Anita Cadoux
email: anita.cadoux@u-psud.fr

Giada Iacono-Marziano

Bruno Scaillet

Alessandro Aiuppa

Tamsin A Mather

David M Pyle

Étienne Deloule

Emanuela Gennaro

Antonio Paonita

Etienne Deloule

The role of melt composition on aqueous fluid vs. silicate melt partitioning of bromine in magmas

Keywords: bromine, fluid/melt partitioning, degassing, arc magmas, atmospheric chemistry

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

Introduction

Volcanic degassing is an important process in sustaining the composition of Earth's atmosphere (e.g., [START_REF] Gaillard | A theoretical framework for volcanic degassing chemistry in a comparative planetology perspective and implications for planetary atmospheres[END_REF][START_REF] Mather | Volcanoes and the environment: Lessons for understanding Earth's past and future from studies of present-day volcanic emissions[END_REF]. Whilst much progress has been made constraining global volcanic fluxes, uncertainties remain regarding the emissions of the key halogen species, especially the trace Br-and I-bearing species [START_REF] Pyle | Halogens in igneous processes and their fluxes to the atmosphere and oceans from volcanic activity: A review[END_REF].

However, improvements in remote sensing techniques and analytical techniques, and their application to an increasing number of active volcanoes, have provided new data on the concentrations of these minor components in volcanic gases (e.g., [START_REF] Gerlach | Volcanic sources of tropospheric ozone-depleting trace gases[END_REF][START_REF] Aiuppa | Emission of bromine and iodine from Mount Etna volcano[END_REF][START_REF] Aiuppa | Degassing of halogens from basaltic volcanism: insights from volcanic gas observations[END_REF][START_REF] Bobrowski | Gas emission strength and evolution of the molar ratio of BrO/SO 2 in the plume of Nyiragongo in comparison to Etna[END_REF], which in turn can be used to better constrain their global fluxes to the atmosphere [START_REF] Pyle | Halogens in igneous processes and their fluxes to the atmosphere and oceans from volcanic activity: A review[END_REF]. Bromine has received particular attention over the last decade, owing to its important role in atmospheric chemistry in general (e.g., [START_REF] Oppenheimer | BrO formation in volcanic plumes[END_REF][START_REF] Roberts | Modelling reactive halogen formation and ozone depletion in volcanic plumes[END_REF]2014) and ozone depletion in the troposphere and stratosphere in particular [START_REF] Von Glasow | The effects of volcanic eruptions on atmospheric chemistry[END_REF][START_REF] Kutterolf | Combined bromine and chlorine release from large explosive volcanic eruptions: A threat to stratospheric ozone?[END_REF][START_REF] Cadoux | Stratospheric Ozone destruction by the Bronze-Age Minoan eruption (Santorini Volcano, Greece)[END_REF]. Global compilations show that Br sources (emissions to the atmosphere) and sinks (removal routes from the atmosphere) are not strictly balanced, hinting at a missing natural source of Br [START_REF] Montzka | Ozone-depleting substances (ODSs) and related chemicals[END_REF]. The direct detection of HBr and BrO in volcanic plumes [START_REF] Bobrowski | Detection of bromine monoxide in a volcanic plume[END_REF][START_REF] Aiuppa | Emission of bromine and iodine from Mount Etna volcano[END_REF] suggests that volcanic activity may be one such a source.

The correct evaluation of the contribution of past volcanic eruptions to atmospheric chemistry depends on our ability to evaluate Br behaviour in magmas, in particular its partitioning between silicate melt and gas phases. So far, only a few experimental studies have been performed on this topic, and have investigated Br behaviour in synthetic albite to rhyolite melt compositions [START_REF] Bureau | Volcanic degassing of bromine and iodine: experimental fluid/melt partitioning data and applications to stratospheric chemistry[END_REF][START_REF] Bureau | An experimental study of bromine behaviour in watersaturated silicic melts[END_REF]. However, natural silicate melt compositions can depart significantly from such model systems, in particular by having elevated contents of Fe, Mg or Ca, which (as Na) can complex with halogens thereby enhancing their solubility in silicate melts [START_REF] Cochain | Bromine speciation in hydrous silicate melts at high pressure[END_REF]. The relationship between halogen solubility and their complexation with cations has been shown for Cl; chlorine solubility in most silicate melts is dominantly controlled by the abundances of Mg ~ Ca > Fe > Na > K > network-forming Al > Li ~ Rb ~ Cs, but Ti, F, and P also have strong influences (e.g., [START_REF] Webster | Chloride and water solubility in basalt and andesite melts and implications for magmatic degassing[END_REF]Webster and De Vivo, 2002). There is thus a need to evaluate the role of melt composition on Br behaviour in magmas, which is the main motivation of the present study. To that end, we have performed fluid/melt partitioning experiments on natural basalt, andesite and rhyodacite compositions under P-T-H 2 O-redox storage conditions relevant to shallow arc magmas. Combining our Br partition coefficient for the basaltic composition, with other experimental data on S and Cl behaviour, and volcanic gas compositions from the literature, we develop a simple first-order model to predict the S-Cl-Br degassing behaviour in mafic systems. We also measure Br contents of melt inclusions from Etna, Stromboli, Merapi and Santorini eruptions and estimate the mass of bromine in the preeruptive magmas, this allows us to address the atmospheric contribution of open-vent mafic volcanoes versus that of intermediate-silicic volcanoes.

Fluid/melt partitioning experiments

Starting material

The selected starting materials are natural volcanic rocks: a hawaiitic basalt from a 2002 Etna eruption (Lesne et al., 2011a, b;[START_REF] Iacono-Marziano | New experimental data and semi-empirical parameterization of H 2 O-CO 2 solubility in mafic melts[END_REF], a calc-alkaline andesite and a rhyodacite from the Santorini Upper Scoria-2 (USC-2) and Minoan eruptions, respectively [START_REF] Cadoux | A new set of standards for in-situ measurement of bromine abundances in natural silicate glasses: application to SR-XRF, LA-ICP-MS and SIMS techniques[END_REF]. The whole-rocks were crushed and ground in an agate mortar. About 10 g of the powders were melted twice (and ground in between), to ensure homogenization, in a platinum crucible at 1400 °C -1 atm for 3-4 hours in a piezoceramic oven, and quenched in cold water. The resulting dry glasses were ground to powder and constituted the starting material for both (i) bromine standard glasses synthesis [START_REF] Cadoux | A new set of standards for in-situ measurement of bromine abundances in natural silicate glasses: application to SR-XRF, LA-ICP-MS and SIMS techniques[END_REF] used to calibrate bromine analyses (section 3) and (ii) partitioning experiments. The compositions of the starting glasses are given in Table 1.

Experimental procedure

Equilibrium partitioning experiments (neglecting kinetic effects) were performed in an Internally Heated Pressure Vessel equipped with a rapid quench device at the Institut des Sciences de la Terre d'Orléans (ISTO, Orléans, France). The chosen experimental T-P-fO 2 conditions are representative of those in shallow crustal reservoirs in volcanic arc contexts [START_REF] Martel | Effects of fO 2 and H 2 O on andesite phase relations between 2 and 4 kbar[END_REF][START_REF] Di Carlo | Experimental crystallization of a high-K arc basalt: the golden pumice, Stromboli volcano (Italy)[END_REF][START_REF] Cadoux | Magma storage conditions of large Plinian eruptions of Santorini Volcano (Greece)[END_REF][START_REF] Kahl | Constraints on the Nature and Evolution of the Magma Plumbing System of Mt. Etna Volcano (1991-2008) from a Combined Thermodynamic and Kinetic Modelling of the Compositional Record of Minerals[END_REF] and are reported in Table 2: T (± 10°C) = 900, 1060 and 1200°C, P (± 2 MPa) = 100 and 200 MPa, and fO 2 estimated around the Ni-NiO (NNO) buffer, on the basis of the partial pressure of H 2 imposed in the vessel (~2 bars;[START_REF] Di Carlo | Experimental crystallization of a high-K arc basalt: the golden pumice, Stromboli volcano (Italy)[END_REF][START_REF] Cadoux | Magma storage conditions of large Plinian eruptions of Santorini Volcano (Greece)[END_REF].

We deliberately used a fluid solely composed of H 2 O and Br. Experiments with simplified fluids are necessary for comparison with future experiments which will include additional volatile species (e.g., CO 2 , S), and will permit us to assess whether or not the presence of other volatile species can modify Br behaviour.

Capsules were always loaded so that the mass ratio between the aqueous fluid and the glass (silicate) phases was equal to or lower than 0.1 (Table 2), which avoids significant silicate dissolution into fluid during experiments. About 50 to 100 mg of glass powder was loaded into Au or Au-Pd capsules (2.5 mm internal diameter, 20-30 mm in length) together with 3-8 mg of a solution composed of distilled water and dissolved NaBr salt. These amounts of solution (6-10 wt%) ensure the attainment of fluid saturation of the silicate melts at the investigated T-P conditions. Different solutions with Br contents between 0.1 and 14 wt% Br were employed. The runs lasted between 24 and 92 hours, depending on the temperature (Table 2). Chlorine partitioning experiments of [START_REF] Alletti | Chlorine partitioning between a basaltic melt and H 2 O-CO 2 fluids at Mount Etna[END_REF] performed at 1200°C with a basaltic melt showed that 3-4 hours were sufficient to attain equilibrium at 1200°C and P > 1 MPa. Considering that Br diffusion coefficient appears to be 2-5 times lower than the other halogens in basaltic melts (at 500 MPa to 1.0 GPa, 1250 to 1450 C and at anhydrous conditions, [START_REF] Alletti | Halogen diffusion in a basaltic melt[END_REF], we chose a 24 hour run duration for our experiments at 1200°C-100 MPa including the basaltic composition, to ensure the attainment of equilibrium.

For our experiments at 900°C-200 MPa including the silicic composition, a run duration of 92 hours was chosen on the basis of previous experiments with chlorine. [START_REF] Kravchuk | Distribution of chloride between aqueous fluids and felsic melts at 2 kbar and 800°C[END_REF] performed partitioning experiments with silicic melts at lower T (800°C) and same P with duration varying between 93h to 1142h, which yielded similar results, thus demonstrating that 93h was sufficient to attain equilibrium (at 800°C).

The run duration for the experiment at 1060°C (48h) with andesite and rhyodacite compositions was chosen as intermediate between that at 900°C and that at 1200°C. Experiments were terminated by drop quench [START_REF] Di Carlo | Experimental crystallization of a high-K arc basalt: the golden pumice, Stromboli volcano (Italy)[END_REF]. Upon opening the capsules, hissing and fluid escape occurred, indicating the presence of excess fluid, and thus that fluid saturation was achieved at the target P-T conditions. All runs produced crystal-free glasses, those of rhyodacitic composition being rich in fluid inclusions (Fig. A.1 in Supplementary Material).

Analytical techniques

Major element analysis

Experimental glasses and natural melt inclusions were analysed for their major elements by electron microprobe (EMP) using the joint ISTO-BRGM SX-Five microbeam facility (Orléans, France). The operating conditions were: 15 kV accelerating voltage, 4-6 nA beam current, 10 seconds counting time on peaks, 5 seconds on background. The standards used were: albite for Si and Na, TiMnO 3 for Ti and Mn, Al 2 O 3 for Al, Fe 2 O 3 for Fe, MgO for Mg, andradite for Ca and orthose for K. Alkalis were analyzed first and a defocused beam was used to minimize alkali migration: 20 µm diameter for experimental glasses, and 6-10 µm for melt inclusions. Between 5 and 10 analyses were performed on each charge. The EMP detection limit for Na (component of the aqueous solution used in the experiments and thus considered in the calculation of the Br partition coefficients; Supplementary Information) was generally < 700 ppm.

Volatile analysis

Br abundances in the experimental glasses were determined either by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) or with a Secondary Ion Mass Spectrometer (SIMS), using Br glass standards synthesized with the same starting compositions as those used for the partitioning experiments presented here (Table 1, [START_REF] Cadoux | A new set of standards for in-situ measurement of bromine abundances in natural silicate glasses: application to SR-XRF, LA-ICP-MS and SIMS techniques[END_REF]. LA-ICP-MS has been shown to be a technique suited to analyse Br contents of hundreds to thousands ppm in experimental glasses, while SIMS and synchrotron X-ray fluorescence (SR-XRF) are more appropriate techniques for lower Br contents [START_REF] Cadoux | A new set of standards for in-situ measurement of bromine abundances in natural silicate glasses: application to SR-XRF, LA-ICP-MS and SIMS techniques[END_REF]. Moreover, the spatial resolutions of SIMS and SR-XRF are significantly higher than that of LA-ICP-MS [START_REF] Cadoux | A new set of standards for in-situ measurement of bromine abundances in natural silicate glasses: application to SR-XRF, LA-ICP-MS and SIMS techniques[END_REF], we therefore analysed bromine contents in melt inclusions from Santorini, Merapi and Etna volcanoes by SIMS and SR-XRF.

The abundance of water dissolved in most of the experimental glasses was determined by SIMS.

Bromine analysis by LA-ICP-MS

LA-ICP-MS analyses were performed at the Istituto Nazionale di Geofisica e Vulcanologia (INGV, Palermo, Italy). The laser used is a Compex Pro 102, 193 nm ArF excimer laser mounted on an ablation system GeoLas Pro, which is connected to an Agilent 7500ce ICP-MS. Analyses were done on polished glass chips set in epoxy resin.

Analyses were performed with a fluence of 15 J/cm 2 and a pulse energy of 100 mJ. The samples were ablated during ~50 seconds on a 90 µm diameter area, with a pulse repetition rate of 10 Hz. Three to ten analyses were collected for every sample, to check sample homogeneity. With this configuration, Br contents of >100 ppm are quantifiable with accuracy generally within 20% [START_REF] Cadoux | A new set of standards for in-situ measurement of bromine abundances in natural silicate glasses: application to SR-XRF, LA-ICP-MS and SIMS techniques[END_REF].

Data reduction was performed using GLITTER™ software (Griffin et al., 2008), using 24 Mg as the reference element (Mg contents from EMP analyses), and in-house bromine glass standards B3000 and B6000 (with 2694 ± 5.1 and 5968 ± 3.5 ppm Br, respectively; [START_REF] Cadoux | A new set of standards for in-situ measurement of bromine abundances in natural silicate glasses: application to SR-XRF, LA-ICP-MS and SIMS techniques[END_REF], as external standards. The error on the measured 79 Br/ 24 Mg ratio was always < 4%.

Bromine analysis by SIMS

Polished chips of experimental glasses were set into indium and coated with gold, while individual crystals from Santorini Minoan eruption, Merapi 2010 eruption and Etna 2006 eruption were mounted in epoxy resin, polished and coated with gold for melt inclusion analysis.

Analyses were conducted at the Centre de Recherches Pétrographiques et Géochimiques (CRPG, Nancy, France) with a Cameca IMS 1280 HR2. The Cs + primary ion beam was accelerated at 10 kV with an intensity of 5 nA, and focused on a 15 µm diameter area. The electron gun was simultaneously used for charge compensation. Negative secondary ions were extracted with a 10 kV potential, and the spectrometer slits set for a mass resolving power (MRP = M/ΔM) of ~20,000. A single collector (EM) was used in ion-counting mode, and the spectrum scanned by peak jumping. Each analysis consisted of 8 or 6 successive cycles. Each cycle began with background measurement at the mass 75.8, followed by We used three different sets of in-house bromine glass standards [START_REF] Cadoux | A new set of standards for in-situ measurement of bromine abundances in natural silicate glasses: application to SR-XRF, LA-ICP-MS and SIMS techniques[END_REF]: a basaltic set containing 1 to 6,000 ppm Br (B1 to B6000), an andesitic set containing 10 to 1,000 ppm Br (A10 to A1000), and a rhyodacitic set containing 10 to 5,000 ppm Br (RD10 to RD5000).

The bromine content of the samples was calculated using the measured 81 Br/ 28 SiO 3 and known Br (ppm)/SiO 2 (wt%) ratios of the standards [START_REF] Cadoux | A new set of standards for in-situ measurement of bromine abundances in natural silicate glasses: application to SR-XRF, LA-ICP-MS and SIMS techniques[END_REF].

The three glass sets define distinct linear calibration curves with slopes decreasing with increasing degree of melt polymerization [START_REF] Cadoux | A new set of standards for in-situ measurement of bromine abundances in natural silicate glasses: application to SR-XRF, LA-ICP-MS and SIMS techniques[END_REF]. The equation of the calibration lines passing through zero is:

where the slope a is a function of SiO 2 content.

The error on the measured 81 Br/ 28 SiO 3 ratio was generally < 3% (most often < 2%).

Br analysis by SR-XRF

Bromine in Etna and Stromboli melt inclusions was analysed via SR-XRF at the UK national synchrotron facility, Diamond Light Source (Didcot, Oxfordshire), on I18, the Microfocus Spectroscopy beamline. Analyses were performed on polished olivine-hosted melt inclusions set in epoxy resin, using a beam of ~55 µm 2 and an analysis time of 120 s (details in [START_REF] Cadoux | A new set of standards for in-situ measurement of bromine abundances in natural silicate glasses: application to SR-XRF, LA-ICP-MS and SIMS techniques[END_REF]. Fluorescence spectra were processed by PyMca (Solé et al., 2007), by identifying the K-lines of Br and applying an iterative Gaussian peak fitting procedure to quantify the net peak areas free of background and interference from other elements.

Background-and baseline-subtracted net peak areas for the MI samples were converted into Br concentrations from comparison with peak areas measured for the basaltic standards of known Br composition [START_REF] Cadoux | A new set of standards for in-situ measurement of bromine abundances in natural silicate glasses: application to SR-XRF, LA-ICP-MS and SIMS techniques[END_REF]. Based on tests made on the standards [START_REF] Cadoux | A new set of standards for in-situ measurement of bromine abundances in natural silicate glasses: application to SR-XRF, LA-ICP-MS and SIMS techniques[END_REF], the Br detection limit is inferred to be < 1 ppm, Br contents ≤ 5 ppm (representative of Br contents in most natural volcanic glasses) are measured with an accuracy < 26% and with a precision of 30%.

Water analysis by SIMS

The analysis of water dissolved in the experimental glasses was performed on the CRPG Cameca 1280 HR2. Spot analyses of secondary ions 17 O, 16 O 1 H, 18 O, 29 Si, 30 Si were obtained using a 3 nA, 20 µm diameter primary beam of Cs + ions. The electron gun was simultaneously used for charge compensation. The measurements were made at a mass resolution of ~7,700 to separate 17 O -and 16 O 1 H -. An energy filtering was set at +30 ±10 eV in moving the energy slit off axis, to minimize both matrix effect and instrumental background.

A 10  10 µm raster was used for 1 minute prior to analysis at each spot in order to presputter through the gold coat and remove surface contamination. The beam position in the field aperture and the magnetic field centering was checked before each measurement. Each analysis on one spot consisted of 18 cycles of measurements, with counting times and switching times of 3 and 1 s respectively at each peak. The errors on the measured

Results

Major element compositions

Average major element compositions of experimental glasses and melt inclusions are presented in Tables 3 andA2, respectively. Comparison of the experimental products (Table 3) with the corresponding starting dry glasses (Table 1) shows a systematic gain in Na 2 O due to the addition of sodium through the H 2 O-NaBr solution in the experimental charges (10 to 153 g of Na were incorporated into the final glass, Table A.1), and a loss in FeO (up to 14% taking into account standard deviations) in experiments ≥ 1060 °C, most likely due to Fe alloying with the AuPd capsule. Other elements were generally comparable with the starting glasses within standard deviations.

H 2 O and Br dissolved in experimental glasses

The concentrations of both H 2 O and Br dissolved in quenched glasses are reported in Table 2. Melt water contents (H 2 O melt ) vary between 2.9 up to 7.3 wt%, encompassing the range of pre-eruptive H 2 O melt of arc magmas (e.g., Scaillet et al., 1998;[START_REF] Di Carlo | Experimental crystallization of a high-K arc basalt: the golden pumice, Stromboli volcano (Italy)[END_REF][START_REF] Cadoux | Magma storage conditions of large Plinian eruptions of Santorini Volcano (Greece)[END_REF]. Bromine concentrations range from 69 (M4-RD1; Table 2) to 9112 ppm (M2-B), comparable to the range explored in previous experimental studies [START_REF] Bureau | Volcanic degassing of bromine and iodine: experimental fluid/melt partitioning data and applications to stratospheric chemistry[END_REF][START_REF] Bureau | An experimental study of bromine behaviour in watersaturated silicic melts[END_REF]. Repeat analyses of either H 2 O (n = 5-7) or Br (n = 3-10) in quenched glasses show them to be homogeneous within analytical uncertainty (including error on calibration and error on sample measurements), indicating that the run durations (1 to 4 days according to T-P conditions; Table 2) were sufficient to attain chemical equilibrium between the co-existing melt and fluid phases.

Fluid/melt bromine partitioning

Assessment of the partition coefficients requires the calculation of the ratio between Br concentration (in ppm) in coexisting aqueous fluid and silicate melt at the experimental conditions. In natural magmatic systems, asides from being dissolved in the silicate melt, Br can exist as a gas species in the vapour phase and/or be dissolved in hydrosaline liquid phase (brine), as has been shown for Cl. This raises the question as to whether a brine rich in Br could have been present in our capsules at run conditions. Solubility experiments of [START_REF] Bureau | An experimental study of bromine behaviour in watersaturated silicic melts[END_REF] have shown that silicic melts can contain up to 7850 ppm Br at P-T conditions of 100-200 MPa and 900-1080°C without brine occurrence. Our experiments were run at similar conditions, with Br contents lower than 2000 ppm (Table 2). We thus conclude that our silicic melts did not co-exist with a brine, and that only a gas phase was present. There is no data about Br solubility in mafic melts but the results of our experiment #3 (basalt, andesite, rhyodacite in the same experimental P-T-H 2 O-[Br°] conditions; Table 2) suggest a higher ability for Br to enter in mafic melts, so that, by comparison with the values for silicic melts given above, we might expect that brine saturation in andesitic and basaltic melts occurs for Br contents higher than 8000 ppm. Hence, as for the silicic melts, we conclude that basalticandesitic charges were co-existing solely with a gas phase during the experiments.

Whereas the amount of Br dissolved in the melt ([Br] melt ) was directly analysed (by SIMS or LA-ICP-MS in run product glasses, section 3; referred to as '[Br] measured in final glass' in for the others): the mass of initial fluid is small relative to the weighing error (Table A1).

Therefore, the calculated mass of fluid has a larger associated relative error of 19% (<15% for the other run products), which increases the error on the calculated value for [Br] The main data of the partitioning experiments are listed in Table 2 (details in Table A1) and displayed on Figures 1 to 5.

The basaltic products of runs #1 to 3 performed at 1200°C, 100 MPa, with various [Br°] contents apparently show a linear relationship between the measured [Br] melt and the calculated [Br] fluid (Fig. 1). Linear regression forced through zero yields a D Br f/m = 5.0 ± 0.3 for the basaltic composition. At the same T-P-[Br°] conditions (experiment #3; Table 2), D Br f/m increases steadily from basalt (D Br f/m = 4.6 ± 2.0), to andesite (D Br f/m = 6.4 ± 3.5), to rhyodacite (D Br f/m = 11.3 ± 0.9). At 1060°C, 200 MPa, ~NNO (Fig. 2), experiments on andesite and rhyodacite melts yields linear trends between [Br] melt and [Br°] (Fig. 2a) or

[Br] fluid (Figs. 2b and2c). Linear regression of the latter data forced through zero yields D Br f/m = 9.1 ± 0.6 for andesite, and D Br f/m = 14.0 ± 0.6 for rhyodacite. At 900°C -200 MPa, (Fig. 3), the same pattern is again observed resulting in D Br f/m = 20.2 ± 1.2 for rhyodacite, slightly higher than that of [START_REF] Bureau | Volcanic degassing of bromine and iodine: experimental fluid/melt partitioning data and applications to stratospheric chemistry[END_REF] for albitic melts (17.5 ± 0.6).

The experiments show a two-fold increase of D Br f/m from basalt (5.0 ± 0.3) to rhyolite melts (11.3 ± 0.9) at 1200°C -100 MPa (Fig. 4). The same trend of an increase of D Br f/m with SiO 2 is noted at 1060°C -200 MPa, with a slightly higher slope. At 900°C, we did not work on basaltic/andesitic compositions because of their extensive crystallization (which would have driven the residual liquids toward higher SiO 2 content). Our data on silicic compositions, along with those of [START_REF] Bureau | Volcanic degassing of bromine and iodine: experimental fluid/melt partitioning data and applications to stratospheric chemistry[END_REF] (Fig. 4), suggest a similar trend of increasing D Br f/m with increasing SiO 2 at 900°C. Figure 4 also suggests a general trend of an increase in D Br f/m as temperature decreases, at least for more silicic compositions. For instance, for rhyolitic melts (71 wt% SiO 2 ), a linear extrapolation of our data set (D Br f/m = -0.0299  T(°C) + 46.673, r 2 = 0.97) yields D Br f/m = 26 at 700°C (Fig. 5).

We did not attempt to explore the effect of pressure in a systematic way. Previous work has shown that D Br f/m strongly increases as pressure decreases, from about 20 at 200 MPa up to over 300 at near atmospheric pressure in silicic melts (synthetic haplogranitic composition; [START_REF] Bureau | Bromine cycle in subduction zones through in situ Br monitoring in diamond anvil cells[END_REF]. In contrast, our experiment at 100 MPa (experiment #3) does not show any significant increase in D Br f/m , with D Br f/m being instead generally lower than those at 200

MPa (experiments #4 and #5; Table 2). However, the fact that temperatures between our 100 and 200 MPa runs are different, does not allow us to make definitive conclusions on this aspect. We suggest that our results should be used to model degassing processes in the crustal reservoir and not for simulating decompression processes between the reservoir and surface. Merapi and Santorini volcano are in pyroxenes and plagioclase crystals, respectively.

Br contents in melt inclusions

Br abundance ranges from 2.5 to 10 ppm, without any clear correlation with melt inclusion major element composition; there is no notable difference between basaltic, andesitic and rhyodacitic melt inclusions. The Br contents of these melt inclusions are comparable to those of submarine back-arc and arc glasses [START_REF] Kendrick | Subduction-related halogens (Cl, Br and I) and H 2 O in magmatic glasses from Southwest Pacific Backarc Basins[END_REF].

Discussion and applications

Halogen behaviour

This section aims to place our novel Br partitioning data in the wider framework of halogen behaviour. Hereafter, we provide a brief, non-exhaustive review of chlorine, fluorine and iodine partitioning and make comparisons with bromine.

Many studies have been dedicated to understanding the partitioning behaviour of halogens between fluids and silicate melts (e.g., [START_REF] Webster | Partitioning of F between H 2 O and CO 2 fluids and topaz rhyolite melt[END_REF][START_REF] Webster | Partitioning of F and Cl between hydrothermal fluids and highly evolved granitic magmas[END_REF]Webster 1992a,b;[START_REF] Webster | Chloride and water solubility in basalt and andesite melts and implications for magmatic degassing[END_REF][START_REF] Signorelli | Solubility and fluid-melt partitioning of Cl in hydrous phonolitic melts[END_REF][START_REF] Bureau | Volcanic degassing of bromine and iodine: experimental fluid/melt partitioning data and applications to stratospheric chemistry[END_REF][START_REF] Botcharnikov | Sulfur and chlorine solubility in Mt. Unzen rhyodacitic melt at 850°C and 200 MPa[END_REF][START_REF] Botcharnikov | The effect of CO 2 on the solubility of H 2 O-Cl fluids in andesitic melt[END_REF][START_REF] Botcharnikov | Solubility and fluid-melt partitioning of H 2 O and Cl in andesitic magmas as a function of pressure between 50 and 500 MPa[END_REF]Dolejs and Baker 2007a,b;[START_REF] Alletti | Experimental investigation of halogen diffusivity and solubility in Etnean basaltic melts[END_REF][START_REF] Stelling | Solubility of H 2 O-and chlorine-bearing fluids in basaltic melt of Mount Etna at T = 1050-1205°C and P = 200 MPa[END_REF]Chevychelov et al., 2008b;[START_REF] Webster | Partitioning behavior of chlorine and fluorine in the system apatite-melt-fluid. II: Felsic silicate systems at 200 MPa[END_REF][START_REF] Borodulin | Experimental study of partitioning of tantalum, niobium, manganese, and fluorine between aqueous fluoride fluid and granitic and alkaline melts[END_REF][START_REF] Alletti | Chlorine partitioning between a basaltic melt and H 2 O-CO 2 fluids at Mount Etna[END_REF][START_REF] Alletti | Chloride partitioning and solubility in hydrous phonolites from Erebus volcano: A contribution towards a multicomponent degassing model[END_REF][START_REF] Beermann | The solubility of sulfur and chlorine in H 2 O-bearing dacites of Krakatau and basalts of Mt[END_REF]Zajacz et al. 2012;[START_REF] Webster | C-O-H-Cl-S-F volatile solubilities, partitioning, and mixing in phonolitic-trachytic melts and aqueouscarbonic vapor ± saline liquid at 200 MPa[END_REF][START_REF] Beermann | Partitioning of sulfur and chlorine between aqueous fluid and basaltic melt at 1050°C, 100 and 200 MPa[END_REF].

Nevertheless, most of these studies have focused on chlorine, mainly because of its importance as a ligand for ore metals (e.g., [START_REF] Carroll | Solubilities of sulfur, noble gases, nitrogen, chlorine, and fluorine in magmas[END_REF][START_REF] Aiuppa | Halogens in volcanic systems[END_REF].

General behaviour

Chlorine partitions preferentially into fluids relative to melts for the vast majority of terrestrial magmas at shallow-crustal pressure and temperature conditions, due to its highly solubility in aqueous and aqueous-carbonic fluids (e.g., [START_REF] Webster | Halogens in Mafic and Intermediate-silica Content Magmas[END_REF] and references therein). The few partitioning experiments performed with bromine and iodine show a similar behaviour [START_REF] Bureau | Volcanic degassing of bromine and iodine: experimental fluid/melt partitioning data and applications to stratospheric chemistry[END_REF][START_REF] Bureau | Modern and past volcanic degassing of iodine[END_REF]this study). In contrast, fluorine concentrations in aqueous and aqueous-carbonic fluids at magmatic conditions are much lower than those of the other halogens (e.g., [START_REF] Carroll | Solubilities of sulfur, noble gases, nitrogen, chlorine, and fluorine in magmas[END_REF], and can be therefore enriched in the silicate melt with respect to the fluid phase (e.g., [START_REF] Webster | Partitioning of F between H 2 O and CO 2 fluids and topaz rhyolite melt[END_REF][START_REF] Webster | Partitioning of F and Cl between hydrothermal fluids and highly evolved granitic magmas[END_REF].

The effect of melt composition

In this study, we show that the partitioning of bromine between aqueous fluid and melt appears to be influenced by the melt composition (Figs. 1, 2, 4). We estimate D Br f/m of 4.6 ± 2.0, 6.4 ± 3.5 and 11.3 ± 0.9 for basaltic, andesitic and rhyodacitic compositions respectively, with [Br°] = 2.4 wt.%, at 1200°C, 100 MPa and ƒO 2 close to NNO (exp. #3, Table 2). If D Br f/m of the basalt and andesite are comparable taking into account the error bars, the difference between the mafic-intermediate compositions on one hand, and the silicic composition on the other hand, is significant. In addition, we observe the same trend at 1060°C, 200 MPa where D Br f/m of the silicic composition (14.0 ± 0.6) is significantly higher than that of the intermediate composition (9.1 ± 0.6). This relationship between melt composition and Br partitioning is consistent with the higher Br solubility in melts with lower SiO 2 observed by [START_REF] Bureau | An experimental study of bromine behaviour in watersaturated silicic melts[END_REF] . The recent study of [START_REF] Cochain | Bromine speciation in hydrous silicate melts at high pressure[END_REF] on Br speciation in hydrous alkali silicic melts at high pressure (up to 7.6 GPa) confirms this trend. Similarly, several studies have demonstrated the strong effect of melt composition on fluid/melt partitioning of chlorine (Webster 1992a,b;[START_REF] Webster | Chloride and water solubility in basalt and andesite melts and implications for magmatic degassing[END_REF]. Like D Br f/m , D Cl f/m also increases with increasing SiO 2 contents of the melts (i.e., with increasing melt polymerization and thus decreasing Br and Cl solubility in melt; e.g., Webster, 1992a,b;[START_REF] Signorelli | Solubility and fluid-melt partitioning of Cl in hydrous phonolitic melts[END_REF][START_REF] Botcharnikov | Sulfur and chlorine solubility in Mt. Unzen rhyodacitic melt at 850°C and 200 MPa[END_REF][START_REF] Webster | The exsolution of magmatic hydrosaline melts[END_REF]Webster et al., 2006;[START_REF] Webster | Partitioning behavior of chlorine and fluorine in the system apatite-melt-fluid. II: Felsic silicate systems at 200 MPa[END_REF]. [START_REF] Webster | Chloride and water solubility in basalt and andesite melts and implications for magmatic degassing[END_REF][START_REF] Stelling | Solubility of H 2 O-and chlorine-bearing fluids in basaltic melt of Mount Etna at T = 1050-1205°C and P = 200 MPa[END_REF]Chevychelov et al., 2008b;[START_REF] Alletti | Chlorine partitioning between a basaltic melt and H 2 O-CO 2 fluids at Mount Etna[END_REF][START_REF] Beermann | The solubility of sulfur and chlorine in H 2 O-bearing dacites of Krakatau and basalts of Mt[END_REF][START_REF] Beermann | Partitioning of sulfur and chlorine between aqueous fluid and basaltic melt at 1050°C, 100 and 200 MPa[END_REF]; with values >160 determined for silicic melts at 200

MPa (Webster, 1992a). Note that D Cl f/m also varies strongly with Cl concentration (Webster 1992a,b;[START_REF] Webster | Chloride and water solubility in basalt and andesite melts and implications for magmatic degassing[END_REF][START_REF] Stelling | Solubility of H 2 O-and chlorine-bearing fluids in basaltic melt of Mount Etna at T = 1050-1205°C and P = 200 MPa[END_REF]) and a decrease of D Cl f/m by up to the order of one magnitude can be observed when the Cl concentration in the system decreases (from Unlike bromine and chlorine, values of D f/m for fluorine are lower in silicic melts (typically well below unity; [START_REF] Webster | Partitioning of F between H 2 O and CO 2 fluids and topaz rhyolite melt[END_REF][START_REF] Webster | Partitioning of F and Cl between hydrothermal fluids and highly evolved granitic magmas[END_REF]Dolejs and Baker 2007a,b;[START_REF] Borodulin | Experimental study of partitioning of tantalum, niobium, manganese, and fluorine between aqueous fluoride fluid and granitic and alkaline melts[END_REF] than in mafic melts coexisting with aqueous fluids (ca. 3 to 38; [START_REF] Alletti | Experimental investigation of halogen diffusivity and solubility in Etnean basaltic melts[END_REF]Chevychelov et al., 2008b).

Temperature and pressure effects

Our data suggest that D Br f/m is sensitive to melt temperature (increase of D Br f/m with decreasing temperature; Figs. 4 and5), though more experiments are required to confirm this trend.

Currently there are insufficient data available to constrain the temperature effect for the other halogens. The few existing data concern Cl in phonolitic and trachybasaltic melts and suggest that there is no strong influence of temperature (Chevychelov et al., 2008a;[START_REF] Stelling | Solubility of H 2 O-and chlorine-bearing fluids in basaltic melt of Mount Etna at T = 1050-1205°C and P = 200 MPa[END_REF].

We do not systematically investigate the effect of pressure on D Br f/m

. Experiments conducted on haplogranite melts coexisting with iodine-bearing aqueous fluids indicate an increase of D f/m of iodine with pressure decrease (from ~2 at 1.5 GPa to 41 at 0.1 GPa; [START_REF] Bureau | Modern and past volcanic degassing of iodine[END_REF]. Contrastingly, D f/m of fluorine decreases with decreasing pressure as suggested by experiments on trachybasaltic melts [START_REF] Alletti | Experimental investigation of halogen diffusivity and solubility in Etnean basaltic melts[END_REF]Chevychelov et al., 2008b). Available data for chlorine show no clear pressure effect on D Cl f/m for most compositions and contrasting effects for phonolitic ones [START_REF] Signorelli | Solubility and fluid-melt partitioning of Cl in hydrous phonolitic melts[END_REF][START_REF] Baker | Fluid saturation and volatile partitioning between melts and hydrous fluids in crustal magmatic systems: The contribution of experimental measurements and solubility models[END_REF][START_REF] Alletti | Chloride partitioning and solubility in hydrous phonolites from Erebus volcano: A contribution towards a multicomponent degassing model[END_REF].

We conclude that more systematic experiments are necessary for all halogens to assess the effect of pressure and temperature on their fluid/melt partitioning, in order to interpret degassing processes of ascending and cooling magmas comprehensively.

Effect of fluid composition

Experiments with trachybasaltic melts coexisting with aqueous fluids have shown that the addition of CO 2 to the system leads significant reductions of D f/m of fluorine.

Several studies have investigated the effect of fluid composition on Cl partitioning in chemically complex O-H ± C ± S ± Cl fluids and show variable influence of CO 2 or S on

D Cl f/m
, no systematic trend appears (e.g., [START_REF] Botcharnikov | Sulfur and chlorine solubility in Mt. Unzen rhyodacitic melt at 850°C and 200 MPa[END_REF][START_REF] Webster | Volatiles, magmatic degassing and eruptions of Mt. Somma-Vesuvius: constraints from silicate melt inclusions, solubility experiments and modeling[END_REF][START_REF] Botcharnikov | The effect of CO 2 on the solubility of H 2 O-Cl fluids in andesitic melt[END_REF][START_REF] Alletti | Chlorine partitioning between a basaltic melt and H 2 O-CO 2 fluids at Mount Etna[END_REF][START_REF] Beermann | The solubility of sulfur and chlorine in H 2 O-bearing dacites of Krakatau and basalts of Mt[END_REF], Zajacz et al. 2012;[START_REF] Webster | C-O-H-Cl-S-F volatile solubilities, partitioning, and mixing in phonolitic-trachytic melts and aqueouscarbonic vapor ± saline liquid at 200 MPa[END_REF]. As we do not explore the effect of other volatile species on Br partitioning in this study, we will not enter into the details of those studies (for a review on this topic, see [START_REF] Webster | Halogens in Mafic and Intermediate-silica Content Magmas[END_REF]. Clearly, further investigations are needed to better understand the effects of other volatile species on halogens partitioning.

Role of ionic radius?

On the basis of their results (D Cl f/m = 8.1, D Br f/m = 17.5, D I f/m =104 with an albitic melt), [START_REF] Bureau | Volcanic degassing of bromine and iodine: experimental fluid/melt partitioning data and applications to stratospheric chemistry[END_REF] suggested that bromine and iodine partitioned even more strongly into the fluid phase than chlorine and that it could be correlated to the increasing ionic radius of the halide ions (Cl -= 1.81 Å, Br -= 1.96 Å, I -= 2.20 Å; [START_REF] Shannon | Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides[END_REF]. Nevertheless, our brief review above shows that chlorine fluid/melt partition coefficients as high as that of iodine may be reached with a rhyodacitic melt (D Cl f/m = 115, Table 2 in Webster at al., 2009) depending on the initial Cl content in the bulk system. In addition, we show in section 5.2.

below that the range of Br and Cl composition of volcanic gases and melts from mafic systems requires a lower D Br f/m (= 5) than the D Cl f/m value (8.6); this questions the general applicability of the higher volatility of Br as shown by the experimental results of [START_REF] Bureau | Volcanic degassing of bromine and iodine: experimental fluid/melt partitioning data and applications to stratospheric chemistry[END_REF] and [START_REF] Mungall | Experimental evidence for the chalcophile behavior of the halogens[END_REF] and attributed to the larger ionic radius of Br vs.

Cl.

S-Cl-Br degassing behaviour in mafic magma systems

Our partition coefficients for Etnean melts set the basis for initializing the first basic models to evaluate Br degassing behaviour in mafic systems. Our aim is to derive modelbased evidence for Br abundance in magmatic gases coexisting with mafic melts at shallow crustal conditions, and to compare this with available information on the measured compositions of volcanic gases, the ultimate product of magmatic degassing. Figure 6a shows a selection of volcanic gas plume compositions (in the S-Cl-Br system) from some open-vent mafic volcanoes (for data provenance, see caption of Figure 6). The wide range of volcanic gas S/Br compositions observed points to a mechanism fractionating Br, relative to sulfur, during magmatic degassing or systematic variations in melt compositions, e.g., between different tectonic settings. In comparison, volcanic gases exhibit a far more restricted range of Cl/Br ratios (see Fig. 6a and [START_REF] Gerlach | Volcanic sources of tropospheric ozone-depleting trace gases[END_REF][START_REF] Aiuppa | Emission of bromine and iodine from Mount Etna volcano[END_REF][START_REF] Aiuppa | Halogens in volcanic systems[END_REF][START_REF] Webster | Halogens in Mafic and Intermediate-silica Content Magmas[END_REF], which suggests that less Cl/Br fractionation takes place during degassing and/or less comparative variation in melt compositions. The relatively constant Cl/Br ratios in our gas dataset also indicate that Cl vs. Br decoupling due to fractionations among coexisting brine and vapour [START_REF] Foustoukos | Trace element partitioning between vapor, brine and halite under extreme phase separation conditions[END_REF][START_REF] Seo | Fractionation of Cl/Br during fluid phase separation in magmatichydrothermal fluids[END_REF], or by halite precipitation [START_REF] Foustoukos | Trace element partitioning between vapor, brine and halite under extreme phase separation conditions[END_REF], are unlikely to occur at the mafic, halogen-poor melt conditions explored here [START_REF] Webster | Partitioning behavior of chlorine and fluorine in the system apatite-melt-fluid. II: Felsic silicate systems at 200 MPa[END_REF][START_REF] Webster | Halogens in Mafic and Intermediate-silica Content Magmas[END_REF]. We consider below our new Br partitioning data, in tandem with previous information on S and Cl from the literature, to provide a simple model verification for these volcanic gas-based inferences.

Rigorous quantitative calculation of magmatic gas compositions would require a theoretical and/or empirical model that describes solubilities, fluid/melt partition coefficient, and diffusivities of all involved volatiles over the range of P-T-X conditions experienced by magmas upon ascent, storage, and eruption. Such quantitative information is increasingly available for S (see review of [START_REF] Baker | Modeling the Solubility of Sulfur in Magmas: A 50-Year Old Geochemical Challenge[END_REF], still limited for Cl [START_REF] Webster | Chloride and water solubility in basalt and andesite melts and implications for magmatic degassing[END_REF][START_REF] Webster | Experimental and modeled chlorine solubilities in aluminosilicate melts at 1 to 7000 bars and 700 to 1250 °C: Applications to magmas of Augustine Volcano, Alaska[END_REF][START_REF] Webster | Halogens in Mafic and Intermediate-silica Content Magmas[END_REF], but virtually absent for Br. Given this limitation, we base our model calculations on a modified version of the empirical degassing model of [START_REF] Aiuppa | S, Cl and F degassing as an indicator of volcanic dynamics: the 2001 eruption of Mount Etna[END_REF] and [START_REF] Aiuppa | Degassing of halogens from basaltic volcanism: insights from volcanic gas observations[END_REF]. The original model described the evolution of the SO 2 -HCl-HF magmatic gas phase exsolved during progressive degassing of a basaltic magma, using a Rayleigh-type open-system degassing model assumption, and with constant S, Cl and F fluid/melt partition coefficients. Based on fair agreement between model results and volcanic gas compositions, it was concluded that a Rayleigh-type open-system process could suitably reproduce the relatively shallow exsolution of halogens from basaltic magmas that often dominates the gas signature [START_REF] Métrich | Volatile abundances in basaltic magmas and their degassing paths tracked by melt inclusions[END_REF][START_REF] Métrich | Crystallisation driven by decompression and water loss at Stromboli volcano (Aeolian Islands)[END_REF][START_REF] Métrich | 2001 flank eruption of the alkali-and volatile-rich primitive basalt responsible for Mount Etna's evolution in the last three decades[END_REF][START_REF] Métrich | Conditions of magma storage, degassing and ascent at Stromboli: new insights into the volcano plumbing system with inferences on the eruptive dynamics[END_REF][START_REF] Spilliaert | S-Cl-F degassing pattern of water rich alkali basalt: modelling and relationship with eruption styles of Mount Etna volcano[END_REF][START_REF] Edmonds | Halogen degassing during ascent and eruption of water-poor basaltic magma[END_REF][START_REF] Webster | Chloride and water solubility in basalt and andesite melts and implications for magmatic degassing[END_REF][START_REF] Webster | Experimental and modeled chlorine solubilities in aluminosilicate melts at 1 to 7000 bars and 700 to 1250 °C: Applications to magmas of Augustine Volcano, Alaska[END_REF][START_REF] Mather | Halogens and trace metal emissions from the ongoing 2008 summit eruption of Kīlauea volcano[END_REF].

Here we adapt and extend the methodology of [START_REF] Aiuppa | Degassing of halogens from basaltic volcanism: insights from volcanic gas observations[END_REF] to bromine, and develop a simple model to account for the variability of S-Cl-Br compositions of volcanic gases (Fig. 6a). We use similar sets of Rayleigh-type open-system equations as in [START_REF] Aiuppa | Degassing of halogens from basaltic volcanism: insights from volcanic gas observations[END_REF] but, contrarily to previous work, we do not derive fluid/melt partition coefficients using an empirical best-fit procedure to volcanic gas data, but rather use independent information (from [START_REF] Alletti | Chlorine partitioning between a basaltic melt and H 2 O-CO 2 fluids at Mount Etna[END_REF][START_REF] Aiuppa | Degassing of halogens from basaltic volcanism: insights from volcanic gas observations[END_REF]and this work) (see below).

We use equations ( 1) and (2) to calculate the evolving S/Cl and S/Br (molar) ratios in the magmatic gas phase produced upon increasing extents of degassing of a mafic silicate melt:

(1)

(2)

where and are the molar volatile ratios in the gas phase; and are the original volatile ratios in the parental (un-degassed) melt; D S , D Cl and D Br are the fluid/melt (molar) partition coefficients for the three volatiles; and R is the residual fraction of sulfur in the melt (ranging from 1 at onset of degassing to 0 if S is totally exsolved from the melt).

To resolve the model equations, and are here set at 1.7 and 1320, respectively, from the characteristic S (0.27 wt%), Cl (0.18 wt%) and Br (5.1 ppm) contents in our most primitive, un-degassed glass inclusions from Etna (inclusion E2 from the 2001 eruption; Table A.2). The molar fluid/melt partition coefficients are obtained from our experimental results on Etnean melts for Br (D Br f/m = 5.0 on weight basis; Fig. 1) and those of [START_REF] Alletti | Chlorine partitioning between a basaltic melt and H 2 O-CO 2 fluids at Mount Etna[END_REF] for Cl (D Cl f/m = 8.6 on weight basis), obtained at the same pressure (100 MPa), temperature (1200°C), redox conditions (NNO) and melt composition. These conditions are appropriate to describe halogen behaviour in mafic magmas at shallow crustal conditions and to extrapolate to shallow degassing, in view of the minor pressure-dependence of halogen fluid/melt partition coefficients [START_REF] Alletti | Chlorine partitioning between a basaltic melt and H 2 O-CO 2 fluids at Mount Etna[END_REF]. It is noteworthy that the experiments of [START_REF] Alletti | Chlorine partitioning between a basaltic melt and H 2 O-CO 2 fluids at Mount Etna[END_REF] were run at the low Cl concentrations (< 0.4 wt %) typical of mafic (basaltic to andesitic) melts [START_REF] Webster | Partitioning behavior of chlorine and fluorine in the system apatite-melt-fluid. II: Felsic silicate systems at 200 MPa[END_REF], similar to those characteristic of the volcanoes we report gas data for in Figure 6. At such Cl-under-saturated conditions (absence of brine formation) the minor (if any) dependence of D Cl f/m on total Cl contents justifies the use of a constant D Cl f/m = 8.6 throughout the entire degassing path. For S, a fluid/melt partition coefficient of 86 (on weight basis) is adopted based on the results of [START_REF] Aiuppa | Degassing of halogens from basaltic volcanism: insights from volcanic gas observations[END_REF], who found that volcanic gas measurements from Etna and several mafic arc volcanoes worldwide can satisfactorily be reproduced with a D S /D Cl ratio (ratio between fluid/melt weight partition coefficients) of 10. Our inferred D S = 86 agrees well with results obtained from S thermodynamic modelling [START_REF] Moretti | Solubility and speciation of sulfur in silicate melts: The Conjugated Toop-Samis-Flood-Grjotheim (CTSFG) model[END_REF] of Etna-like melts at P ≤ 100 MPa, ~ NNO and ~ 3 wt.% H 2 O [START_REF] Aiuppa | Forecasting Etna eruptions by real-time observation of volcanic gas composition[END_REF], and is within the range of D S values (3-236, by weight) obtained by [START_REF] Beermann | Partitioning of sulfur and chlorine between aqueous fluid and basaltic melt at 1050°C, 100 and 200 MPa[END_REF] in their partitioning experiments between fluid and Etna-like melts at 100-200 MPa, and at either reducing or oxidizing redox conditions. We are aware that, at redox conditions close to the sulfide-sulfate transition, such as at ~ NNO [START_REF] Jugo | Sulfur K-edge XANES analysis of natural and synthetic basaltic glasses: implications for S-speciation and S content as function of oxygen fugacity[END_REF], D S can exhibit large variations for even subtle redox variations [START_REF] Beermann | Partitioning of sulfur and chlorine between aqueous fluid and basaltic melt at 1050°C, 100 and 200 MPa[END_REF], and that therefore keeping D S constant in our model is an over-simplification that does not completely reflect the real S degassing behaviour in natural (basaltic) systems. However, incorporation of such complexities (including the nonlinear S partition behaviour, and its dependence on the total S content in the system; [START_REF] Beermann | Partitioning of sulfur and chlorine between aqueous fluid and basaltic melt at 1050°C, 100 and 200 MPa[END_REF] into an S-Cl-Br degassing model is currently hampered by our very preliminary understanding of Br partitioning behaviour. In view of this, we find it safer to assume a constant D S value (of 86) in our preliminary model, keeping in mind that our inferred D S /D Cl ratio is derived from empirical fitting of hundreds of volcanic gas data, and is thus likely to describe the "averaged" S degassing behaviour in mafic systems at shallow (<< 3 km; [START_REF] Spilliaert | S-Cl-F degassing pattern of water rich alkali basalt: modelling and relationship with eruption styles of Mount Etna volcano[END_REF] conditions relevant to halogen degassing.

With these numbers, and with R varied from 1 (start of degassing) to 0 (complete S exsolution from the melt), the magmatic vapour model line shown in Figure 6a is obtained. The evolving volatile composition of the coexisting melt is obtained by mass balance (e.g., by subtracting from the initial volatile contents, at each degassing step, the volatile fractions partitioned into the vapour phase), and is illustrated by the melt model line (solid red line) in Fig. 6b.

Our model results predict that the coexisting magmatic gas and melt (Fig. 6a,b) should both evolve with increasing degassing, from S-rich (early gas and early melt) to Cl-Br rich (relative to S) (late gas and late melt). The vapour model line reproduces the observed compositional range of volcanic gas samples from Etna and other mafic systems well (Fig. 6a). Our calculations, therefore, provide a first, though simplistic, model to interpret Br abundance in volcanic gases from basaltic systems. We propose that high S/Br (along with S/Cl ratios; Aiuppa, 2009) gas compositions reflect shallow degassing of fertile (volatile-rich) magmas in basaltic volcano plumbing systems; while more soluble Cl and Br will prevail in gas released by later degassing stages (e.g., during near-surface syn-eruptive degassing). Our conclusions are opposite to those of [START_REF] Bobrowski | Bromine monoxide / sulphur dioxide ratios in relation to volcanological observations at Mt. Etna 2006-2009[END_REF] who, based on observational evidence and use of BrO gas measurements (that under-estimate total Br),

proposed that low S/Br ratios mark "deep" degassing episodes of fresh basaltic magmas (at Etna). We stress, instead, that our model calculations more closely reproduce the similar shallow degassing behaviour of Cl and Br, which is supported by the limited variability of Cl/Br volcanic gas ratios (Fig 6a). We caution, however, that additional experimental observations, especially at low pressure, and rigorous thermodynamic models, are required to more fully constrain the fate of Br during ascent and degassing of mafic melts.

Our melt model line also suitably reproduces the compositional trends exhibited by Etna's and Stromboli's melt inclusions (data from Table A.2). Curiously, a set of model calculations initialised as above but with initial volatile contents from Stromboli's most primitive inclusion (ST82c 137; S = 0.2 wt.%; Cl = 0.17 wt.%; and Br = 4.8 ppm; Table A.2) output a melt model line (orange line) that is very close to the Etna-like model trend above (Fig. 6b).

An additional set of two model lines, calculated using slightly different initial Br contents to encompass the whole range of glass inclusion compositions observed, are also illustrated in the Figure 6b (dashed lines).

Bromine contribution of volcanism to the atmosphere

Global compilations show that Br sources and sinks are not strictly balanced, hinting at a missing natural source of Br [START_REF] Khalil | Atmospheric methyl bromide: Trends and global mass balance[END_REF][START_REF] Montzka | Ozone-depleting substances (ODSs) and related chemicals[END_REF]. Methyl bromide CH 3 Br (mainly produced by marine phytoplankton, biomass burning and fumigants in agriculture) is the largest source of bromine to the atmosphere, and is believed to play a key role in tropospheric and stratospheric ozone depletion (e.g., Mano and Andreae, 1994;[START_REF] Warwick | Global modeling of biogenic bromocarbons[END_REF]. However, methyl bromide alone cannot explain the total amount of active Br species involved in the ozone destruction process (e.g., [START_REF] Warwick | Global modeling of biogenic bromocarbons[END_REF].

Following the first detection of bromine monoxide (BrO) in a volcanic plume [START_REF] Bobrowski | Detection of bromine monoxide in a volcanic plume[END_REF], volcanic degassing (both passive and active) has been recognized as a potentially major source of reactive bromine species to the atmosphere (e.g., [START_REF] Gerlach | Volcanic sources of tropospheric ozone-depleting trace gases[END_REF][START_REF] Oppenheimer | BrO formation in volcanic plumes[END_REF].

Possible approaches to quantify the volcanogenic bromine contribution to the atmosphere include: (i) direct measurements from volcanic fumaroles and plumes or (ii) calculation from bromine contents of pre-eruptive melts (i.e., undegassed crystal-hosted melt inclusions).

Below we apply the second approach to Etna, Merapi and Santorini volcanoes, and compare to direct gas measurements when possible.

Bromine emission from an open-vent mafic volcano: the case of Mount Etna

Mount Etna is a persistently degassing basaltic volcano with frequent eruptive activity. We measured the Br contents of olivine-hosted melt inclusions from the trachybasaltic magma erupted during the 2006 Etna eruptions (Table A.2). This eruptive period began in mid-July 2006 and continued intermittently for 5 months [START_REF] Neri | Continuous soil radon monitoring during the July 2006 Etna eruption[END_REF][START_REF] Behncke | Pyroclastic density currents resulting from interaction of basaltic magma with hydrothermally altered rock: An example from the 2006 summit eruptions of Mount Etna (Italy)[END_REF]; it was characterized by strombolian and effusive activity along fissures and at different vent locations and by a short episode of lava fountaining (more details in Behncke et al., 2009 and references therein).

Br in the pre-eruptive magma versus Br released in the atmosphere Taking into account (i) an average Br content of 5.6 ppm dissolved in the pre-eruptive melt (Table A.2), (ii) a 'dense-rock equivalent' (DRE) erupted volume of 0.012-0.013 km 3 (Supplementary Information) and (iii) 25 vol% of phenocrysts [START_REF] Ferlito | Relationship between the flank sliding of the South East Crater (Mt. Etna, Italy) and the paroxysmal event of November 16, 2006[END_REF], we estimate that 125-141 tons of Br were dissolved in the melt prior to the eruption (SI).

The bromine output (as BrO) of this eruptive period, calculated from gas monitoring data (using an average SO 2 flux of 3444 tons/day, from [START_REF] Aiuppa | Total volatile flux from Mount Etna[END_REF]; and a molar volcanic gas BrO/SO 2 ratio of 1.1 × 10 -4 ; from [START_REF] Bobrowski | Bromine monoxide / sulphur dioxide ratios in relation to volcanological observations at Mt. Etna 2006-2009[END_REF] was 85 tons. However, this is a minimum estimate since BrO is not emitted directly from the magma, but forms by conversion from HBr after emission (e.g. [START_REF] Oppenheimer | BrO formation in volcanic plumes[END_REF][START_REF] Martin | The implications of H 2 S and H 2 stability in high-T mixtures of magmatic and atmospheric gases for the production of oxidized trace species (e.g., BrO and NOx)[END_REF][START_REF] Von Glasow | Atmospheric chemistry in volcanic plumes[END_REF][START_REF] Roberts | Reactive bromine chemistry in Mount Etna's volcanic plume: the influence of total Br, high-temperature processing, aerosol loading and plume-air mixing[END_REF]. Thermodynamic equilibrium calculations indicate that HBr is the primary Br species at Etnean magmatic temperatures (in the 500-1100 °C temperature range and 0.1 MPa pressure; [START_REF] Aiuppa | Emission of bromine and iodine from Mount Etna volcano[END_REF]. The HBr output was unfortunately not determined during the 2006 eruption. If we assume the same mean HBr/SO 2 (7  10 -4 by mass) as measured in 2004 [START_REF] Aiuppa | Emission of bromine and iodine from Mount Etna volcano[END_REF] then this yields a Br emission of 425 tons.

It is preferable to base the estimate on data from the actual 2006 eruption; however the percentage of BrO of the total emitted bromine is difficult to determine. BrO/SO 2 depends on factors including the plume age (distance from the vent, wind velocity), meteorology, time of day, etc (e.g., [START_REF] Bobrowski | Bromine monoxide / sulphur dioxide ratios in relation to volcanological observations at Mt. Etna 2006-2009[END_REF]. Observations and models suggest that BrO contents may represent 20 to ~50 % of total bromine within a few tens of minutes after plume release [START_REF] Von Glasow | Atmospheric chemistry in volcanic plumes[END_REF][START_REF] Roberts | Reactive bromine chemistry in Mount Etna's volcanic plume: the influence of total Br, high-temperature processing, aerosol loading and plume-air mixing[END_REF]. The total mass of bromine emitted during the 2006 Etna eruption would therefore be between 170 and 425 tons, which is comparable to or larger than the mass of bromine in the pre-eruptive melt (125-141 tons, see above and SI), suggesting that Br was efficiently degassed from the basaltic melt.

Estimate of Br annual flux at Mount Etna

On the basis of the 2006 BrO gas monitoring data encompassing non-eruptive and eruptive periods (i.e., [START_REF] Aiuppa | Emission of bromine and iodine from Mount Etna volcano[END_REF][START_REF] Bobrowski | Bromine monoxide / sulphur dioxide ratios in relation to volcanological observations at Mt. Etna 2006-2009[END_REF], we calculate a time-averaged Br emission rate of 0.7 kt/yr (assuming that BrO = 40% of Br total, [START_REF] Oppenheimer | BrO formation in volcanic plumes[END_REF]SI). This is similar to the estimate for the 2004 eruption from [START_REF] Aiuppa | Emission of bromine and iodine from Mount Etna volcano[END_REF]. However, as highlighted by [START_REF] Collins | Melt inclusions track pre-eruption storage and dehydration of magmas at Etna[END_REF], the 2004 and 2006 eruptions were "gas-poor eruptions" thus 0.7 kt/yr should be considered as a minimum Br annual flux for Etna.

Bromine emission from an andesitic volcano: the 2010 Merapi plinian eruption

Merapi volcano (Java, Indonesia) is one of the most active and hazardous volcanoes in the world. The 2010 eruption (VEI 4; [START_REF] Solikhin | High-spatial-resolution imagery helps map deposits of the large (VEI 4) 2010 Merapi Volcano eruption and their impact[END_REF] was the volcano's largest since 1872.

In contrast to the prolonged and effusive dome-forming eruptions typical of Merapi's activity of the last decades, the 2010 eruption began explosively, before a new dome was rapidly emplaced. This new dome was subsequently destroyed by explosions, generating pyroclastic density currents. The initial explosive phase generated an ash plume that rose to 18 km altitude [START_REF] Solikhin | High-spatial-resolution imagery helps map deposits of the large (VEI 4) 2010 Merapi Volcano eruption and their impact[END_REF]. The entire eruption released ~0.44 Tg of SO 2 (cumulative SO 2 output based on satellite observations; [START_REF] Surono | The 2010 explosive eruption of Java's Merapi volcano -A '100-year' event[END_REF], much more than previous Merapi eruptions (from 1992to 2007;[START_REF] Surono | The 2010 explosive eruption of Java's Merapi volcano -A '100-year' event[END_REF]. The SO 2 emission rates of the 2010 eruption greatly exceed background and eruptive emissions recorded at Merapi between 1986 and 2007 [START_REF] Nho | Source strength assessment of volcanic trace elements emitted from the Indonesian Arc[END_REF][START_REF] Humaida | Aktivitas Merapi 2006 dan Pemantauan Emisi SO 2 dengan COSPEC, in Erupsi Merapi 2006. Laporan dan Kajian Vulkanisme Erupsi[END_REF][START_REF] Surono | The 2010 explosive eruption of Java's Merapi volcano -A '100-year' event[END_REF]. On the basis of the 'petrological method', [START_REF] Surono | The 2010 explosive eruption of Java's Merapi volcano -A '100-year' event[END_REF] and [START_REF] Preece | Pre-and syn-eruptive degassing and crystallisation processes of the 2010 and 2006 eruptions of Merapi volcano, Indonesia[END_REF] calculated that the magma volume needed to account for the amount SO 2 released is at least an order of magnitude higher than the estimated DRE volume of magma erupted. They inferred the existence of an exsolved S-rich fluid phase in the pre-eruptive magma body, consistent with the conclusion given by Scaillet et al. (1998b[START_REF] Scaillet | Petrological and volcanological constraints on volcanic sulfur emissions to the atmosphere. Volcanism and the Earth's Atmosphere[END_REF] and [START_REF] Keppler | Experimental Evidence for the Source of Excess Sulfur in Explosive Volcanic Eruptions[END_REF] to explain the common excess of sulfur upon explosive eruptions. According to VolatileCalc modelling by [START_REF] Preece | Pre-and syn-eruptive degassing and crystallisation processes of the 2010 and 2006 eruptions of Merapi volcano, Indonesia[END_REF], the vapour phase would have represented 1 wt% of the magma and degassing occurred in closed-(i.e., gas bubbles remained in physical contact and equilibrium with their host melt)

rather than in open-system conditions prior to the explosive phase of the 2010 eruption.

The GOME-2 satellite instrument measured SO 2 SCD (slant column densities) of up to 8.9×10 18 molecules.cm -2 (paroxysmal phase of November 5, 2010; Hormann et al., 2013), while BrO/SO 2 ratios were extremely low (8×10 -6 maximum), indicating that Br was virtually absent. Yet, considering a magma density of 2550 kg/m 3 , 55 wt% of phenocrysts [START_REF] Preece | Pre-and syn-eruptive degassing and crystallisation processes of the 2010 and 2006 eruptions of Merapi volcano, Indonesia[END_REF] and an average Br content of 9 ppm in the melt inclusions (this study, Table A.2), 433 tons of Br were available in the pre-eruptive melt. In addition, if we consider the presence a free fluid phase in the reservoir (1 wt%; [START_REF] Preece | Pre-and syn-eruptive degassing and crystallisation processes of the 2010 and 2006 eruptions of Merapi volcano, Indonesia[END_REF] and the D Br f/m = 9.1 in andesitic melt (Fig. 2b), 73 tons of Br were stored in the fluid and hence immediately available during eruption. Note that we observe the same large discrepancy between the satellite-based estimate of the chlorine yield and the petrological one (see SI). In our opinion, the two most probable explanations are: (1) the paroxysmal phase of the eruption being ash-rich (opacity)

and occurring in the middle of the night, the production of BrO was prevented until many hours later (as the reactions are UV-enabled) and is probably lower than in ash-poor plumes

(2) satellite instruments measure gases which reached the stratosphere more effectively than those that remain lower in the atmosphere (significant amount of bromine might have been scavenged in the troposphere). Additional causes might include: (i) preferential S degassing owing to kinetic factors (e.g., [START_REF] Fiege | Kinetic vs. thermodynamic control of degassing of H 2 O-S±Cl-bearing andesitic melts[END_REF], (ii) Br uptake by brine saturation during magma uprise, (iii) the involvement of other volatile species (e.g. CO 2 ) which may alter Br partitioning.

The case of Merapi 2010 eruption hints at the need of studies on Br speciation in ash-rich volcanic plumes and additional experimental constraints, in particular on the effect of volatiles other than H 2 O on Br systematics.

Bromine emission during the cataclysmic Minoan eruption of Santorini volcano

The Late-Bronze age Minoan eruption discharged 38-86 km 3 DRE of rhyodacitic magma (e.g. [START_REF] Pyle | New volume estimates for the Minoan eruption[END_REF][START_REF] Johnston | Revised estimates for the volume of the late Bronze Age Minoan eruption, Santorini[END_REF]. Petrological studies have shown that the pre-eruptive melt was rich in halogens, particularly in chlorine (2500-6000 ppm), and was most probably in equilibrium with an exsolved H 2 O-Cl-rich fluid phase [START_REF] Cadoux | Magma storage conditions of large Plinian eruptions of Santorini Volcano (Greece)[END_REF][START_REF] Cadoux | Stratospheric Ozone destruction by the Bronze-Age Minoan eruption (Santorini Volcano, Greece)[END_REF][START_REF] Druitt | Magma Storage and Extraction Associated with Plinian and Interplinian Activity at Santorini Caldera (Greece)[END_REF]. Here, we have measured for the first time the Br content of plagioclase-hosted melt inclusions from the Minoan plinian fallout deposit (Table A.2). They

give an average value of 7.3 ± 0.8 ppm, which multiplied by the D Br f/m of 20.2 (obtained in this study for a rhyodacitic composition at 900°C, 200 MPa and ~NNO; Fig. 3), indicate that the pre-eruptive fluid phase contained 147 ppm Br.

Assuming a minimum erupted volume of 39 km 3 DRE and a magma crystallinity of 10%, the Minoan pre-eruptive melt would have contained 0.6 Mt of Br. Recent studies have shown that, in silicic magma systems, Br is efficiently degassed with water during eruption [START_REF] Bureau | Bromine cycle in subduction zones through in situ Br monitoring in diamond anvil cells[END_REF][START_REF] Cochain | Bromine speciation in hydrous silicate melts at high pressure[END_REF]. If we assume that all the Br was degassed from the melt (i.e. we consider 0 ppm of Br in the interstitial melt), the Br output of the Minoan eruption was 0.6 Mt. If we add the contribution of the fluid phase (assuming that it represents 5 wt% of the magma mass, as in [START_REF] Cadoux | Stratospheric Ozone destruction by the Bronze-Age Minoan eruption (Santorini Volcano, Greece)[END_REF], then the total Br output would have reached 1.3

Mt. These Br yields are consistent with previous estimates of 0.1-1.5 Mt [START_REF] Cadoux | Stratospheric Ozone destruction by the Bronze-Age Minoan eruption (Santorini Volcano, Greece)[END_REF] obtained by multiplying the chlorine yields by the mean molar Br/Cl ratio of 0.0022 of volcanic arc gases [START_REF] Gerlach | Volcanic sources of tropospheric ozone-depleting trace gases[END_REF].

The estimated Br output of this single large explosive event (VEI 6-7) is > 100 times higher than the annual Br flux at a persistently degassing volcano such as the Etna (0.0007 Mt, see before) and the estimated global Br flux at volcanic arcs (0.005-0.015 Mt/yr; [START_REF] Pyle | Halogens in igneous processes and their fluxes to the atmosphere and oceans from volcanic activity: A review[END_REF].

Conclusions

Determining halogen behaviour in magmatic systems is important to understand their role in the Earth's element cycles and to provide reliable constraints on the contribution of volcanism to atmospheric and ocean chemistry. The behaviour of the heavier halogens such as Br in magmatic systems is less well understood than that of Cl and F. We MPa -900°C for the rhyodacite. Our data confirm previous experimental constraints on synthetic model magma compositions [START_REF] Bureau | Volcanic degassing of bromine and iodine: experimental fluid/melt partitioning data and applications to stratospheric chemistry[END_REF]. They also show that D Br f/m increases with increasing SiO 2 content of the melt (as for chlorine) and it also appears to be sensitive to melt temperature (increase of D Br f/m as temperature decreases). These results suggest that the Br yield into atmosphere from relatively cold and silicic magmas will be much larger than that from hotter and more mafic magmas. The partition coefficients of this study will permit better estimates of the Br yield of past explosive eruptions, provided their pre-eruptive temperature is well known.

Our Br partition coefficient for Etna basalt, together with literature data on S and Cl behaviour, and S-Cl-Br volcanic gas compositions in mafic volcanic systems, allow first order quantitative modelling of S-Cl-Br degassing behaviour in shallow magma reservoirs, permitting a better interpretation of gas-monitoring data.

Zajacz, Z., Candela, P.A., Piccoli, P.M., Sanchez-Valle, C., 2012. The partitioning of sulfur and chlorine between andesite melts and magmatic volatiles and the exchange coefficients of major cations. Geochim Cosmochim Acta 89, 81-101.

.

  This observation is consistent with the uncertainty analysis for the mass balance calculations of Zajacz et al. (2012), which shows that the determination of D for elements with low D values in experiments with low volatile/mass ratio may bear large uncertainties due to error propagation. Their modeling (Fig. 1 in Zajacz et al., 2012) predicts relative error of 10 up to 100% for volatile/mass ratios lower than 0.1. The error on the D Br f/m estimated for M1-B being > 100% (D Br f/m = 3.8 ± 4.5), we conclude that this value, taken alone, is meaningless. Overall, the error on D Br f/m is particularly sensitive to the error on the measured [Br] melt (i.e., Br content of the run product glasses). The precision of the Br measurements is crucial for the accuracy of D Br f/m .

  Most experimental values of D Cl f/m for basaltic systems are <10 (Stelling et al 2008; Beermann 2010; Baker and Alletti 2012), but Alletti et al. (2009) observed D Cl f/m of 8-34 in trachybasaltic melt in equilibrium with aqueous fluids at ƒO 2 near NNO. Values of D Cl f/m for intermediate (andesitic and phonolitic) and silicic melts exceed those for mafic melts (e.g.,

  several wt.% to <1 wt.% Cl). At low Cl system concentrations (<1 wt% Cl in andesite,Zajacz et al., 2012; or in basalt, Beermann et al., 2015;[START_REF] Stelling | Solubility of H 2 O-and chlorine-bearing fluids in basaltic melt of Mount Etna at T = 1050-1205°C and P = 200 MPa[END_REF], D Cl f/m seems to achieve values close to or even below unity. From our data, we do not observe any systematic relationship between Br concentration in the system ([Br°]) and D Br f/m : the apparent decrease of D-values for andesite (M4-A) relative to [Br°] at 1060°C and 200 MPa (Table2) is actually not significant taking into account the errors on D. More experiments are necessary to investigate the potential relationship between D Br f/m and the Br concentration in the system.

  have experimentally determined the fluid/melt partitioning of bromine at shallow crustal pressure and temperature conditions (100-200 MPa, 900-1200°C) with mafic, intermediate and silicic natural melts. D Br f/m values range from 5.0 ± 0.3 at 100 MPa -1200°C for the basalt to 20.2 ± 1.2 at 200

Figure 1 .

 1 Figure 1. Partitioning of bromine between melt and fluid in the run products at 1200°C, 100 MPa and fO 2 ~NNO. The D Br f/m of the basaltic composition, determined by linear regression

Figure 2 .

 2 Figure 2. (a) Melt Br contents versus bulk Br contents (ppm) for the andesitic and rhyodacitic

Figure 3 .

 3 Figure 3. Partitioning of bromine between melt and fluid for the rhyodacitic composition, at 900°C, 200 MPa, ~NNO. At lower temperature, the D Br f/m of the rhyodacite increases: 20.2 ±

Figure 4 .

 4 Figure 4. D Br f/m as a function of SiO 2 (wt%) of the run products of this study. The data for the

  Figure 1

Table 2 )

 2 , the Br amount of the coexisting aqueous fluid phase([Br] Br] melt ), and assuming that the difference between these two figures represents the amount of Br left over for the fluid phase.(details of the calculation are given in Supplementary Information and in TableA.1). For each run product, we estimated the errors on the calculated[Br] fluid and D Br f/m by propagating the errors coming from the preparation of the experimental charges to the analysis of Br, Na and H 2 O contents of the glasses (Supplementary Information and Table A.1). The resulting errors on [Br] fluid are < 51% (and mostly < 34%), except for one run-product (M1-B). As the main source of error for D Br f/m is [Br] fluid , the resulting errors on D Br f/m are close to those for [Br] fluid : they do not exceed 54% and are mostly <45%, except for

fluid ) was determined by mass balance, knowing the original bulk Br content (the amount of Br loaded in the capsule as H 2 O+NaBr solution; hereafter [Br°]), the measured amount of Br dissolved in the final glass ([M1-B (112%; Table A.1). Note that, as underlined in previous studies (e.g., Alletti et al., 2014), such rigorous error propagation overestimates the real errors, due to the complex covariation of individual errors. The large error associated with the D Br f/m estimated for M1-B is due to the low fluid/melt mass ratio of the experimental charge (0.06 compared to 0.08-0.11

Table A

 A

.2 reports the Br contents of melt inclusions from (i) basaltic magmas erupted at Mount Etna and Stromboli, (ii) the andesitic magma erupted in 2010 at Mount Merapi, and (iii) the rhyodacitic magma of the 1613 BC Minoan eruption of Santorini volcano. Melt inclusions from Mt. Etna and Stromboli are hosted by olivine crystals, while those from Mt.

Table 1 .

 1 Major element composition of the starting dry glasses used for the partitioning experiments Major element analyses performed by electron microprobe a: from Iacono-Marziano et al. (2012) b: from Cadoux et al. (2017), recalculated to 100% c: from[START_REF] Cadoux | Magma storage conditions of large Plinian eruptions of Santorini Volcano (Greece)[END_REF][START_REF] Cadoux | A new set of standards for in-situ measurement of bromine abundances in natural silicate glasses: application to SR-XRF, LA-ICP-MS and SIMS techniques[END_REF], recalculated to 100% n: number of analyses, and : standard deviation of the average of n analyses nd: not determined These dry glasses were also used to synthesize Br standards characterized in[START_REF] Cadoux | A new set of standards for in-situ measurement of bromine abundances in natural silicate glasses: application to SR-XRF, LA-ICP-MS and SIMS techniques[END_REF]

	Volcano: Eruption	Etna: 11/22/2002 Santorini: USC-2 Santorini: Minoan
	Sample name	ET02PA27 a	S09-22 b	S82-30 c
	Major oxides (wt%)	n = 32	± 1	n = 8	± 1	n = 22	± 1
	SiO 2	47.95	0.82	58.88	0.43	71.24	0.26
	TiO 2	1.67	0.11	1.28	0.05	0.45	0.04
	Al 2 O 3	17.32	0.27	16.16	0.17	14.87	0.15
	FeO tot	10.24	0.13	8.18	0.25	2.85	0.18
	MnO	nd	nd	0.20	0.09	0.08	0.05
	MgO	5.76	0.28	2.77	0.09	0.73	0.05
	CaO	10.93	0.37	6.46	0.12	2.34	0.14
	Na 2 O	3.45	0.16	4.07	0.15	4.24	0.08
	K 2 O	1.99	0.10	1.67	0.06	3.08	0.11
	P 2 O 5	0.51	0.12	0.31	0.06	0.13	0.04
	Original sum	99.82		96.66		98.40

Table 2 .

 2 Results of the fluid/melt partitioning experiments

	[Br°] a	fluid/melt	[Br] melt	b	± 1 [H 2 O] melt c	± 1 [Br] fluid	d	±	D Br	f/m	±
	(ppm)	mass ratio	(ppm)		(wt%)	(ppm)		error			error
								(ppm
								)
	Experiment #1

: Au-Pd capsule, 1200°C, 100 MPa, ~NNO (pH 2 = 2 bars), 24 hours

	Run product # (B for basalt)
	M1-B	72181	0.06	3753	168	n.d.		14298	3.8	4.3
								8
	Experiment #2: Au-Pd capsule, 1200°C, 100 MPa, ~NNO (pH2 = 2 bars), 24 hours
	M2-B	14251	0.08	9112	464	n.d.		47027	5.2	1.4
		5						7
	Experiment #3: Au-Pd capsules, 1200°C, 100 MPa, ~NNO (pH 2 = 2 bars), 24 hours
	Run product # (B for basalt, A for andesite, RD for rhyodacite)
	M3-B	24222	0.12	2034	290	3.4	0.0	9370	4.6	2.0
	M3-A	24222	0.12	1912	376	4.8	0.1 12298 6.4	3.5
	M3-RD	24222	0.12	1426	42	2.9	0.0 16065 11.3 0.9
	Experiment #4

: Au-Pd capsules, 1060°C, 200 MPa, ~NNO (pH 2 = 2 bars), 48 hours

	M4-A1	999	0.11	69.5	0.1	7.2	0.1	948	138 13.7 2.0
	M4-A2	4968	0.11	359	48	7.3	0.1	4439	12.3 4.5
	M4-A3	9874	0.11	733	79	7.3	0.2	8396	11.5 3.6
	M4-A4	24222	0.12	1960	34	7.2	0.1 16908 8.6	1.3

Acknowledgements A.C. thanks N. Bouden (CRPG, Nancy) for his assistance during H 2 O SIMS measurements, and S. Erdmann (ISTO, Orléans) who provided crystal mounts from Merapi andesite for melt inclusions analysis. A.C. is also grateful to Y. Missenard and P. Sarda (GEOPS, Orsay) for their help and discussion about error propagation. N. Metrich (IPGP, Paris) and A. Bertagnini (INGV, Pisa) provided melt inclusions from Stromboli. I. Di Carlo (ISTO, Orléans) and L. Brusca (INGV, Palermo) are acknowledged for their assistance with EMP and LA-ICPMS analyses, respectively This work was partially supported by the 'Laboratoire d'Excellence VOLTAIRE' (University of Orléans, France), the French agency for research [ANR project #12-JS06-0009-01] and the European Research Council [ERC grant agreement n°305377]. A.A., T.A.M. and D.M.P. acknowledge the Diamond Light Source for time on Beamline I18 [Proposal sp8797]. We thank two anonymous reviewers for their insights on an earlier draft of this paper.