N Sainlot

I Vlastelic

S Moune

E F Rose-Koga

F Schiavi

S Valade

F Aguilera

Uptake of gaseous thallium, tellurium and vanadium into anhydrous alum, Lascar volcano fumaroles, Chile

Formation of secondary sulphate minerals during the reaction between volcanic gases and rocks modulates the composition and flux of gaseous emanations. We report on the sub-surface formation of anhydrous alum (M I M III (X VI O4)2 with M I = NH4 + , Na + , K + ; M III = Al 3+ , Fe 3+ and X VI = S 6+ ) in the 330°C fumaroles of the Lascar volcano (Chile). The alum occurs as a few millimetres thick crust that grew internally by two-way diffusion of reaction gases and diffusive influx of rock cations within the crust. The average growth rate is estimated at ca. 0.3 m/day, based on the 19-year-long activity of the degassing fracture hosting the crust. The growth rate is controlled by the slow migration of the rock cations and decreases towards crust rim.

The crust selectively concentrates Tl, V and Te (thousands of g/g) and to a lesser extent Mo (hundreds of g/g). The uptake of gaseous Tl, V and Mo is not due to saturation of the gas with Tl, V and Mo compounds but to the possibility for the elements to enter the crystal structure of alum: Tl + , V 3+ and Mo 6+ might substitute for the M I , M III and X VI ions, respectively. The process of Te uptake remains uncertain, but must be related to the incorporation of Tl and V with which Te tightly correlates.

Thallium, V and Te concentrations increase by a factor > 20 from core to rim, where they reach 8760, 8508, and 1687 g/g respectively. This trend correlates with the decrease of crust growth rate. This indicates that the low rate of rock cations supply to the outer edge of the crust favours the substitution of Tl, V and Te in the crystal network. Such surface enrichment does not occur for Mo, because Mo competes with S, another element from the gas. This suggests that the surface of mature alum crust has a high adsorption capacity for those gaseous metals able to compensate for the lack of rockderived cations.

Based on the composition of gases escaping from the fracture hosting the crust, it is estimated that the partition coefficients of Tl (3.310 7 ), V (1.110 7 ) and Te (0.610 7 ) between crust surface and gases are two to four orders of magnitude higher than for other volatile metals and metalloids. It follows that gases equilibrating with anhydrous alums lose between 77 and 95% of their initial Tl content, but less than 1% of Pb. Given the Tl emission rate of Lascar volcano (5 g/day), between 17 and 104 g of toxic Tl would deposit every day if all Lascar gases were to equilibrate with anhydrous alums.

INTRODUCTION

Sub-aerial degassing of high-temperature magmas directly releases large amount of sulfur, metals and metalloids to the atmosphere (e.g., [START_REF] Allard | Acid gas and metal emission rates during long lived basalt degassing at Stromboli volcano[END_REF][START_REF] Moune | Trace elements in the particulate phase of the plume of Masaya Volcano, Nicaragua[END_REF][START_REF] Mather | Halogens and trace metal emissions from the on-going 2008 summit eruption of Kīlauea volcano, Hawai'i[END_REF][START_REF] Zelenski | Trace elements in the gas emissions from the Erta-Ale volcano, Afar, Ethiopia[END_REF][START_REF] Gauthier | Elevated gas flux and trace metal degassing from the 2014-2015 fissure eruption at the Bárðarbunga volcanic system, Iceland[END_REF]. Conversely, gases continuously emanating from the crustal magma reservoirs feeding stratovolcanoes have a more complex history, and only a fraction escapes from the ground. The other fraction is trapped within the volcanic edifices, due to the dissolution of magmatic gases into groundwater [START_REF] Doukas | Sulfur Dioxide Scrubbing During the 1992 Eruptions of Crater Peak, Mount Spurr Volcano, Alaska[END_REF] and/or hydrothermal system [START_REF] Symonds | Origin, speciation, and fluxes of traceelement gases at Augustine volcano, Alaska: Insight into magma degassing and fumarolic processes[END_REF], formation of secondary sulphate minerals during gas-rock reactions [START_REF] Zimbelman | Origin of secondary sulfate minerals on active andesitic stratovolcanoes[END_REF], condensation of sulpho-salt melts during gas expansion in fractures [START_REF] Henley | Nature's refineries-Metals and metalloids in arc volcanoes[END_REF] and precipitation of gas sublimates at fumaroles vents as gas cools and mixes with air [START_REF] Bunsen | Ueber die Processe der vulkanischen Gesteinsbildungen Islands[END_REF][START_REF] Stoiber | Fumarole incrustations at active Central American volcanoes[END_REF]. The extent of element uptake in the ground has implications for volcano monitoring [START_REF] Symonds | Magmatic gas scrubbing: implications for volcano monitoring[END_REF][START_REF] Guern | Characterization and modelling of the complete volcanic gas phase[END_REF], and also controls the global versus local environmental impact of magmatic gases, especially regarding toxic heavy metals.

Several studies have collected gas sublimates and condensates at the mouth of fumaroles to document the condensation sequence of elements (e.g., [START_REF] Bernard | Condensation of volatile elements in hightemperature gases of Mount St. Helens[END_REF][START_REF] Symonds | Volatilization, transport and sublimation of metallic and non-metallic elements in high temperature gases at Merapi Volcano, Indonesia[END_REF][START_REF] Taran | Chemistry and mineralogy of high-temperature gas discharges from Colima volcano, Mexico. Implications for magmatic gas-atmosphere interaction[END_REF]. In agreement with thermochemical data, they showed that semi-volatile elements (Cd, Pb, Bi, Cu, Zn) condense over a wide temperature range, from 800 to less than 400°C, depending on gas composition and oxidation state (e.g., [START_REF] Symonds | Volatilization, transport and sublimation of metallic and non-metallic elements in high temperature gases at Merapi Volcano, Indonesia[END_REF][START_REF] Guern | Characterization and modelling of the complete volcanic gas phase[END_REF][START_REF] Churakov | Evolution of composition of high-temperature fumarolic gases from Kudryavy Volcano, Iturup, Kuril Islands: the thermodynamic modeling[END_REF]. Fewer studies noted that fumarolic incrustations are also enriched in some volatile metals (e.g. Cu, Tl) that are not saturated in the gas phase [START_REF] Naughton | Incrustations and fumarolic condensates at kilauea volcano, Hawaii: field, drill-hole and laboratory observations[END_REF][START_REF] Kodosky | Fumarole distribution, morphology, and encrustation mineralogy associated with the 1986 eruptive deposits of Mount St. Augustine, Alaska[END_REF][START_REF] Kodosky | Fumarole distribution, morphology, and encrustation mineralogy associated with the 1986 eruptive deposits of Mount St. Augustine, Alaska[END_REF][START_REF] Okrugin | Smoking gun for thallium geochemistry in volcanic arcs: Nataliyamalikite, TlI, a new thallium mineral from an active fumarole at Avacha Volcano, Kamchatka Peninsula, Russia[END_REF]. They suggested that gaseous elements partition into, or adsorb on the alteration minerals coating degassing conduits. There is growing evidence that underground gas-rock reactions significantly influence the chemistry of volcanic gases ultimately released in the atmosphere [START_REF] Henley | Nature's refineries-Metals and metalloids in arc volcanoes[END_REF][START_REF] Henley | Gas-solid reactions in arc volcanoes: Ancient and modern[END_REF][START_REF] King | Gas-solid reactions: Theory, experiments and case studies relevant to earth and planetary processes[END_REF][START_REF] Palm | Unravelling the Consequences of SO2-Basalt Reactions for Geochemical Fractionation and Mineral Formation[END_REF]. For instance, [START_REF] Henley | Sulfosalt melts and heavy metal (As-Sb-Bi-Sn-Pb-Tl) fractionation during volcanic gas expansion: the El Indio (Chile) paleo-fumarole[END_REF] emphasized the fractionation of chalcophile elements (As-Sb-Bi-Sn-Pb-Tl) during sub-surface precipitation of sulphosalts and suggested that over 90% of As might be trapped underground.

This study focuses on the partitioning of gaseous elements into secondary sulphates, which commonly form when acid gases interact with basaltic or more evolved rocks rich in aluminium and alkalis [START_REF] Getahun | Mount St. Augustine volcano fumarole wall rock alteration: mineralogy, zoning, composition and numerical models of its formation process[END_REF][START_REF] Africano | Acidic alteration in the fumarolic environment of Usu volcano, Hokkaido, Japan[END_REF][START_REF] Zimbelman | Origin of secondary sulfate minerals on active andesitic stratovolcanoes[END_REF]. The range of secondary sulphate minerals is wide due to the numerous possibilities of combining rock-derived cations (K + , Na + , Ca 2+ , Mg 2+ , Al 3+ , Fe 3+ ) with gasderived ions (NH4 + , SO4 2-). In addition, trace ions with valence ranging from +1 to +6 might substitute for major cations, further increasing mineral variety [START_REF] Lipson | The crystal structure of the alums[END_REF][START_REF] Giester | Crystal structure of anhydrous alum RbFe 3+ (SeO4)2[END_REF][START_REF] Zunic | Dorallcharite, Tl0.8K0.2Fe3(SO4)2(OH)6, a new member of the jarosite-alunite family[END_REF][START_REF] Dutrizac | Jarosites and their application in hydrometallurgy[END_REF]. For instance, more than one hundred of sulphate minerals occur in the fumarolic incrustations of Tolbachik volcano, Kamchatka (Mindat.org database, 2019).

The geochemical study of gas-rock interaction meets with the difficulty of accessing the active areas of stratovolcanoes and collecting underground samples. To circumvent this difficulty, [START_REF] Henley | Sulfosalt melts and heavy metal (As-Sb-Bi-Sn-Pb-Tl) fractionation during volcanic gas expansion: the El Indio (Chile) paleo-fumarole[END_REF] studied paleo-fumarole conduits, now exposed at the surface by tectonics. Here, we use a sub-surface fumarole crust showing growth layers to reconstruct the temporal evolution of the gas-rock reaction within a degassing fracture of the Lascar volcano (Chile). This evolution is studied by measuring in situ major-trace element concentrations and oxygen isotopes along core-rim profiles. The bulk Pb and Sr isotopic composition of the crust is also measured to constrain the origin of the crust. The composition of the outermost layer of the crust is compared to the composition of gases emanating from the same fracture [START_REF] Menard | Gas and aerosol emissions from Lascar volcano (northern Chile): insights into the origin of gases and their links with the volcanic activity[END_REF], in order to assess the partition coefficients of elements between gas and crust. A major result of this study is the recognition of selective incorporation of gaseous Tl, V and possibly Te in the crystal structure of the sulphate. This effect potentially depletes volcanic emanations in these elements and contributes to concentrate them within the edifice.

BRIEF OVERVIEW OF LASCAR VOLCANO

Lascar (23°22'S, 67°44'W, 5590m) is one of the most active stratovolcanoes of the Central Volcanic Zone (CVZ) of the Andes (Fig. 1a) [START_REF] Stern | Active Andean volcanism: its geologic and tectonic setting[END_REF]. The volcano rests on a pre-Cenozoic basement, made of Devonian to early Carboniferous sandstones (Lila formation), Permian granites and volcanic rocks (Cas formation), Permo-Trias volcanic products (Peine strata and Cerro Negro Strata) [START_REF] Gardeweg | Evolution of Lascar Volcano[END_REF][START_REF] Gardeweg | Geología del volcán Lascar, Región de Antofagasta[END_REF]. The volcano is composed of two cones, the westernmost crater of the eastern edifice being the current focus of activity (Fig. 1b). The volcano has produced about 30 to 40 km 3 of magma during its 220 ka history. Since the nineteenth century Lascar produced ca. 30 eruptions andesitic-to-dacitic in composition [START_REF] Gardeweg | Evolution of Lascar Volcano[END_REF][START_REF] Gardeweg | Geología del volcán Lascar, Región de Antofagasta[END_REF].

Petrochemical modelling indicates the occurrence of large magmatic reservoir at 6 km depth, which is mobilized during dacitic eruptions, and several shallower reservoirs sampled by smaller but more frequent mafic eruptions [START_REF] Matthews | The Piedras Grandes-Soncor eruptions, Lascar Volcano, Chile; evolution of a zoned magma chamber in the Central Andean upper crust[END_REF]. Between the 1984 reactivation and the last major eruption in 1993, Lascar showed a cyclic activity including sequences of dome growth, degassing pulse, dome subsidence, and explosive events [START_REF] Matthews | The 1984 to 1996 cyclic activity of Lascar volcano, northern Chile: cycles of dome growth, dome subsidence, degassing and explosive eruptions[END_REF]. The 1993 eruption (VEI 4) produced a 25 kmhigh sub-plinian ash column whose collapse generated pyroclastic flows up to 8.5 km NW from the summit. The eruption deposited 10 9 tons of tephra, coated by gypsum likely recycled from tertiary sediments [START_REF] Risacher | Geochemistry of ash leachates from the 1993 Lascar eruption, northern Chile. Implication for recycling of ancient evaporates[END_REF]. Since 1993, Lascar alternated passive degassing and vulcanian eruptions, and produced its last eruption in October 2015.

A major feature of Lascar volcano is the high fumarolic activity that feeds a persistent gas plume. Sampling of the active crater fumaroles between 2002 and 2006 revealed temperatures in the range of 30-385°C and the contribution of magmatic gases, hydrothermal fluids, and meteoric water to volcanic emanations [START_REF] Tassi | The magmatic-and hydrothermal-dominated fumarolic system at the Active Crater of Lascar Volcano, northern Chile[END_REF]. The hottest fumaroles (250-385°C) occurred within a narrow area to the northeast of the active crater. [START_REF] Menard | Gas and aerosol emissions from Lascar volcano (northern Chile): insights into the origin of gases and their links with the volcanic activity[END_REF] sampled the gas plume between 2009 and 2012, and estimated that the volcano releases 150-940 t/d SO2, 170-210 t/d HCl, and up to 100 t/d HF. They also identified little volatile (i.e., Co, V, Sc, REE, Y, HFSE, Th, U, alkali-earth and alkaline metals), moderately volatile (Cu, Cr, W, In, Mo, Sn, Pb), and highly volatile (Cd, Zn, B, Tl, Sb, Bi, Se, As, Te) groups of trace elements.

FUMAROLE CRUST SAMPLE

A fumarole crust was sampled in November 2012 in the arcuate fracture located along an older crater wall east of the active crater, at an elevation of 5375 m (Fig. 1b). This fracture opened following the 1993 eruption [START_REF] Matthews | The 1984 to 1996 cyclic activity of Lascar volcano, northern Chile: cycles of dome growth, dome subsidence, degassing and explosive eruptions[END_REF]. Gases emanating from the fracture were also sampled in 2012 through filter packs ca. 2m from the fracture (Fig. 2a) and analysed by [START_REF] Menard | Gas and aerosol emissions from Lascar volcano (northern Chile): insights into the origin of gases and their links with the volcanic activity[END_REF]. The gases have a low SO2/HCl molar ratio (0.21) compared to plume aerosols (average of 1.7) and fumaroles from the active crater (average of 3.0) [START_REF] Menard | Gas and aerosol emissions from Lascar volcano (northern Chile): insights into the origin of gases and their links with the volcanic activity[END_REF][START_REF] Tassi | The magmatic-and hydrothermal-dominated fumarolic system at the Active Crater of Lascar Volcano, northern Chile[END_REF]. Such low SO2/HCl ratio is indicative of extensive gas-water-rock interactions. The temperature of the bottom of the fracture where the crust formed, measured by thermal camera, was 330°C (Fig. 2b). This temperature is unexpectedly elevated given the distance from the active crater (ca. 500 m) and the hydrothermal signature of the gases.

The studied crust was attached to the lower surface of a lava block that obstructed the fracture (Fig. 2cd). It was collected ca. 30 cm underground. The crust was initially fully green, but some parts became blue a few minutes after sampling as the crust equilibrated with air. Crust colours faded during the first year that followed sampling.

The crust coats the rock over ca. 100 cm 2 and is a few mm thick (Fig. 2d). It includes several outgrowths, more or less cracked and hollow (Fig. 2e). Inspection of different cross sections under a binocular microscope revealed that they formed via concentric addition of growth layers. A globular outgrowth, ca. 8 mm in diameter and filled with crust material was selected for this study (Fig. 2f).

METHODS

All analyses were done at Laboratoire Magmas et Volcans (LMV), Clermont-Ferrand, except Secondary Ion Mass Spectrometry (SIMS) measurements which were carried out at CRPG, Nancy.

Electron Microscopy

The selected crust chip was embedded in resin, polished on SiC discs using ethanol to preserve water-soluble phases. An area of 2.01.5 mm showing well-formed growth layers was selected for chemical analysis (Fig. 2f). All analyses, with the exception of oxygen isotopic composition, were performed on this reference area (referred to as side A). This area was first surveyed by Scanning Electron Microscopy (SEM, JSM 5910LV) operating in Backscattered Electron (BSE) imaging mode. Detected elements (S, O, Al, Fe, Mg, Na, Ca and K) were mapped by Energy Dispersive Spectroscopy (EDS). Quantitative analyses of major elements, with the exception of O, were subsequently performed by Electron Probe Microanalysis (EPMA, Cameca SX 100) along two core-rim profiles, referred to as A1 and A2 (location shown in Table S1), with a 15-kV accelerating voltage, 10-nA beam current, and 30µm spot size. Relative in-run errors (2) are typically 4% for Al and S, 12% for Fe, Mg and Na, 20% for K and 30% for Ca.

Laser ablation inductively coupled plasma mass spectrometry

Concentrations of minor and trace elements (Li, Sc, V, Cr, Co, Ni, Cu, Zn, Ga, Ge, As, Rb, Sr, Y, Zr, Nb, Mo, Cd, In, Sn, Sb, Te, Cs, Ba, Hf, Ta, Tl, Pb, Bi, Th, U and Rare Earth Elements) were measured on the EPMA spots by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) using an excimer 193nm Resonetics M-50E laser system and an Agilent 7500cs quadrupole ICPMS. Measurements were done with a dwell time of 25 ms, spot size of 27 µm, laser repetition rate of 3 Hz, He and N2 flow rates of 575 and 2 ml/min, respectively. Backgrounds were measured for 30s before each ablation. Oxide production as measured by ThO/Th on the NIST 610 standard glass was 0.9%.

Aluminium content was used for internal normalization, with reference to EPMA data.

The NIST 610 was measured several times at the beginning of the session and then every 25 spots. As no significant drift occurred, the average of the NIST 610 measurements were used for signal calibration, using the GeoRem database values [START_REF] Jochum | GeoReM: A new geochemical database for reference materials and isotopic standards[END_REF]. Relative error on the NIST 610 is less than 1% for each measured trace elements. The GLITTER software was used for the trace-element data reduction.

Detection limit was <1ppm for all elements except Se, which was measured by wet chemistry (cf. 4.5).

Raman spectroscopy

Raman spectroscopy was used to characterize crust mineralogy. Raman spectra were collected using an InVia confocal Raman micro-spectrometer manufactured by Renishaw and equipped with a 532 nm diode laser, a CCD detector, an edge filter, a motorized XYZ stage and a Leica DM 2500M optical microscope. Laser power on the sample was reduced to ~ 1 mW and slit aperture was set to 65 μm. A 50× microscope objective and a 2400 l/mm grating were used. Spectral resolution was better than 1 cm -1 . Daily calibration of the spectrometer was performed based on a Si 520.5 cm -1 peak. The spectra were recorded in the 60-1320 and 2920-3740 cm -1 wavenumber ranges using Wire 4.2 software.

Secondary Ion Mass Spectrometry

Following Raman analysis, the surface of the sample became crackled. A 1 mm-thick slice was sawed to perform oxygen isotope analysis by SIMS on the opposite side of the slice (referred to as side B). The use of a thin slice also limits degassing of the epoxy resin and of the hydrated sulphate sample within the mass spectrometer. Oxygen isotopes (δ 18 O) measurements were carried out with a SIMS 1270. The slice was coated with three 4nm-thick layers of gold to assure a good electrical conduction of the porous sample. The same protocol has been applied to standard samples. Oxygen isotope compositions were measured along 3 core-to-rim profiles in the same area as majortrace element analysis. Profile B1 and B3 were perpendicular, and profile B2 was in between profile 1 and 3 (locations reported in Table S2). The spot size was 15 µm and the distance between spots was 40µm in the first and the second profile, and 56.6 µm for the third profile.

The SIMS was set in multi-collection mode with 16 O, 17 O and 18 O isotopes counted in L2, H1 and axial faraday cups, respectively. The field aperture and MRP were set at 2000 and 6000, respectively. We applied a 30µm raster during the 120 s pre-sputtering, and a 25µm raster during data acquisition. The total analytical time for one measurement was 6 min. The calcite CCcigA reference material with δ 18 O =18.94‰ ±0.14(2) [START_REF] Pfister | Freshwater pearl mussels as a stream water stable isotope recorder[END_REF] was first repeatedly analysed in order to assure the SIMS measurements stability. Instrumental mass bias and drift were corrected by measuring at the beginning and at the end of each profile the composition of a sulphate standard with homogeneous and known isotopic composition (CRPG internal standard BaSO4 from Maïza, with δ 18 O of 14.45‰). The in-run error was on average 0.25‰ (2).

N, H, S and Se bulk analysis

Bulk concentrations of N, H, S and Se were measured on finely crushed surface scrapings. Nitrogen, H and S were analysed using a Flash 2000 CHNS-O elemental analyser from Thermo Fisher Scientific. Circa 1 mg of powder and a similar amount of V2O5 were precisely weighted in a tin capsule using a microbalance, and introduced into the 950°C high-temperature reactor. The combustion gases (N2, H20 and SO2) were separated in the coupled chromatography column and quantified by thermal conductivity. Cystine (C6H12NO4S2) was used for calibration. Selenium was measured by ICPMS after sample dissolution in 6M HCl, evaporation and dilution in 0.5M HNO3.

Pb and Sr isotope bulk analysis

Four crust chips were powdered in an agate mortar. Circa 50 mg of sample were dissolved and Pb and Sr were extracted using anion exchange resin (AG1-X8) and Sr chelating resin (Sr Spec), respectively, following the methods described in Vlastelic et al. (2013). The isotopic compositions were measured on a Neptune Plus MC-ICPMS (Pb) and on a Triton TIMS (Sr). Instrumental mass bias correction is detailed in Table S3.

RESULTS

Crust mineralogy

Electron microscopy survey of the crust section shows a finely crystallized sulphate of Al, Fe, Na, Mg, K and Ca. Electron microprobe analyses performed along the two core-rim profiles A1 and A2 give between 19.5 and 23 wt% S, between 5.4 and 9.3 wt% Al, and between 0.5 and 4.2 wt% Fe (Table S1). Sodium ranges from 0.6 to 3.9 wt%, Mg from 0.7 to 3 wt% and K from 0.2 to 1.6 wt%. Calcium is generally less than 1 wt%, but frequent spikes (up to 8.5 wt%) occur due to the presence of anhydrite nuggets. Aluminium negatively correlates with Fe and Mg, whereas K negatively correlates with Na. Atomic abundances follow the relation: S/(Al+Fe)=0.16(Ca+Mg)+1.9, which is consistent with the occurrence of a double sulphate of Al and Fe.

Raman spectra (Fig. 3) show the characteristic vibration bands of (1) anhydrous alums, with major occurrence of (NH4)Al(SO4)2 (godovikovite bands at 318, 483, 610 and 653, 1076, 1287, 3148, 3208, 3331 cm -1 ), and minor occurrence of (NH4)Fe(SO4)2 (sabieite main band at 1033 cm -1 ) and (NH4)2Mg2(SO4)3 (efremovite main band at 1049 cm -1 ) [START_REF] Kosek | Raman spectroscopic study of six synthetic anhydrous sulfates relevant to the mineralogy of fumaroles[END_REF]; (2) hydrated sulphates including MgSO4•6H2O (exahydrite bands at 460±5, 610, 983-984, 1080 and 3435 cm -1 ), mirabilite (Na2SO4•10H2O) and/or hydrated alums (band at 990/991 cm -1 ); and (3) CaSO4 (anhydrite bands at 416, 499, 609, 627, 675, 1017 and 1129 cm -1 ) [START_REF] Frezzotti | Raman spectroscopy for fluid inclusion analysis[END_REF]. The average K/Al atomic ratio of 0.10±0.05 (1) is consistent with minor occurrence of K in godovikovite [START_REF] Shimobayashi | Ammonium sulfate minerals from Mikasa, Hokkaido, Japan: boussingaultite, godovikovite, efremovite and tschermigite[END_REF]. Bulk volatile element analysis in the surface scrapings yields 1.37-1.51 wt% N, 3.19-3.32 wt% H and 18.10-19.15 wt% S, which indicates relatively low proportion of godovikovite (<25 wt%) and high proportion of hydrated phases in the outer layer of the crust. Inspection of the unpolished surface of the crust by electron microscopy (Supplementary material S4) reveals the occurrence of 5-10 m Te-rich crystals embedded within fine-grained Na2SO4. Once corrected for background Na, S and O (assuming all Na signal is from background Na2SO4) the composition of these crystals turns out to be TeO2 with minor amount of S. Other surface minerals are anhydrite, quartz, Ba sulphate, and a Tl-rich Al sulphate whose composition cannot be precisely determined due to its very small size (ca. 1 m).

Crust layering and compositional zonation

The SEM image of the crust section shows at least 11 concentric growth layers (U1-U11) whose thickness (500-30 m) generally decreases towards crust rim (Fig. 4a). The growth layers do not have the same thickness in all directions indicating an anisotropic growth. Element mapping (Fig. 4b-d) shows that Al tends to increase towards the layer boundaries, the boundaries themselves being enriched in Fe. Anhydrite occurs as particles, whose number increases towards crust rim as their size decreases. Generally major elements do not show core-rim trends beyond a slight decrease of S and K, and increase of Na in the external shells (U1 to U3). The sum of the measured major elements, plus O, N and H calculated taking into account anhydrous minerals only (godovikovite, sabieite, efremovite and anhydrite) decreases from 90 to 80 wt.% from core to rim (Table S1). This indicates an increase of water content and/or porosity towards crust rim. The first possibility is supported by the 40% increase of the EDS oxygen signal from core to rim, and the elevated H content (>3 wt%) in the surface scrapings. It is not known whether the outer shell of the crust was initially hydrated, or hydrated between sampling (2012) and analysis (2014).

In situ trace element concentrations are reported in Table S1. Many trace elements commonly transported by magmatic gases (Pb, Bi, Zn, Cu, Cd, Sn, Rb) occur in the crust in concentrations that are similar to or lower than those of Lascar lavas, and do not show core-rim trends (Fig. 5). Tellurium, thallium and molybdenum stand out, with concentrations between 4.10 2 and 10 7 times those of lavas (Fig. 5). Selenium, arsenic and vanadium show second order enrichments (between 22 and 37) principally at crust rim. Vanadium together with Tl and Te show a marked zonation across the crust, with concentrations increasing from 297, 210 and 71 ppm in crust core to 8508, 8760 and 1687 ppm at crust rim, respectively (Fig. 6a). This increase is the most pronounced near the rim of the crust, between U6 and U1, with a local high in layers U3-U4 (1897 ppm V, 2622 ppm Tl and 944 ppm Te). Molybdenum does not show such zonation, but slightly higher concentrations in the middle of the crust (600-924 ppm) compared to core and rim (205-710 ppm). Despite their large ranges of variation, Tl, V and Te are little fractionated from each other, with Tl/V=1.4±1.1(2), Tl/Te=3.6±2.7(2) and V/Te=2.9±2.4 (2) (Fig. 6b).

Oxygen isotope profiles

In situ δ 18 O values measured along the three profiles of side B are reported in Table S2.

δ 18 O compositions calibrated with the BaSO4 standard from Maïza show similar ranges of variation along B1 (-1.2 to +8.2‰), B2 (-1.6 to +5.4‰) and B3 (-2.2 to +6.4‰) profiles. These variations are clearly outside the errors of individual measurements, which do not exceed 0.4‰. δ 18 O does not show systematic core-rim trend but highfrequency fluctuations (Fig. 7). Depending on the profile, δ 18 O display between 12 and 20 oscillations that are not obviously linked to the growth layers. Profile B2 tends to show a smooth evolution of δ 18 O between +2.5 and 5.4‰, on which superimpose low δ 18 O spikes. The average δ 18 O of the three profiles (+3.5 ±1.8 (), n=165) is identical to the composition of Lascar gas condensates (+3.5±1.2() n=2) [START_REF] Tassi | The magmatic-and hydrothermal-dominated fumarolic system at the Active Crater of Lascar Volcano, northern Chile[END_REF]. Thus, despite the small-scale heterogeneity of 18 O/ 16 O within the crust, no net isotopic fractionation occurs between vapour and the bulk sulphate crust.

Bulk Pb and Sr isotope composition

The bulk Pb and Sr isotope compositions of the four crust chips are reported in Table S3.

The variations of 206 Pb/ 204 Pb (18.803-18.821), 207 Pb/ 204 Pb (15.648-15.661), 208 Pb/ 204 Pb (38.783-38.830), and 87 Sr/ 86 Sr (0.706329-0.706482) are small and identical, within error, to the composition of lavas from the 1993 eruption ( 206 Pb/ 204 Pb=18.817-18.819; [START_REF] Rosner | Slab-derived boron isotope signatures in arc volcanic rocks from the Central Andes and evidence for boron isotope fractionation during progressive slab dehydration[END_REF].

DISCUSSION

This study investigates a time series of metal variations in a zoned sulphate crust that grew on a lava block continuously exposed to fumarole gas flux from Lascar volcano.

The following discussion will focus on crust formation processes and time-scale, the extent of element extraction from the lava block relative to the gas flux, and the extensive uptake of V, Tl and Te from the gas phase.

Crust formation processes

Anhydrous alums frequently occur at the degassing vents of burning coal-dumps (BCDs) (see review by Kosek el al., 2018). There, godovikovite commonly forms stalactite or thick crusts in subsurface cavities. Anhydrous alums are less common in volcanic fumaroles: godovikovite has been described in the Phlegrean fields and La Fossa crater on the island of Vulcano (Mindat.org database, 2019), while its hydrated form (Tschermigite) has been found in the thermal fields of Southern Kamchatka Peninsula [START_REF] Zhitova | Tschermigite from thermal fields of Southern Kamchatka: hightemperature transformation and peculiarities of IR-spectrum[END_REF], East African rift [START_REF] Lowenstern | A geochemical reconnaissance of the Alid volcanic center and geothermal system, Danakil depression, Eritrea[END_REF] and possibly Merapi volcano, Indonesia [START_REF] Kavalieris | High Au, Ag, Mo, Pb, V and W content of fumarolic deposits at Merapi volcano, central Java, Indonesia[END_REF]. Steklite, KAl(SO4)2, the K-analogue of godovikovite, has been identified in the fumarole deposits of Tolbachik volcano, Kamchatka [START_REF] Murashko | Steklite, KAl(SO4)2: A finding at the Tolbachik Volcano, Kamchatka, Russia, validating its status as a mineral species and crystal structure[END_REF]. Godovikovite is stable between 210 and 390°C [START_REF] Zhitova | Tschermigite from thermal fields of Southern Kamchatka: hightemperature transformation and peculiarities of IR-spectrum[END_REF], in agreement with the temperature (330°C) of the Lascar degassing fracture where the studied crust was sampled. Godovikovite hydrates into an amorphous phase below 200°C, and into tschermigite below 60°C. Conversely, it loses ammonium above 380°C to form millosevichite [START_REF] Zhitova | Tschermigite from thermal fields of Southern Kamchatka: hightemperature transformation and peculiarities of IR-spectrum[END_REF]. The occurrence of godovikovite throughout the crust indicates that the temperature has remained relatively stable during crust formation history. The suspected hydration of the outer shell of the sulphate crust is likely a post-formation process, as observed in a BCDs crust [START_REF] Parafiniuk | Ammonium minerals from burning coal-dumps of the Upper Silesian Coal Basin (Poland)[END_REF]. [START_REF] Symonds | Origin, speciation, and fluxes of traceelement gases at Augustine volcano, Alaska: Insight into magma degassing and fumarolic processes[END_REF] estimated that even under highly oxidizing conditions (fO2 of 10 - with less than 0.01% moles S, i.e. more than 10 times the S content of Lascar gases [START_REF] Tassi | The magmatic-and hydrothermal-dominated fumarolic system at the Active Crater of Lascar Volcano, northern Chile[END_REF]. Thus, the Lascar crust must have formed by the interaction between gases and silicate rocks. This happens either through the reaction between SO3(g) and silicate without change in S oxidation state, or through the disproportionation of SO2(g) to SO4 2-that bonds with rock derived cations, and reduced S released as S2(g) or H2S(g) [START_REF] Henley | Porphyry copper deposit formation by sub-volcanic sulphur dioxide flux and chemisorption[END_REF][START_REF] Renggli | SO2 Gas Reactions with Silicate Glasses[END_REF]. Because this reaction requires diffusion of elements towards the gas-rock interface, sulphates of Ca, Na and to a lesser extent Mg preferentially form, while sulphates of less mobile networkforming cations (Al, Fe) are rarely observed in experiments [START_REF] Renggli | SO2 Gas Reactions with Silicate Glasses[END_REF][START_REF] King | Gas-solid reactions: Theory, experiments and case studies relevant to earth and planetary processes[END_REF]. For instance, anhydrite might form through chemisorption of SO2(g) on anorthite [START_REF] Henley | Porphyry copper deposit formation by sub-volcanic sulphur dioxide flux and chemisorption[END_REF]: 3CaAl2Si2O8 + 4SO2(g) + H2O(g) → 3CaSO4 + 3Al2SiO5 + 4SiO2 + H2S(g)

(1) [START_REF] Johnson | SO2-rock interaction on Io: Reaction under highly oxidizing conditions[END_REF] nevertheless reported the formation of a mixture K-Na anhydrous alum and thenardite following one week reaction at 850°C between crystalline albite (NaAlSi3O8) and a gas mixture with SO2/O2=1 obtained by thermal decomposition of K2S2O8. This experiment demonstrates that, in oxidizing conditions, gas-derived (K and S) and silicate-derived (Na and Al) elements combine to form anhydrous alum. The layered structure of anhydrous alums, which results from stacking thin layers of Al(SO4)2 -and layers of large cations (K + , NH4 + ) [START_REF] West | Structural disorder, octahedral coordination and two-dimensional ferromagnetism in anhydrous alums[END_REF][START_REF] Murashko | Steklite, KAl(SO4)2: A finding at the Tolbachik Volcano, Kamchatka, Russia, validating its status as a mineral species and crystal structure[END_REF][START_REF] Zhitova | Tschermigite from thermal fields of Southern Kamchatka: hightemperature transformation and peculiarities of IR-spectrum[END_REF], suggests a two step formation process. One possibility is that the anion layer first forms by chemisorption of SO2(g) on residual silicate but is weakly bonded to H + , allowing the sorption of larger species from the gas subsequently:

Al2SiO5 + 6SO2(g) + H2O(g) → 2[H + Al(SO4)2 -] + S2(g) + SiO2 (2) [H + Al(SO4)2 -] + NH3(g) → NH4Al(SO4)2 (3)
and generally, for any monovalent cation M I transported as chloride in the gas,

[H + Al(SO4)2 -] + M I Cl(g) → M I Al(SO4)2 + HCl(g) (4)
The formation of godovikovite requires a sustained supply of gaseous NH3. Nitrogen may originate from the devolatilization of sediments subducted into magmas source [START_REF] Snyder | Sources of nitrogen and methane in Central American geothermal settings: Noble gas and 129 I evidence for crustal and magmatic volatile components[END_REF] and/or unusually elevated nitrate concentration in Atacama terrains due to the hyperarid climate [START_REF] Ericksen | The Chilean nitrate deposits[END_REF].

After the initial stage of gas-rock interaction, growth of a sulphate crust requires transport of elements through the sulphate. Different growth textures will form depending on whether rock cations migrate towards the sulphate-gas interface, or gasderived elements migrate towards the sulphate-silicate interface, or rock and gas elements both migrate within the sulphate. [START_REF] Renggli | SO2 Gas Reactions with Silicate Glasses[END_REF] studied the formation of anhydrite coating during short (24-72H) high-temperature (700-800°C) reaction between SO2(g) and Fe-free glass. They observed that the thickest coatings (ca. 200 m)

tend to detach from the glass surface and to wrinkle and buckle, which they ascribed to stress accumulation within the coating due to internal sulphate growth. They suggested that the empty space between the buckled coating and silicate substrate might fill with sulphate material in case of longer durations. The outgrowths of the Lascar crust very likely form via a similar process (Fig. 8ab), although the aluminous composition and thickness of the sulphate crust indicate that the extent of reaction is much higher. In particular, the prolonged gas-rock reaction results in the formation of growth layers.

The layers are ascribed to a decreasing growth rate towards crust surface due to the slow diffusion of rock-cations within the altered rock and the sulphate. Element migration occurs through evolving vacancies, defects and grain boundaries, and is much less efficient for Al than for Ca, Na, Mg and other divalent cations [START_REF] Henley | Porphyry copper deposit formation by sub-volcanic sulphur dioxide flux and chemisorption[END_REF][START_REF] Palm | Unravelling the Consequences of SO2-Basalt Reactions for Geochemical Fractionation and Mineral Formation[END_REF]. As the crust grows, the progressive exhaustion of cations in the reacted rock also requires cations to diffuse over longer distances. The thin layers at crust outer edge are consistent with a slow growth due to a low rate of rock cations supply. As shown on Figure 8c, the structure of the crust is consistent with a model where all layers grow simultaneously, but at decreasing rate towards the rim.

In the internal growth model, the small-scale 18 O/ 16 O variations across the crust have no temporal significance. They result from the stacking of the temporal O isotope signal of the gas. The substantial isotope variations nevertheless require the input of sources with different compositions, such as meteoric water with δ 18 O of -19‰ at Lascar summit [START_REF] Tassi | The magmatic-and hydrothermal-dominated fumarolic system at the Active Crater of Lascar Volcano, northern Chile[END_REF] and magma-derived water with δ 18 O between +6 and +12‰ [START_REF] Giggenbach | Isotopic shifts in waters from geothermal and volcanic systems along convergent plate boundaries and their origin[END_REF][START_REF] Taran | Deuterium and oxygen-18 in fumarolic steam and amphiboles from some Kamchatka volcanoes: "andesitic waters[END_REF]. Part of the 18 O/ 16 O variations can also result from equilibrium isotopic fractionation between the different sulphate minerals that are finely distributed within the crust.

Two observations suggest that the crust started to grow ca. 19 years before being sampled in 2012: (1) The fracture hosting the crust opened following the 1993 eruption [START_REF] Matthews | The 1984 to 1996 cyclic activity of Lascar volcano, northern Chile: cycles of dome growth, dome subsidence, degassing and explosive eruptions[END_REF].

(2) Lead and Sr isotope compositions indicate that the crust is genetically linked to the 1993 eruption, either through degassing of un-erupted magmas, or acid alteration of the rock on which the crust grew. The 2-3 mm thickness of the crust then requires an average growth rate of 0.3 -0.4 m/day. By comparison, the growth rates of sulphates formed in 850°C experiments are between less than 1m/day for natural basalts and 2-7m/day for glass [START_REF] Johnson | SO2-rock interaction on Io: Reaction under highly oxidizing conditions[END_REF].

Selective uptake of Tl, Te and V

The concentrations of Tl and Te are very elevated in the sulphate crust, but they are not unusual for fumarole deposits that commonly host Tl and Te gas sublimates. For instance, Tl and Te concentrations in excess of 2000 and 70 ppm, respectively, have been reported for Merapi (Indonesia), El Misti (Peru) and Vulcano (Italy) fumarole deposits [START_REF] Kavalieris | High Au, Ag, Mo, Pb, V and W content of fumarolic deposits at Merapi volcano, central Java, Indonesia[END_REF][START_REF] Birnie | The geochemistry of El Misti volcano, Peru fumaroles[END_REF][START_REF] Fulignati | Presence of native gold and tellurium in the active high-sulfidation hydrothermal system of the La Fossa volcano (Vulcano Italy)[END_REF]. However, Tl and Te generally condense together with other metals of comparable volatility, such as Pb, Bi, Cd, Cu or Zn, which is not the case for the Lascar crust. Consequently ratios such as Tl/Pb (125-4558) and Te/Pb (28-2023) are orders of magnitude higher in the Lascar crust than in worldwide fumarolic gases, condensates and solid deposits, where they rarely exceed 3 and 10, respectively (Fig. 9). Only condensates from the fumaroles of the Avacha volcano (Kamchatka Peninsula, Russia), which are abnormally enriched in Tl [START_REF] Okrugin | Smoking gun for thallium geochemistry in volcanic arcs: Nataliyamalikite, TlI, a new thallium mineral from an active fumarole at Avacha Volcano, Kamchatka Peninsula, Russia[END_REF], have Tl/Pb (up to 73) and Te/Pb (up to 36) ratios close to the Lascar crust values. On the other hand, Tl and Te concentrate in the crust together with V, which generally behaves as a non-volatile element in volcanic gases [START_REF] Symonds | Volatilization, transport and sublimation of metallic and non-metallic elements in high temperature gases at Merapi Volcano, Indonesia[END_REF][START_REF] Mather | Halogens and trace metal emissions from the on-going 2008 summit eruption of Kīlauea volcano, Hawai'i[END_REF], except in highly oxidizing conditions [START_REF] Hughes | Vanadium sublimates from the fumaroles of Izalco volcano, El Salvador[END_REF][START_REF] Taran | Chemistry and mineralogy of high-temperature gas discharges from Colima volcano, Mexico. Implications for magmatic gas-atmosphere interaction[END_REF].

Generally, the trace element chemistry of the crust appears unrelated to the chemistry of Lascar gases, which are enriched in As, Se, Bi, B, Zn, Pb, Sn, Mo, In, in addition to Te and Tl, and depleted in V and other refractory lithophile elements [START_REF] Menard | Gas and aerosol emissions from Lascar volcano (northern Chile): insights into the origin of gases and their links with the volcanic activity[END_REF]. This strongly suggests that the Tl, Te and V enrichments in the crust are not inherited from the gas phase -in the sense that similar Tl-Te-V enrichments relative to other metals do not occur in gases-, but result from crust formation process. Both the gas phase and the host rock can supply Tl, Te and V to the crust. Although a dominant contribution from the gas phase is expected given the extreme enrichment of these elements in the crust, especially regarding Te that occurs at the ppb level in rocks, a contribution from rock cannot be ruled out. This possibility is taken into account in the calculation of the distribution coefficient of elements between crust and vapours (DCrust/Gas ): [SO2]Gas is the mass fraction of SO2 estimated at 10% based on the composition of gases emanating from the active crater fractures with similar temperatures (285-385°C) [START_REF] Tassi | The magmatic-and hydrothermal-dominated fumarolic system at the Active Crater of Lascar Volcano, northern Chile[END_REF]. The mass fraction of element originating from the gas phase

DCrust/Gas = [X]Cc/[X]Gas (5) with [X]Cc= [X]C -[Zr]C . [X]R/[Zr]R
([X]Cc/[X]C
) is estimated at < 30% for U and Hf, 50% for Pb and Rb, 66-82% for Co, Cu, Ni, Cr, Nb and Cs, 90-98% for Li, Zn, Bi, Cd and Sb, and >99% for As, Se, V, Mo, Tl, Te and S.

[X]Cc/[X]C is plotted against DCrust/Gas on Figure 10 for elements with [X]Cc > 0. The generally high to very high values of DCrust/Gas reflect the very low content of trace metals in gases, in the range of 10 -2 -10 1 ppb. DCrust/Gas is ca. 4 for S, in the range of 1 -510 3 for Se, As and Sb, 10 4 -10 5 for Pb, U, Zn, Hf, Cu, Rb, and 1-410 5 for Cs, Mo, Cr, Ni, Co, Cd, Li and Nb. Tellurium, vanadium and thallium stand out, with DCrust/Gas between 610 6 and 3.310 7 . This order clearly does not reflect the volatility sequence of elements, but the ability of elements to enter the surface of the crust. For most elements DCrust/Gas does not markedly change if the inner layer (U11) composition is used in the calculation.

However, for Tl, V and Te DCrust/Gas decreases proportionally to the rim/core ratio (>20), and remains distinctly elevated only for Tl (110 6 ).

The enrichment of Tl and Te in the crust relative to the gas is comparable to that existing between Fe-Mn crust and seawater, in the range of 10 7 -510 8 [START_REF] Hein | Global occurrence of tellurium-rich ferromanganese crusts and a model for the enrichment of tellurium[END_REF]. The enrichment of V is higher in the fumarole crust than in seawater Fe-Mn crust (210 5 ).

The enrichment of Tl, Te and other metals in marine Fe-Mn crust has been ascribed to oxidative sorption on, or structural incorporation in Mn oxides and Fe oxy-hydroxide (Hein et al., 2003, Peacock and[START_REF] Peacock | Oxidative scavenging of thallium by birnessite: explanation for thallium enrichment and stable isotope fractionation in marine ferromanganese precipitates[END_REF][START_REF] Kashiwabara | Chemical processes for the extreme enrichment of tellurium into marine ferromanganese oxides[END_REF]. Unlike marine crusts that concentrate 30+ elements by 6 orders of magnitude or more, the enrichment of Tl, Te and V in the fumarole crust is very selective and requires a very specific enrichment process.

Isomorphic substitution of Tl and V

Within the crust, Tl and V bearing phases are not detected at the highest resolution of scanning electron microscopy, indicating that Tl and V are in solid solution in the sulphate, or occur in the form of nanoparticles. The former possibility is strongly supported by the layered structure of alums that facilitates element substitution.

Anhydrous alums are a series of more than 30 double sulphates sharing the general formula [START_REF] Giester | Crystal structure of anhydrous alum RbFe 3+ (SeO4)2[END_REF][START_REF] West | Structural disorder, octahedral coordination and two-dimensional ferromagnetism in anhydrous alums[END_REF][START_REF] Kosek | Raman spectroscopic study of six synthetic anhydrous sulfates relevant to the mineralogy of fumaroles[END_REF]:

M I M III (X VI O4)2
where M I is an univalent species (K + , Na + , Rb + , Cs + , Ag + , Tl + , NH4 + ), M III is a trivalent metal (Al 3+ , Fe 3+ , Cr 3+ , V 3+ , Sc 3+ , Mn 3+ , Ga 3+ , Rh 3+ , In 3+ , Tl 3+ ), X VI is an hexavalent element (S 6+ , Se 6+ , Cr 6+ , Mo 6+ , W 6+ ). The structure of anhydrous alums consists of parallel sheets of M III O6 octahedra and X VI O4 tetrahedra, separated by large 12-fold coordinated M I ions [START_REF] West | Structural disorder, octahedral coordination and two-dimensional ferromagnetism in anhydrous alums[END_REF][START_REF] Murashko | Steklite, KAl(SO4)2: A finding at the Tolbachik Volcano, Kamchatka, Russia, validating its status as a mineral species and crystal structure[END_REF][START_REF] Zhitova | Tschermigite from thermal fields of Southern Kamchatka: hightemperature transformation and peculiarities of IR-spectrum[END_REF]. Isomorphic substitutions play an important role in alum composition and can be predicted from the ionic radii of elements [START_REF] Shannon | Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides[END_REF]. Previous studies showed that univalent Tl (r=1.70Å) and trivalent V (r=0.64Å) tend to substitute for K + (r=1.64Å) and Al 3+ (r=0.535 Å) respectively (see synthesis by [START_REF] Giester | Crystal structure of anhydrous alum RbFe 3+ (SeO4)2[END_REF]. In godovikovite, Tl + might substitute for NH4 + (r=1.67Å for 12-fold coordination) [START_REF] Sidey | On the effective ionic radii for ammonium[END_REF]. The Tl-rich sulphate crystal lying on crust surface (Supplementary material S4) provides evidence for Tl incorporation into the crystal structure of alum. At magmatic temperature and oxygen fugacity, Tl and V dominantly occur in the gas phase as +1 (TlCl or Tl2Cl2) and +3 ions (VF3 or VOCl), respectively [START_REF] Hughes | Vanadium sublimates from the fumaroles of Izalco volcano, El Salvador[END_REF][START_REF] Churakov | Evolution of composition of high-temperature fumarolic gases from Kudryavy Volcano, Iturup, Kuril Islands: the thermodynamic modeling[END_REF]. The enrichment of Tl and V in the Lascar crust is thus consistent with incorporation of gas derived Tl + and V 3+

in the M I and M III sites, respectively, without change in valence state. Coupled incorporation of Tl and V might be energetically favourable since TlV(SO4)2 is a stable compound [START_REF] Perret | Sur les « aluns anhydres » de vanadium. Préparation et étude cristallographique de NH4V(SO4)2, TlV(SO4)2, NaV(SO4)2 et AgV(SO4)2[END_REF]. This may result from Tl + and V 3+ being both slightly larger than the ions they replace, limiting the distortion of the crystal network that may arise from individual substitution. Trivalent Tl, although generally of minor occurrence, can also probably significantly enter the M III site as it does in potassium jarosite [START_REF] Dutrizac | The behaviour of thallium(III) during jarosite precipitation[END_REF].

Structural incorporation versus adsorption of Te

Unlike Tl and V, tellurium is not reported to enter the crystal structure of anhydrous alum, alum or alunite. This reflects the fact that Te + and Te 3+ are unstable and Te 6+ (0.43 Å), unlike Se 6+ , is too large to substitute for four-fold coordinated S 6+ (0.12 Å) in SO4 2- [START_REF] Shannon | Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides[END_REF]. Moreover, as discussed later for Mo (section 6.6.), Te concentration would not show a drastic increase towards crust rim if it were to substitute for sulphur.

Tellurium is expected to be dominantly transported as Te2(g) [START_REF] Grundler | Speciation of aqueous tellurium(IV) in hydrothermal solutions and vapors, and the role of oxidized tellurium species in Te transport and gold deposition[END_REF] in Lascar eccentric degassing system where redox conditions are governed by the Fe II -Fe lII buffer of the rocks [START_REF] Tassi | The magmatic-and hydrothermal-dominated fumarolic system at the Active Crater of Lascar Volcano, northern Chile[END_REF], Giggenbach, 1987). Two processes of Te2(g) incorporation into the crust are envisioned. (a) The fact that Te correlates with Tl and V suggests that Te is incorporated by coupled substitution. For instance, Te2(g) could enter the M I site via coupled substitution with trivalent Tl, in which two large Te 0 atoms (2.06

Å radius) and one small atom of Tl 3+ (< 1 Å radius) substitute for three M I atoms (ca. 1.7 Å radius). If this substitution controls the Te and Tl 3+ budget of the crust, the atomic abundance of Te must be twice that of Tl 3+ . The average measured (Tl + + Tl 3+ )/Te ratio of 2.2 then requires a somewhat realistic Tl + / Tl 3+ atomic ratio of 3.3. (b) The occurrence of TeO2 crystals on crust surface suggests that gaseous Te is adsorbed at the surface of anhydrous alum. Tellurium adsorption processes have been mainly described in seawater and soils, where Te IV and Te VI species adsorb at the surface of Fe (III) hydroxides and illite via the formation of inner-sphere complexes [START_REF] Hein | Global occurrence of tellurium-rich ferromanganese crusts and a model for the enrichment of tellurium[END_REF][START_REF] Kashiwabara | Chemical processes for the extreme enrichment of tellurium into marine ferromanganese oxides[END_REF][START_REF] Qin | Tellurium Distribution and Speciation in Contaminated Soils from Abandoned Mine Tailings: Comparison with Selenium[END_REF]. Adsorption of gaseous Te at the surface of mordenite zeolite has also been reported (Kodaira and Ikeda, 2014). This reaction occurs via sorption of nano-chains of Te atoms on dipoles formed by AlO2 -and Na + in mordenite crystalline structure. The layers of Al(SO4)2 -and cations in anhydrous alums could represent reactive sites allowing Te sorption as for mordenite, but the occurrence of Te IV on crust surface requires that tellurium oxidizes in a second step. The scenarios (a) and (b) are certainly not incompatible, because the adsorption process is the precursor to the chemisorption reaction, during which elements are incorporated in the crustal structure of sulphate [START_REF] King | Gas-solid reactions: Theory, experiments and case studies relevant to earth and planetary processes[END_REF].

Growth rate control on Tl-V-Te zonation

The concentrations of Tl, V and Te increase by a factor >20 from the core to the outer edge of the sulphate crust. This trend correlates with a decrease of the thickness of the growth layers, pointing out a control of growth rate on Tl, V and Te content (Fig. 11).

Such growth rate control on metal abundance has been described in marine Fe-Mn crusts (e.g., [START_REF] Hein | Global occurrence of tellurium-rich ferromanganese crusts and a model for the enrichment of tellurium[END_REF] where metal-rich hydrogenic phases are diluted by metalpoor detrital phases. The growth rate of the sulphate crust is controlled by the slow diffusion of rock cations by comparison to gas species. Rock cation supply is critical for the outermost layer of the mature crust because rock cations have to diffuse over long distances in the sulphate but also in the thick rock layer already depleted in cations (Fig. 8c). A straightforward consequence of the low rate of cation supply to the outermost layer of the crust is to favour the substitution of gas derived Tl, V and Te in the crystal structure of alum. The extremely elevated Tl-Te-V concentrations at crust rim thus first result from the lack of cations with which they compete for the M I and M III sites of anhydrous alum. In addition, as discussed for Te, it is possible that elements are adsorbed to some degree at the surface of the crust.

The molybdenum issue

Molybdenum stands out from Tl, V and Te because its elevated concentration in the crust (205-924 ppm) reflects a high concentration in the gas (32 ng/m 3 ) [START_REF] Menard | Gas and aerosol emissions from Lascar volcano (northern Chile): insights into the origin of gases and their links with the volcanic activity[END_REF], and not an anomalously high crust/gas ratio (Fig. 10). The high Mo concentration of the gas might be related to its enrichment in HCl, which promotes Mo volatility in the form of MoO2Cl2 [START_REF] Rempel | The solubility of molybdenum dioxide and trioxide in HCl-bearing water vapour at 350°C and pressures up to 160 bars[END_REF]. In addition, Mo does not show a comparable zonation as Tl-V-Te across the crust, but tends to correlate with S. This highlights a fundamental difference in the process by which Tl-Te-V and Mo are incorporated into the crust. In the absence of Mo mineral, Mo must also enter the crystal network of anhydrous alum. Unlike Te, Mo possibly enters the X VI site [START_REF] Giester | Crystal structure of anhydrous alum RbFe 3+ (SeO4)2[END_REF] although Mo 6+ (r=0.41 Å) is barely smaller than Te 6+ (r=0.43 Å) in tetrahedral coordination [START_REF] Shannon | Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides[END_REF]. Molybdenum essentially competes with S for this site. Because both Mo and S entirely originate from the gas phase (Fig. 10), the Mo content of the crust first depends on the Mo/S ratio of the gas, which has seemingly little varied during crust formation history (Mo/S=27±1210 -4 ). Thus, the major difference between Tl-V-Te and Mo is that the formers compete with rock-derived cations, whose rate of supply decreases towards crust rim, whereas the second competes with another element of the gas.

Implications for trace element degassing

Figure 9 shows that the aerosols collected within the Lascar plume have four orders of magnitude lower Tl/Pb and Te/Pb compared to the fumarole crust, but remarkably similar Tl/Te ratio (1.5±0.6 (2) versus 3.6±2.7(2)). Likewise, aerosols and crust also have similar Tl/V ratio (0.54±0.42 (2) and 1.4±1.1(2), respectively). At Lascar, the small fractionation between Tl, Te and V contrasts with the large fractionation existing between Tl-Te-V and other volatile elements. This supports the idea that the underground partitioning of Tl, Te and V into anhydrous alums, such as godovikovite, significantly influences the abundance of these three elements in the gas plume. In the following, we evaluate quantitatively the effect of preferential incorporation of Tl over Pb into anhydrous alums on the Tl/Pb ratio. The trace element content of a gas equilibrating with a rock follows the general law of fluid/rock interaction:

[𝑋] 𝐺 𝑓 = [𝑋] 𝑅 𝑖 +𝑁[𝑋] 𝐺 𝑖 𝑁+𝐷 𝑆/𝐺 𝑋 (6)
where [𝑋] 𝐺 𝑓 and [𝑋] 𝐺 𝑖 are the initial and final concentrations of element X in gas, [𝑋] 𝑅 𝑖 is the initial concentration of X in rock, 𝐷 𝑆/𝐺 𝑋 is the partition coefficient between solid and gas, and N is the gas/rock mass ratio. In the case of extensive gas-rock interaction,

[𝑋] 𝑅 𝑖 ≪ 𝑁[𝑋] 𝐺 𝑖 , and thus:

[ 𝑇𝑙 𝑃𝑏 ] 𝐺 𝑓 = 𝑁+𝐷 𝑆/𝐺 𝑃𝑏 𝑁+𝐷 𝑆/𝐺 𝑇𝑙 × [ 𝑇𝑙 𝑃𝑏 ] 𝐺 𝑖 (7)
Using distribution coefficients between crust surface and gas (DS/G Tl =3.310 7 and DS/G Pb =1.110 4 ) calculated as described in section 6.2, a gas/rock mass ratio between 1.610 6 and 1.010 7 is needed to decrease Tl/Pb from 1-5 in the initial gas [START_REF] Johnson | The degassing behavior of Au, Tl, As, Pb, Re, Cd and Bi from silicate liquids: experiments and applications[END_REF] to 0.23 in average Lascar aerosols [START_REF] Menard | Gas and aerosol emissions from Lascar volcano (northern Chile): insights into the origin of gases and their links with the volcanic activity[END_REF]. The sulphate crust formed by the interaction between gas and rock uptakes between 77 and 95% of the initial Tl content of the gases, but less than 1% of Pb. The fraction of Tl trapped in the ground translates into a daily deposition rate between 17 and 104 g, based on Lascar average Tl emission rate of 5 g/day (recalculated from Tl/SO2 mass ratio of 10 -8 in the gas plume and mean SO2 flux of 500t/day from [START_REF] Menard | Gas and aerosol emissions from Lascar volcano (northern Chile): insights into the origin of gases and their links with the volcanic activity[END_REF] who erroneously calculated metal emission rates). The estimated fraction of Tl trapped underground should be considered as an upper bound, because it is likely that gases do not fully equilibrate with sulphates, and anhydrous alums might not form everywhere in the degassing network. For instance, 3 out of the 11 fumaroles sampled by [START_REF] Tassi | The magmatic-and hydrothermal-dominated fumarolic system at the Active Crater of Lascar Volcano, northern Chile[END_REF] within Lascar active crater have temperature within the stability field of godovikovite.

Godovikovite has been discovered in burning coal-dumps in 1988, thus after the main surveys of fumarolic incrustations [START_REF] Stoiber | Fumarole incrustations at active Central American volcanoes[END_REF][START_REF] Naughton | Incrustations and fumarolic condensates at kilauea volcano, Hawaii: field, drill-hole and laboratory observations[END_REF][START_REF] Oskarsson | The chemistry of Icelandic lava incrustations and the latest stages of degassing[END_REF]. This raises the possibility that godovikovite is more frequent than described in early reports. In support of this idea, godovikovite and isostructural steklite have been recently found in the fumarolic fields of Tolbachik, Mutnovsky and Avachinsky volcanoes, Kamchatka (Murashko et al., 2013 and M. Zelenski, personal communication). Generally, trapping of Tl in the ground is probably a widespread phenomenon due to the frequent occurrence of K-Na sulphates. For instance, [START_REF] Okrugin | Smoking gun for thallium geochemistry in volcanic arcs: Nataliyamalikite, TlI, a new thallium mineral from an active fumarole at Avacha Volcano, Kamchatka Peninsula, Russia[END_REF] noted that the altered rocks of Vulcano contain more than three orders of magnitude more Tl (up to 280 ppm) than the fumarolic fluids (14-82 ppb Tl) [START_REF] Cheynet | Trace elements from fumaroles at Vulcano Island (Italy): Rates of transport and a thermochemical model[END_REF][START_REF] Boyce | Fluids in early stage hydrothermal alteration of high-sulfidation epithermal systems: A view from the Vulcano active hydrothermal system (Aeolian Island, Italy)[END_REF][START_REF] Fulignati | Presence of native gold and tellurium in the active high-sulfidation hydrothermal system of the La Fossa volcano (Vulcano Italy)[END_REF]. We hypothesize that Tl might substitute for K in alunite and for Na in natroalunite, which are common alteration phases at Vulcano [START_REF] Boyce | Fluids in early stage hydrothermal alteration of high-sulfidation epithermal systems: A view from the Vulcano active hydrothermal system (Aeolian Island, Italy)[END_REF]. Selective uptake of gaseous elements by isomorphic substitution in fumarolic minerals is not limited to Tl, Te and V. At Kilauea volcano, K and Cu might substitute for Na into thenardite [START_REF] Naughton | Incrustations and fumarolic condensates at kilauea volcano, Hawaii: field, drill-hole and laboratory observations[END_REF]. At Mount St. Augustine, Alaska, Cr and V might substitute for Fe in fumarole incrustations [START_REF] Kodosky | Fumarole distribution, morphology, and encrustation mineralogy associated with the 1986 eruptive deposits of Mount St. Augustine, Alaska[END_REF][START_REF] Kodosky | Fumarole distribution, morphology, and encrustation mineralogy associated with the 1986 eruptive deposits of Mount St. Augustine, Alaska[END_REF]. In high-sulphidation epithermal deposits, Ag + -As 3+ , and possibly Au 3+ -Cu + ion pairs substitute for Fe 2+ into pyrite [START_REF] Chouinard | Crystallographic controls on trace-element incorporation in auriferous pyrite from the Pascua epithermal highsulfidation deposit, Chile-Argentina[END_REF][START_REF] Scher | Fumarolic Activity, Acid-Sulfate Alteration, and High Sulfidation Epithermal Precious Metal Mineralization in the Crater of Kawah Ijen Volcano, Java, Indonesia[END_REF].

Selective entrapment of some gaseous elements into alteration minerals has significant consequences: (1) As modelled above, they can modify extensively key trace element ratios, such as Tl/Pb. At global scale, the Tl/Pb ratio of volcanic aerosols shows two order of magnitude variations (0.03 to 3.3), with no systematic difference between arc and hotspots volcanoes [START_REF] Baker | Investigation of thallium fluxes from subaerial volcanism-Implication for the present and past mass balance of thallium in the oceans[END_REF][START_REF] Edmonds | A distinct metal fingerprint in arc volcanic emissions[END_REF]. Our results on the Lascar volcano suggest that Tl entrapment in alteration sulphates might explain part of the Tl/Pb variations in volcanic emanations, in addition to gas source heterogeneity [START_REF] Okrugin | Smoking gun for thallium geochemistry in volcanic arcs: Nataliyamalikite, TlI, a new thallium mineral from an active fumarole at Avacha Volcano, Kamchatka Peninsula, Russia[END_REF], diffusive fractionation of Pb and Tl at the melt-gas interface [START_REF] Johnson | The degassing behavior of Au, Tl, As, Pb, Re, Cd and Bi from silicate liquids: experiments and applications[END_REF], and chemical evolution of gas during cooling [START_REF] Churakov | Evolution of composition of high-temperature fumarolic gases from Kudryavy Volcano, Iturup, Kuril Islands: the thermodynamic modeling[END_REF].

(2) Substitution reactions potentially deplete volcanic emanations in some elements, which lead to underestimate the volatility of these latter. For instance V is the second most depleted element in the Lascar plume [START_REF] Menard | Gas and aerosol emissions from Lascar volcano (northern Chile): insights into the origin of gases and their links with the volcanic activity[END_REF] although there is evidence from the studied crust that V is significantly transported by subsurface gases.

(3) Highly toxic elements such as Tl are immobilized within volcanic edifices instead of being released in the atmosphere, in the same manner as heavy metals are retained by alunite group minerals in polluted sites [START_REF] Kolitsch | Crystal chemistry of the crandallite, beudantite and alunite groups: a review and evaluation of the suitability as storage materials for toxic metals[END_REF][START_REF] Figueiredo | The Positive environmental contribution of jarosite by retaining lead in acid mine drainage areas[END_REF]. Thallium pollution of soils around Lascar volcano is an example of such local impact [START_REF] Queirolo | Thallium levels and bioaccumulation in environmental samples of Northern Chile: Human health risks[END_REF].

7) Conclusions

This study of the fine-scale chemical and isotopic zonation of an underground fumarolic crust from the Lascar volcano reached the following conclusions:

(1) Magmatic-hydrothermal gases from the Lascar volcano, 330°C in temperature, precipitate anhydrous alums underground as they react with rocks. The studied specimen occurs as a few millimetres thick crust that grew internally by polydiffusion reactions within the sulphate. The growth rate is on average 0.3 m/day, but decreases towards crust rim due to the slow diffusive influx of rock cations.

(2) The crust selectively uptakes gaseous Tl, V and Te and, to a lesser extent Mo.

This uptake results from the incorporation of the elements into the crystal structure of alum: Tl + , V 3+ and Mo 6+ substitute for K + , Al 3+ and S 6+ into the M I , M III and X VI sites of alum, respectively. The process of Te uptake remains uncertain, but must be tightly related to Tl-V incorporation.

(3) The extent of Tl, V and Te uptake is the highest at the outer edge of the crust, where the low rate of rock cations supply favours the substitution process. Such surface enrichment does not occur for Mo, because Mo competes with S, which is continuously supplied from the gas. Thus, the extreme enrichment of Tl, V and Te at the surface of the mature alum crust first reflects their ability to substitute for rock-derived cations and to compensate for their lack.

(4) Gases equilibrating with anhydrous alums lose a large amount of Tl, V and Te, but insignificant amount of other volatile trace elements (Pb, Cu, Zn, Bi, Sb, As, Se). This dramatically fractionates key trace elements ratios in volcanic emanations, such as Pb/Tl, or Se/Te. This also decreases the amount of Tl, V and Te released in the atmosphere. Immobilization of toxic Tl within the Lascar volcano contributes to the local Tl soil-pollution.

(5) Sub-surface gas-rock interaction within stratovolcanoes acts as a filter stripping some elements from the gas phase. A better understanding of the mechanism and extent of incorporation of metals and metalloids in the wide range of fumarole alteration minerals is needed to evaluate quantitatively the impact of underground gas-rock interaction on the degassing fluxes of volcanoes. [START_REF] Tassi | The magmatic-and hydrothermal-dominated fumarolic system at the Active Crater of Lascar Volcano, northern Chile[END_REF]. Compositions of local meteoric water [START_REF] Tassi | The magmatic-and hydrothermal-dominated fumarolic system at the Active Crater of Lascar Volcano, northern Chile[END_REF] and andesitic water [START_REF] Giggenbach | Isotopic shifts in waters from geothermal and volcanic systems along convergent plate boundaries and their origin[END_REF][START_REF] Taran | Deuterium and oxygen-18 in fumarolic steam and amphiboles from some Kamchatka volcanoes: "andesitic waters[END_REF] are indicated. Te/Pb versus Tl/Pb plot comparing the signature of Lascar crust and aerosols to those of worldwide volcanic aerosols, fumarole condensates and solid deposits. Propagated error on trace element ratios is within symbol size (Lascar data). Lascar aerosol data are from [START_REF] Menard | Gas and aerosol emissions from Lascar volcano (northern Chile): insights into the origin of gases and their links with the volcanic activity[END_REF]. Other data are from [START_REF] Kavalieris | High Au, Ag, Mo, Pb, V and W content of fumarolic deposits at Merapi volcano, central Java, Indonesia[END_REF], [START_REF] Moune | Trace elements in the particulate phase of the plume of Masaya Volcano, Nicaragua[END_REF], [START_REF] Zelenski | Trace elements in the gas emissions from the Erta-Ale volcano, Afar, Ethiopia[END_REF][START_REF] Zelenski | Emissions of trace elements during the 2012-2013 effusive eruption of Tolbachik volcano, Kamchatka: enrichment factors, partition coefficients and aerosol contribution[END_REF], [START_REF] Chaplygin | Geochemistry of volcanic gas at the 2012-13 New Tolbachik eruption, Kamchatka[END_REF], [START_REF] Taran | Geochemistry of magmatic gases from Kudryavy volcano, Iturup, Kuril Islands[END_REF], [START_REF] Zelenski | Sublimate speciation at Mutnovsky volcano, Kamchatka[END_REF], [START_REF] Okrugin | Smoking gun for thallium geochemistry in volcanic arcs: Nataliyamalikite, TlI, a new thallium mineral from an active fumarole at Avacha Volcano, Kamchatka Peninsula, Russia[END_REF], [START_REF] Fulignati | Presence of native gold and tellurium in the active high-sulfidation hydrothermal system of the La Fossa volcano (Vulcano Italy)[END_REF], [START_REF] Boyce | Fluids in early stage hydrothermal alteration of high-sulfidation epithermal systems: A view from the Vulcano active hydrothermal system (Aeolian Island, Italy)[END_REF], [START_REF] Gauthier | Elevated gas flux and trace metal degassing from the 2014-2015 fissure eruption at the Bárðarbunga volcanic system, Iceland[END_REF] and [START_REF] Mather | Halogens and trace metal emissions from the on-going 2008 summit eruption of Kīlauea volcano, Hawai'i[END_REF].

  and [X]Gas = {X}Gas/{SO2}Gas . [SO2]Gas where [X]Cc is the element concentration in the crust corrected for inputs from the rock, assuming all Zr originate from the altered rock. Zirconium is used because high-field strength elements are generally strongly depleted in magmatic gases. [X]C and [X]R are the measured concentrations in the crust outer layer (U1) and in 1993 lavas[START_REF] Menard | Gas and aerosol emissions from Lascar volcano (northern Chile): insights into the origin of gases and their links with the volcanic activity[END_REF], respectively. [X]Gas is the mass fraction of elements in gases. {X}Gas and {SO2}Gas are the volume-normalized masses of trace metal and SO2 in the gas escaping from the fracture hosting the crust, respectively (sample Las 18 fromMenard et al., 2014) (Fig 2a).

Figure captions Figure 1

 1 Figure captions

Figure 2

 2 Figure 2 Sampling site and crust sample. (a) Thermal image showing the location of gas sampling (filter packs) ca. 2m from the degassing hole. (b) Thermal image of the degassing hole where the crust was sampled in November 2012. (c) Photo of the degassing hole. Arrow indicates the lava block under which the crust was sampled. (d) Drawing of the crust and the host rock, respecting size proportions. (e) Photo of crust surface showing several outgrowths, more or less cracked and hollow. (f) Polished section of the studied outgrowth showing growth layers and the area selected for in-situ chemical analyses.

Figure 3

 3 Figure 3Raman spectra of the three major phases. (a) 100-1300 cm -1 range. (b) 2900-3700 cm -1 range. Characteristic vibration bands of (NH4)Al(SO4)2 (godovikovite), MgSO4•6H2O (exahydrite), CaSO4 (anhydrite) and hydrated alums (990/991 cm -1 ) are indicated.

Figure 4 SEM

 4 Figure 4 SEM image of the studied crust section. (a) Concentric growth layers and spot locations of EPMA and ICPMS analyses are highlighted. (b-d) Mapping of Al, Fe and Ca.

Figure 5 Lava

 5 Figure 5 Lava-normalized trace element concentrations. Normalized concentrations are shown for crust core (average of unit 11) and crust rim (average of unit 1). Because Se was not measured by LA-ICPMS, the bulk Se concentration in crust surface scrapings (2.4±1.2 ppm) is used. Concentrations of the 1993 lava used for normalization are from Menard et al. (2014). Elements are sorted from left to right according to their increasing

Figure 6

 6 Figure 6 Chemical variations across the crust. (a) Thallium, vanadium and tellurium concentrations plotted against distance to crust rim. Data are from profile A1. Profile A2 (not plotted) shows the same variations. Error on trace element concentrations is within symbol size. (b) Tl/V, Tl/Te and V/Te concentration ratios plotted against distance to crust rim.

Figure 7

 7 Figure 7Core-rim  18 O profiles. The three profiles (B1, B2 and B3) are normalized to the length of the A1 profile. The average in-run error (0.25‰, 2) is indicated. The grey band indicates the δ 18 O of Lascar fumaroles condensates[START_REF] Tassi | The magmatic-and hydrothermal-dominated fumarolic system at the Active Crater of Lascar Volcano, northern Chile[END_REF]. Compositions of

Figure 8

 8 Figure 8 Schematic drawing of the processes leading to the formation of a thick alum crust. (a) A sulphate layer grows via diffusive influx of rock cations and two-way diffusion of reaction gases within the sulphate. Gaseous NH3 and trace metals (Tl, V and Te) are incorporated into the crust following Eqs. 2-4. (b) Internal sulphate growth results in stress accumulation within the coating (). The thickening sulphate layer detaches from the rock surface, wrinkles and buckles. The layer also starts to split into two layers due to higher growth rate near the sulphate-rock interface. (c) As the sulphate crust grows, the exhaustion of cations in the reacted rock requires cations to migrate over longer distances. The growth rate decreases towards crust surface due to the low rate of rock cations supply. This results in the formation of thinner growth layers at the outer edge of the crust. The lack of rock cations also favours the substitution of Tl, V and probably Te into the crystal structure of alum, yielding the extreme enrichment observed at crust rim. (a-c) Grey scale indicates the depletion of major cations in the silicate, white colour corresponding to SiO2. (a) and (b) are modified from[START_REF] Renggli | SO2 Gas Reactions with Silicate Glasses[END_REF].

Figure 9

 9 Figure 9

Figure 10 Crust

 10 Figure 10Crust/gas distribution coefficient plotted against element fraction originating from the gas. Distribution coefficients are calculated by normalizing the concentration of elements in the outer shell of the Lascar crust (U1 layer) to their abundance in the gas escaping from the fracture hosting the crust. Crust concentrations are corrected for rock inputs, as explained in text.

Figure 11 Correlation between 1 /

 111 Figure 11 Correlation between 1/Tl and growth layer thickness, a proxy for crust growth rate. Median values are plotted for each unit. Error bars on x and y axis indicate upper and lower bounds for each layer.

  Figure1

Pb/ 204 Pb=38.795-38.803; 87 Sr/ 86 Sr=0.706337-0.706438)

), sulphuric acid and its hygroscopic forms are undersaturated above 202 °C in a gas

Acknowledgements J.L. Devidal, J.M. Henot, N. Bouden, C. Fonquernie, D. Auclair and A. Gannoun contributed to data acquisition. We thank M. Zelenski and two anonymous reviewers for their constructive comments, and G. Pokrovski for handling the manuscript. This work benefited from the financial support from the Institut de Recherche pour le Développement (ECOS-sud project no. C11U01, P.I. O. Roche), the French Government Laboratory of Excellence initiative n°ANR-10-LABX-0006, the Région Auvergne and the European Regional Development Funds. This is Laboratory of Excellence ClerVolc contribution number XXX.

Supplementary material


L Abi Nassif

S Rioual

W Farah

C Hellio

M Fauchon

R Trepos

M Abboud

E Diler

D Thierry

B Lescop

Reduction of potential ennoblement of stainless steel in natural seawater by an ecofriendly biopolymer

Keywords: Biopolymer, Steel, Ennoblement, Antimicrobial, Alginate, Biofilm

The effect of biofilm formation on passive stainless steel is of primary importance since it leads to potential ennoblement of the surfaces and subsequently to localized corrosion such as pitting and crevice corrosion. This study therefore aims at developing an ecofriendly biopolymer which inhibits this effect. For this purpose, an alginate biopolymer containing both calcium and zinc ions localized in the vicinity of the steel surface was tested and reduced significantly the ennoblement process in natural and renewed seawater, during one month. The influence of the two types of cations on the reduction of the ennoblement is further discussed.

Introduction

In a natural environment, microorganisms can grow, develop and then form biofilms on any available natural and man-made surfaces. The presence of such biofilm on passive alloys such as stainless steels in natural seawater strongly enhances the cathodic reaction of oxygen [START_REF] Mollica | Biofilm and corrosion on active-passive alloys in seawater[END_REF]. This leads to an increase of the open circuit potential (so called ennoblement) by about 350 mV [START_REF] Acuña | Biofilm colonization dynamics and its influence on the corrosion resistance of austenitic UNS S31603 stainless steel exposed to Gulf of Mexico seawater[END_REF][START_REF] Mattila | Biofilm development during ennoblement of stainless steel in Baltic Sea water: A microscopic study[END_REF][START_REF] Scotto | The influence of marine aerobic microbial film on stainless steel corrosion behaviour[END_REF][START_REF] Shams El Din | Biofilm formation on stainless steels in Arabian Gulf water[END_REF][START_REF] Wang | Influence of biofilms growth on corrosion potential of metals immersed in seawater[END_REF][START_REF] Thierry | Corrosion potential and cathodic reduction efficiency of stainless steel in natural seawater[END_REF] and consequently to the initiation of pitting corrosion showed by Xiangbo et al. [START_REF] Xiangbo | Ennoblement of stainless steel in natural seawater -A new explanation[END_REF], the ennoblement process is highly dependent on the growth of bacteria on the metal surface and thus follows an exponential law. This phenomenon was used for decades to monitor the biofilm growth in seawater at its initial phase, and thus trigger some water treatments [START_REF] Mollica | Simple Electrochemical Sensors for Biofilm and MIC Monitoring and In-Field Experience, Microbially Influ. Corros. Ind. Mater. Biocorrosion Network, Brite-Euram III Themat[END_REF][START_REF] Mollica | On-line biofilm monitoring by "BIOX" electrochemical probe[END_REF][START_REF] Diler | Biofilm sensor for deep sea[END_REF][START_REF] Werwinski | Electrochemical sensing of aerobic marine bacterial biofilms and the influence of nitric oxide attachment control[END_REF][START_REF] Werwinski | Electrochemical Sensing for Aerobic Marine Bacterial Biofilms within Seawater Piping Systems[END_REF][START_REF] Pires | Online Monitoring of Biofilm Growth and Activity Using a Combined Multi-channel Impedimetric and Amperometric Sensor[END_REF]. However, the mechanisms responsible for the ennoblement process are quite complex. Among them, the oxygen reduction reactions in the biofilm seems to play a major role [START_REF] Dickinson | Evidence for surface changes during ennoblement of type 316L stainless steel: dissolved oxidant and capacitance measurements[END_REF][START_REF] Washizu | Role of H2O2 in microbially influenced ennoblement of open circuit potentials for type 316L stainless steel in seawater[END_REF]. The influence of enzymes and more recently electron transfer on the process was also clearly demonstrated [START_REF] Landoulsi | Enzymeinduced ennoblement of AISI 316L stainless steel: focus on pitting corrosion behavior[END_REF][START_REF] Trigodet | Electroactive Bacteria Associated With Stainless Steel Ennoblement in Seawater[END_REF].

Traditional chemical treatments of biofilm are based on the use of biocides. They are particularly efficient when applied at the initial phase of the biofilm formation. Indeed, bacteria in biofilm can be a thousand times more resistant than their planktonic counterparts [START_REF] Rasmussen | Quorum-sensing inhibitors as anti-pathogenic drugs[END_REF].

However, such treatments lead to a biocide dosage escalation and consequently to environmental issues and propagation of resistance [START_REF] Xu | Advances in the treatment of problematic industrial biofilms[END_REF]. Thus there is an urgent need for the development of new eco-friendly materials with antifouling properties [START_REF] Elshaarawy | Surface functionalization of chitosan isolated from shrimp shells, using salicylaldehyde ionic liquids in exploration for novel economic and ecofriendly antibiofoulants[END_REF] and to explore novel strategy to combat this phenomenon [START_REF] Elshaarawy | Novel high throughput mixed matrix membranes embracing poly ionic liquid-grafted biopolymer: Fabrication, characterization, permeation and antifouling performance[END_REF][START_REF] Elshaarawy | Hybrid molecules comprising 1, 2, 4-triazole or diaminothiadiazole Schiff-bases and ionic liquid moieties as potent antibacterial and marine antibiofouling nominees[END_REF]. Biopolymer integrating silver nanoparticles can be produced for that purpose [START_REF] Jian | Fabrication of highly stable silver nanoparticles using polysaccharide-protein complexes from abalone viscera and antibacterial activity evaluation[END_REF][START_REF] Dechatre | Characterization of exopolysaccharides after sorption of silver ions in aqueous solution[END_REF]. However, the eco-friendly nature of such material is still debatable [START_REF] Pandiarajan | Properties, synthesis and toxicity of silver nanoparticles[END_REF]. Recently, we proposed a solution based on a biopolymer, e.g. alginate, containing several types of cations [START_REF] Abi Nassif | Development of alginate hydrogels active against adhesion of microalgae[END_REF]. The anti-adhesive and biocidal properties of this material was assessed in laboratory towards microalgae strains, and promising results were obtained against Cylindrotheca closterium which acts as pioneer microalga strain in surface colonization [START_REF] Briand | Pioneer marine biofilms on artificial surfaces including antifouling coatings immersed in two contrasting French Mediterranean coast sites[END_REF][START_REF] Luciana | The effect of biofouling on localized corrosion of the stainless steels N08904 and UNS S32760[END_REF]. This previous study showed that a mixed zinc/calcium alginate protects the surfaces by blocking the microalgae adhesion together with a low toxicity, an essential property for environmental friendly applications. The present study aims at demonstrating the ability of proposed alginate-based solution to reduce the ennoblement process of stainless steel in a natural and renewed seawater i.e. in conditions representative of real applications. For this purpose, the measurement of the corrosion potential of stainless steel was performed with and without alginate active materials in the vicinity of the samples. Moreover, to highlight the influence of non-toxic calcium ions in the reduction of the ennoblement process, alginates containing only zinc or zinc and calcium ions are considered.

Materials and methods

Alginate is a copolymer composed of α-L-glucuronic acid (G) and β-D-mannuronic acid (M) linked together in different sequences via 1-4bonds. The formation of zinc alginate and calcium/zinc alginate from a sodium alginate solution is based on the adsorption of positive ionic species on anionic groups of the alginate polymer. The reaction is governed by the ion exchange process between sodium cations and the other cationic species, in this case: Zn 2+ and Ca 2+ . This leads to the jellification of the biopolymer. Briefly, a solution of sodium alginate (Sigma Aldrich 71238, M/G ratio of 0.85) was prepared with a concentration of 0.25 mol.L -1 by magnetic stirring during 5 hours. Solutions of CaCl2 and ZnC4H6O4 were added to the sodium alginate solution in order to modify the initial sodium alginate. A concentration of 0.1 mol.L -1 of Zn(C4H6O4) was used for zinc alginate (ZnAlg) while concentrations of 0.27 and 0.05 mol.L -1 of CaCl2 and Zn(C4H6O4) were used for calcium/zinc alginate (CaZnAlg). The elaborated hydrogels were rinsed with deionized water to remove the excess of ions on the surface. UNS S32205 duplex stainless-steel (DSS) coupons were polished and subsequently immersed in 3 liters seawater tanks. The natural seawater was continuously renewed at 1.5 L/h using peristatic pumps. A tank containing two UNS S32205 coupons without any alginate was used as reference. In two other tanks, four cubes of alginate (1 cm 3 ) were positioned near the coupons (at 3 cm), without a direct contact. The open circuit potential or corrosion potential (Ecorr) was monitored during 30 days using a voltmeter with a high input impedance (> 10 8 ) EPC8 from nke Instrumentation. Saturated Calomel Electrode (SCE) reference electrode from Mettler Toledo were used as reference. Following these measurements, the morphology and the chemical composition of the biofilms were determined using Scanning Electron Microscope (SEM) imaging. The crystalline structure of the material was analysed by X-ray diffraction (XRD) with an Empyrean PANalytical apparatus using the CuK radiation. The chemical characterization of the polymer was obtained by X-ray photoelectron spectroscopy (XPS).

Briefly, the experimental apparatus consisted of an Al-K ray source (Thermo VG) and a cylindrical mirror analyzer from RIBER.

Results and discussion

The surface morphology and chemical composition of the dried hydrogels was studied before their immersion by SEM/EDX. As shown on Fig. 1 of supplementary information, the surface of the material is very smooth. The hydrogel's composition is presented in Table 1 : the materials are mainly composed of carbon and oxygen atoms due to the hydroxyl and carboxyl groups of the alginate matrix. The presence of zinc and calcium cations is clearly observed and explained by the ion exchange process between Na + present in the initial alginate and the cations present in the solution. Atomic percent of 1.6 % of Zn 2+ was found for the ZnAlg. Values of 4.1% and 0.8% of Ca 2+ and Zn 2+ were measured for CaZnAlg. Note that the decrease of zinc content in CaZnAlg with respect to ZnAlg is in agreement with the zinc concentration in the two solutions used to produce the materials. XPS measurements, not shown here, proved the presence of Ca 2+ and Zn 2+ species in agreement with the exchange process. During sorption of ionic species by hydrogels, as observed for silver ions, nanoparticles may form [START_REF] Dechatre | Characterization of exopolysaccharides after sorption of silver ions in aqueous solution[END_REF] steel in natural seawater [START_REF] Marconnet | Stainless steel ennoblement in fresh water: from exposure tests to mechanisms[END_REF] and corresponds to the well-known ennoblement process. As stated by Xiangbo et al. [START_REF] Xiangbo | Ennoblement of stainless steel in natural seawater -A new explanation[END_REF], an exponential law correlated to the development of bacteria on the surface describes the potential variation. The blue line in Fig. 1(a) displays the potential variation when zinc enriched alginate cubes are placed in the vicinity of the steel coupons. In this case, the potential ennoblement is clearly delayed in time, demonstrating thereby the ability of ZnAlg alginate to inhibit the biofilm growth during the initial stage of its formation. This confirms the biocidal action of the material observed in laboratory conditions [START_REF] Abi Nassif | Development of alginate hydrogels active against adhesion of microalgae[END_REF] on adhesion of microalga on glass and on their growth in Guillard's F/2 medium at 2% [START_REF] Guillard | Studies of marine planktonic diatoms, I: cyclotella nana (Hustedt) and Detonula confervacea (Cleve)[END_REF]. The use of zinc/calcium (CaZnAlg) alginate leads to a different behavior. Firstly, an additional delay is observed in the ennoblement process. Secondly, the increase of the Ecorr potential remains very low with respect to the two previous cases. This reduction of the ennoblement is of first relevance when considering the possibility of pitting corrosion since it remains even after 30 days of immersion time showing the long-term efficiency of the CaZnAlg alginate.

The reduction of the ennoblement process is associated with the release of the calcium and zinc cations in the surrounding environment, as it is also made for the development of controlled release systems for drugs in medical applications. The biocidal action of the modified alginate is attributed to the release of Zn 2+ , calcium ions being ineffective regarding this effect. In contrast, both calcium and zinc cations inhibit the adhesion of microalgae on surfaces as demonstrated in laboratory tests [START_REF] Abi Nassif | Development of alginate hydrogels active against adhesion of microalgae[END_REF]. Due to this mode of action, the efficiency of the materials should be highly dependent on their solubility. To investigate such property, the mass of the alginate cubes was measured before and after the 30 days of immersion. Mass losses of 4% and 38% were determined for CaZnAlg and ZnAlg, respectively. As the consequence, the higher efficiency and long term activity of CaZnAlg with respect to ZnAlg seems therefore associated to the low solubility of the material in seawater despite a lower content of biocidal agent. To investigate the surface of the steel coupons after the ennoblement process, SEM images of surfaces after biofilm formation were analysed (see Fig. 2 of supplementary data). CaZnAlg in the surrounding environment might therefore be associated with a different composition of the biofilm with more bacteria and less diatoms. However, a full quantification of microorganism would be necessary to confirm this finding. Note that, the influence of diatoms on the ennoblement process with respect to that of bacteria was reported in the pioneering investigation of Ishihara and Tsujikawa [START_REF] Ishihara | Effect of diatoms on ennoblement of electrode potential for stainless steels in natural sea water[END_REF][START_REF] Ishihara | Effect of bacteria combined with diatom on ennoblement of electrode potential for stainless steels in natural sea water[END_REF]. Indeed, these authors demonstrated the strong increase of the Ecorr potential when considering diatoms plus bacteria within the biofilm instead of only bacteria. However, as pointed out by Landoulsy et al. [START_REF] Landoulsi | Dupres Review -Interactions between diatoms and stainless steel: focus on biofouling and biocorrosion[END_REF][START_REF] Richard | Diagnosis of the Diatom Community upon Biofilm Development on Stainless Steels in Natural Freshwater[END_REF], separating the influence of both species on the ennoblement process is not straightforward.

Conclusion

As a conclusion, the reduction of the ennoblement process on stainless steel is clearly highlighted when positioning CaZnAlg material near the steel surface in natural and renewed seawater. A mass loss of only 4% is reported after one month indicating the long-term activity of the biomaterial. This result greatly exceeds our expectations following our laboratory tests, carried out in a well-controlled and closed environment, and opens up prospects for new developments of this environmentally friendly hydrogel for in situ applications in real conditions. As demonstrated, a higher content of biocide agent in the alginate does not provide a better efficiency. In contrast, the influence of the anti-adhesive property of CaZnAlg alginate associated with its low solubility seems to be the most important criterion for the development of such materials. However, characterization of the biofilm property and it's composition will be our next goal for research as these data will be key to further understand and characterize the interaction between the biofilm and the active material.

Fig. 1 (

 1 Fig. 1(a) displays the variations of the Ecorr potential of steel coupons with the immersion time. Without any alginate hydrogels in the surrounding environment, as shown by the pink curve, a potential ennoblement with values above 0.3 V/SCE is obtained after 12 days of immersion time. This behavior is frequently observed during the biofilm formation on stainless

Fig. 1 .

 1 Fig. 1. a) Ecorr potential variation versus immersion time in natural seawater for a steel coupon without any additional material (reference), with zinc alginate (ZnAlg) and calcium/zinc alginate (CaZnAlg) in vicinity of the coupons. b,c,d) SEM images of these three steel coupons showing the presence of diatoms. e) marine bacteria visible on the film treated by CaZnAlg.

  Large area images show that the surfaces are fully covered by a biofilm. This leads to an increase of the roughness with respect to the initial surface. Several regions of the surfaces were probed at the micrometer scale. Fig.1(b,c,d) displays some representative images of the surfaces for the three cases discussed above. As observed, the surfaces are covered by a complex matrix composed of biomacromolecules and microorganisms. Pennate diatoms dominate and this is in agreement with Landoulsy et al. [33 and references therein]. Pennate diatoms have the ability to attach to the substrate via extracellular polymeric substances which are secreted by their raphe slits. In particular, Actinocyclus exiguous and Rhizosoleniineae were identified in the present work. The influence of CaZnAlg on the biodiversity within the biofilm is depicted in Fig 1(e). Indeed, as observed, the SEM image does not display only diatoms but also the presence of bacteria in the surface. Such bacteria were absent from the two other surfaces (untreated and treated by zinc alginate). The lower value of Ecorr achieved with the presence of

Table 1 .

 1 . In this case, XRD patterns reveal the presence of nanoparticles. In the present work, XRD patterns were measured but do not present any structure indicating the absence of such nanoparticles. EDX results achieved on ZnAlg and CaZnAlg.

		C	O	Zn	Ca	Cl
	ZnAlg	65.8	32.6	1.6
	CaZnAlg	58.0	34.0	0.8	4.1	3.1

Acknowledgements

This work is supported by the European Union through the European Regional Development Fund (ERDF), the Ministry of Higher Education and Research, the Région Bretagne, Biogenouest, the Conseil général du Finistère and Brest Métropole Océane, through the CPER Project 2015-2020 MATECOM. This work was supported by the Francophone University Agency and the research council of the Saint Joseph University.