20 1.24 2.92 4.11 100.00		0.02 0.02 27.23 0.50 0.28 0.08 0.03 0.05 0.01 0.02 10.83 0.33 5.36 0.20 0.15 0.03 97.78 An52	0.09 0.04 0.52 0.14 25.62 0.49 0.90 0.09 17.37 0.34 1.56 1.08 0.04 0.04 0.01 0.02 97.90 En52	9.31 0.51 1.60 0.11 76.21 3.70 0.36 0.09 0.79 0.07 0.07 0.06 0.02 0.02 0.01 0.01 88.55 Mag71	41.35 0.15 47.69 0.51 1.73 0.00 0.00 0.00 91.43 Ilm83	0.11 0.03 0.89 0.07 10.93 0.36 0.46 0.13 12.57 0.04 20.90 0.22 0.26 0.04 0.00 0.00 97.59 Wo44	0.25 13.10 0.76 0.02 0.09 1.80 3.68 2.81 100.00	0.02 0.00 12.48 0.47 0.42 0.03 0.03 0.04 0.01 0.01 1.31 0.42 3.18 0.35 3.20 0.07 100.00
	n SiO	1 76.86		9 53.88 0.68	4 51.79 0.13	9 0.19 0.40	1 0.00	2 51.49 0.31	1 77.50	2 79.36 1.09
		GI (Ilm)	Grand Bay (DOM31)	Plg (rim)	Opx	Mt	Ilm	Cpx	RG	GI (Cpx)

All oxides in wt%; n indicates the number of EMP analyses on which the composition has been averaged. Numbers in italic give the 1σ deviation on the mean value; Plg plagioclase, Opx orthopyroxene, amph amphibole, Mt titanomagnetite, Ilm ilmenite (with Ilm* representing a composition averaged on two ilmenites of the Layou PDC overlying the plinian fallout deposit), RG matrix glass, GI glass inclusion (host crystal in bracket); Calc gives the calculated anorthite content (An, mol%) for Plg, enstatite content (En, mol%) for Opx, magnesium number (Mg#) for Amph, magnetite content (Mag, mol%) for Mt., ilmenite content (Ilm, mol%) for Ilm

Table 3

 3 Experimental conditions and phase proportions Run # Sample X H2Oin Major phase (proportions in wt%) Gl/cryst (vol%) H 2 O m (wt%) H 2 O c (wt%) Gl glass, Plg plagioclase, Opx orthopyroxene, Cpx clinopyroxene, Amph amphibole, Mt titanomagnetite, Ilm ilmenite. Phase proportion calculated using mass balance calculation (where lacking Mt composition, one in the charge with the closest P-T-X H2Oin conditions has been used) and determined by image analysis in vol% (assuming area and volume proportion compares) with Gl/cryst giving the volume proportion of glass over crystals. H 2 O m is melt H 2 O content (wt%) determined by the EMP by-difference method and H 2 O c is melt H 2 O content calculated after the formulation of

	Run 1, 850 °C, 275 MPa, ~NNO + 1, 240 h
	850-275-L1	LAY	1	Gl(73) + Plg(12) + Amph(13) + Mt(2)	77/23	8.9	7.4
	850-275-L0.8	LAY	0.78	Gl(53) + Plg(36) + Opx(10) + Mt(1)	60/40	6.2	5.8
	850-275-R1	ROS	1	Gl(75) + Plg(11) + Amph(11) + Mt(3)	74/26	9.5	7.4
	850-275-R0.8	ROS	0.78	Gl(54) + Plg(36) + Opx(10) + Mt(1)	66/34	6.7	5.8
	Run 2, 850 °C, 190 MPa, ~NNO + 1, 208 h
	850-190-L1	LAY	1	Gl(73) + Plg(17) + Opx(7) + Mt(3)	71/29	8.7	5.8
	850-190-L0.8	LAY	0.76	Gl + Plg + Opx + Mt	72/28	6.4	4.4
	850-190-R1	ROS	1	Gl(72) + Plg(18) + Opx(7) + Mt(3)	62/38	9.3	5.8
	850-190-R0.8	ROS	0.81	Gl(46) + Plg(47) + Opx(7) + Mt(2)	57/43	6.2	4.7
	Run 3, 825 °C, 285 MPa, ~NNO + 1, 172 h
	825-285-L1	LAY	1	Gl(73) + Plg(14) + Amph(10) + Mt(3)	78/22	8.4	7.6
	825-285-L0.8	LAY	0.80	Gl(55) + Plg(35) + Opx(10) + Mt(< 1)	55/45	-	6.1
	825-285-R1	ROS	1	Gl(69) + Plg(17) + Opx(2) + Amph(9) + Mt(3)	73/27	9.1	7.6
	825-285-R0.8	ROS	0.79	Gl(59) + Plg(30) + Opx(10) + Mt(1)	56/44	-	6.0
	Run 4, 825 °C, 195 MPa, ~NNO + 1, 242 h
	825-195-L1	LAY	1	Gl + Plg + Opx + Mt	71/29	9.6	5.9
	825-195-L0.8	LAY	0.82	Gl(54) + Plg(36) + Opx(8) + Mt(1) + Ilm(< 1)	60/40	-	4.8
	825-195-R1	ROS	1	Gl + Plg + Opx + Mt	74/26	8.7	5.9
	825-195-R0.8	ROS	0.78	Gl + Plg + Opx + Mt	63/37	-	4.6
	Run 5, 850 °C, 375 MPa, ~NNO + 1, 162 h
	850-375-L1	LAY	1	Gl(74) + Plg(9) + Amph(14) + Mt(3)	78/22	9.8	9.9
	850-375-L0.8	LAY	0.78	Gl(62) + Plg(25) + Amph(7) + Opx(5) + Mt(1)	68/32	7.9	7.7
	850-375-R1	ROS	1	Gl(73) + Plg(10) + Amph(13) + Mt(3)	79/21	9.2	9.9
	850-375-R0.8	ROS	0.79	Gl(58) + Plg(30) + Opx(10) + Mt(1) + Ilm(< 1)	67/33	8.2	7.8
	Run 6, 850 °C, 225 MPa, ~NNO + 1, 190 h
	850-225-L1	LAY	1	Gl(76) + Plg(11) + Opx(2) + Amph(13) + Mt(2)	70/30	8.6	6.7
	850-225-L0.9	LAY	0.89	Gl(64) + Plg(27) + Opx(8) + Mt(1)	64/36	-	6.0
	850-225-L0.8	LAY	0.81	Gl(57) + Plg(32) + Opx(9) + Mt(1)	65/35	5.7	5.4
	850-225-R1	ROS	1	Gl(73) + Plg(15) + Opx(5) + Amph(5) + Mt(2)	76/24	8.7	6.7
	850-225-R0.9	ROS	0.89	Gl(61) + Plg(30) + Opx(8) + Mt(1)	72/28	-	6.0
	850-225-R0.8	ROS	0.80	Gl(62) + Plg(28) + Opx(9) + Mt(1) + Ilm	67/33	-	5.4
	Run 7, 900 °C, 300 MPa, ~NNO + 1, 190 h
	900-300-L1	LAY	1	Gl(90) + Amph(6) + Mt(4)	90/10	-	7.7
	900-300-L0.8	LAY	0.80	Gl(70) + Plg(20) + Opx(8) + Mt(2)	69/31	-	6.2
	900-300-R1	ROS	1	Gl(90) + Opx(7) + Mt(3)	89/11	-	7.7
	900-300-R0.8	ROS	0.82	Gl(71) + Plg(19) + Opx(7) + Mt(3)	70/30	-	6.3
	Run 8, 825 °C, 400 MPa, ~NNO + 1, 192 h
	825-400-L1	LAY	1	Gl(77) + Plg(11) + Amph(10) + Mt(2)	77/23	-	10.0
	825-400-L0.8	LAY	0.80	Gl(49) + Plg(36) + Amph(<1) + Opx(13) + Mt(< 1) + Ilm(1)	56/44	-	8.1
	825-400-R1	ROS	1	Gl(70) + Plg(15) + Amph(12) + Mt(3)	75/25	-	10.1
	825-400-R0.8	ROS	0.81	Gl(51) + Plg(34) + Opx(14) + Mt(< 1) + Ilm(1)	51/49	-	8.2
	Run 9, 875 °C, 400 MPa, ~NNO + 1, 215 h
	875-400-L1	LAY	1	Gl(88) + Amph(10) + Mt(2)	92/8	-	9.9
	875-400-L0.8	LAY	0.82	Gl(72) + Plg(18) + Amph(5) + Opx(3) + Mt(2)	75/25	-	8.1
	875-400-R1	ROS	1	Gl(79) + Plg(9) + Amph(9) + Mt.(3)	83/17	-	9.9
	875-400-R0.8	ROS	0.78	Gl(54) + Plg(33) + Opx(10) + Cpx(1) + Mt(1) + Ilm(1)	62/38	-	7.7
	Run 10, 800 °C, 150 MPa, ~NNO + 2, 120 h
	800-150-GS1	GS	1	Gl + Pl + Amph + Opx? + Mt	~50	-	5.1
	800-150-GB1	GB	1	Gl + Pl + Amph/Opx? + Mt	~50	-	5.1
	X H2Oin = X H2O /(X H2O + X CO2 ) initial .

  ; SM1). It is likely that the bulk ROS is slightly too high in Al 2 O 3 and CaO than the reactive magma

		250 300 350 400	Opx (a) LAY XH2O=1	An71 Liq73(9.9) (30) Liq73(7.6) (22) An62 Am10 (22) An74 Am10 Mag81 Plg An65 Am10 Mag77 Liq72(10.1) (23)	An55 (35) Liq76(7.7) Mag74 (32) An53 Am10 En50 Liq76(8.1) Liq76(6.1) En47 An58 (45) Amph Pl Opx Am Ox Pl Am Ox Opx Mag72-74 Liq70(7.7) Am? (10) Layou (30+5) An50-54 Am6-7 En50-52 Liq77-80 Ilm85-86 +Qtz Liq70(9.9) (23) An71 Am10 Mag78 Liq73(7.4) Ilm88 Mag83 Am10 En43 Am9 An52 (8) (44) (b) LAY Mt XH2O=0.8	Liq76(5.8) En52 An54 Liq72(6.2) (40) Mag68 En64 An63 (31) M t ( + I l m ) + P l g + O p x Liq73(8.1) Mag77 En52 (25) An67 Am10
	Total pressure (MPa)	200 150 350 400	Am9 En61 Mag79 Liq72(6.7) (29) An? En58 Mag79 Liq76(5.9) (21) An75 Am10 Mag76 Liq74(9.9) (29) An70 En67 Liq73(5.8) Plg (27) (17) An87 Liq71(9.9) Mag81 Am10 Am11 Mag70 ? [GS] Am12 (~50) An54 Amph Mag81? Liq78(5.1) Opx Liq73(10.1) (25) An75 (c) ROS XH2O=1	Mt	En52-53 Mag73-75 Liq76-80 Ilm84-85 Roseau (30+5) An53-58 Am Ox Pl Opx Pl Ox Opx Opx	En55 Mag67 Liq74(5.4) (33) An57 En54 Ilm84 Liq76(7.8) (40) An54 En52 Ilm85 Liq79(4.8) (49) An62 Liq76(8.2) Ilm89 En44 (d) ROS XH2O=0.8	(28) An? En59 Liq75(4.4) Liq75(7.7) Ilm81 Mag73 En53 Wo24? An64 (38) ) + P lg + O p x M t ( + I lm C p x ?
		300		An63 Am9 En56 Mag78 Liq74(7.6)	(24)	?	(26) An79 Am8 En? Mag78 Liq74(7.4)	Liq69(7.7) (11) En68		(44) An66 En48 Mag76 Liq76(6.0)	(33)	(34) An? En52 Liq73(5.8)	Mag65 (30) En65 An67
		250			Am9 An75							En55 An63	Liq72(6.3)
					En61							Ilm85
					Mag79							Liq74(5.4)
				Liq73(6.7)
		200 150	Amph (~50) [GB] An? ? Am/En? ? Mag84?	(26) An? En57 Liq76(5.9)	(38) An75 En66 Mag78 Liq73(5.8)			(37) An? En51 Liq76(4.6) experimental accuracy		(43) An69 En60 Mag72 Liq75(4.7)
			Liq?(5.1)
			775	800	825	850	875		900	775	800	825	850	875	900
								Temperature (°C)

Acknowledgements We would like to thank Michel Fialin and Nicolas Rividi (service CAMPARIS) and Ida di Carlo (ISTO) for assistance during microprobe analyses and Omar Boudouma (SEM, Sorbonne Université) for help during SEM imaging. We thank Vincent Christmann who started the study during his master's thesis. We are grateful to the reviewers, Jenny Riker, Maxim Gavrilenko, Elena Melekhova, the Associate Editor, Maxim Portnyagin and the Executive Editor, Andrew Harris, for their comments that significantly improved the manuscript. This work was financially supported by the French MNRT doctoral grant (to C. Solaro), the European FP7_VUELCO project (J. Gottsmann, PI; grant 282759) and the French INSU Action Incitative (G. Boudon, PI).