# Behaviour of bromine in Cl- and F-bearing alkali-rich felsic magmas at crustal depth: An experimental study at 800–1100 °C, 10–200 MPa

Carmela Federica Faranda, Gaëlle Prouteau, Bruno Scaillet, Joan Andújar

► To cite this version:

Carmela Federica Faranda, Gaëlle Prouteau, Bruno Scaillet, Joan Andújar. Behaviour of bromine in Cl- and F-bearing alkali-rich felsic magmas at crustal depth: An experimental study at 800–1100 °C, 10–200 MPa. Geochimica et Cosmochimica Acta, 2025, 390, pp.117-144. 10.1016/j.gca.2024.11.024. insu-04848984

HAL Id: insu-04848984

https://insu.hal.science/insu-04848984v1

Submitted on 19 Dec 2024

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
Journal Pre-proofs

<figure><img src="image_2.png" /></figure>

Geochimica et
Cosmochimica Acta

JOURNAL OF THE GEOCHEMICAL SOCIETY
AND THE METEOROLOGICAL SOCIETY

Behaviour of bromine in Cl- and F-bearing alkali-rich felsic magmas at crustal depth: An experimental study at 800–1100 °C, 10–200 MPa

Carmela Federica Faranda, Gaëlle Prouteau, Bruno Scaillet, Joan Andújar

PII: S0016-7037(24)00605-7

DOI: https://doi.org/10.1016/j.gca.2024.11.024

Reference: GCA 13641

To appear in: *Geochimica et Cosmochimica Acta*

Received Date: 7 July 2023

Accepted Date: 23 November 2024

Please cite this article as: Faranda, C.F., Prouteau, G., Scaillet, B., Andújar, J., Behaviour of bromine in Cl- and F-bearing alkali-rich felsic magmas at crustal depth: An experimental study at 800–1100 °C, 10–200 MPa, *Geochimica et Cosmochimica Acta* (2024), doi: https://doi.org/10.1016/j.gca.2024.11.024

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

© 2024 Elsevier Ltd. All rights are reserved, including those for text and data mining, AI training, and similar technologies.
# Behaviour of bromine in Cl- and F-bearing alkali-rich felsic magmas at crustal depth: an experimental study at 800–1100°C, 10–200 MPa

Carmela Federica Faranda¹, Gaëlle Prouteau ¹*, Bruno Scaillet ¹, Joan Andújar ¹

¹ Institut des Sciences de la Terre d'Orléans (ISTO), UMR 7327, Université d'Orléans, CNRS, BRGM, F-45071 Orléans, France

* Corresponding author. E-mail: gaelle.prouteau@cnrs-orleans.fr

## ABSTRACT

Bromine, although a minor component in volcanic gases, has received increasing interest in recent studies due to its high atmospheric ozone depletion potential, but its behaviour in alkali-rich felsic hydrous magmas remains unexplored. In this study, fluid-melt partitioning experiments were carried out using natural, Cl- and F-bearing silicate glasses with phonolitic, comenditic and pantelleritic compositions. For each composition, experiments were performed with a range of Br concentrations, at P-T conditions simulating isothermal decompression degassing or isobaric equilibrium cooling at shallow crustal depth (800–1000°C, 10–200 MPa, oxidising and reducing conditions). The major element, Cl and F concentrations of the run-product glasses were determined by electron microprobe and the Br concentrations by LA-ICPMS. Volatile concentrations in the fluid were determined by mass balance calculations. The experimental results show that more Br partitions into the fluid phase with increasing bulk halogen concentration. The most Br-doped experiments are typically saturated with a vapor phase and a hydrosaline liquid. Experiments at the lowest Br concentrations, closest to natural systems, show that the pressure dependence of vapor-melt Br partitioning is complex, with a minimum vapor-melt partition coefficient observed at 50 MPa in the phonolitic composition (1.8 ± 0.9 at 1000°C and oxidising conditions). Our results indicate that the eruptive degassing of Br from peralkaline felsic magmas is restricted to the shallowest levels of the magmatic plumbing system and is likely to occur at much lower pressures than in metaluminous magmas. This observation is consistent with the composition of melt inclusions preserved in alkaline silicic magmas. An important finding is that the vapor-melt partitioning of Br and Cl decreases with increasing temperature. In phonolite, at 200 MPa and oxidising conditions, the Br vapor-melt partition coefficient decreases from 18.5 ± 3.6 at 900°C to 3.8 ± 1.5 at 1000°C. As the ratio between the Br and Cl vapor-melt partition coefficients is not conservative, the Br/Cl ratio in the vapor phase is likely to increase during isobaric cooling and degassing. The vapor-melt partition coefficients of Br and, to a lesser extent Cl also increase with decreasing fO₂ in the phonolitic system, and the Br vapor-melt partition coefficient for a reduced alkaline magma is close to that for an oxidised calc-alkaline magma. Our results also show that during protracted storage at shallow levels, oxidised alkali- and F-rich rhyolites coexist with vapor and brine. This suggests that high F concentration promotes unmixing of the halogen-bearing phase coexisting with such melts. The exsolution of immiscible vapor and brine efficiently removes Br from peralkaline magmas and probably limits the flux of Br to the atmosphere from such magmas.
KEY WORDS: experimental petrology; bromine; magmatic degassing; F- and Cl-bearing alkali-
rich felsic magma; fluid

# 1. Introduction

## 1.1. Br behaviour in magma and aim of this study

Volcanic eruptions can release enormous amounts of climate-sensitive species into the
atmosphere, among which CO2 and SO2 figure prominently as illustrated by the Pinatubo 1991
eruption (e.g., Bluth et al., 1992). Halogen elements (F, Cl, Br and I) are minor volatile
constituents in natural magmas but they play a critical role in magmatic degassing (e.g., Aiuppa
et al., 2009, Webster et al., 2018 and references therein) and in terms of metal transport and
ore deposit formation (e.g., Williams-Jones & Heinrich, 2005; Simon et al., 2006; Zajacz et al.,
2011; Seward et al., 2014; Tattitch and Blundy, 2017; Lecumberri-Sanchez and Bodnar, 2018;
Tattitch et al., 2021). Among the halogens, Br plays a critical role in atmospheric chemistry
(e.g., Oppenheimer et al., 2006; Roberts et al., 2009, 2014) and most importantly in
tropospheric and stratospheric ozone depletion (von Glasow et al., 2009; Kutterolf et al., 2013;
Cadoux et al., 2015; Brenna et al., 2020).

In contrast to Cl and F (e.g., Holland, 1972; Shinohara et al., 1989; Webster and Holloway,
1990; Kravchuk and Keppler, 1994; Webster et al., 1999; Signorelli and Carroll, 2000, 2002;
Botcharnikov et al., 2004; Chevychelov et al., 2005; Dolejš and Baker, 2007; Stelling et al.,
2008; Alletti et al., 2009, 2014; Webster et al., 2009; Zajacz et al., 2012; Webster et al., 2014;
Beermann et al., 2015; Botcharnikov et al., 2015, Hsu et al. 2019; Tattitch et al., 2021; Cassidy
et al., 2022), experimental data for Br are still very limited (Bureau et al., 2000, 2010; Cadoux
et al., 2018; Cassidy et al., 2022), due mainly to the analytical challenges posed by the low
concentration of Br in magmas. Available data indicate that halogens significantly favour the
fluid as the ionic radii increase, their behaviour being also sensitive to melt composition in
magmas of calc-alkaline affinity (Bureau et al., 2000; Cadoux et al., 2018; Cassidy et al., 2022).
Previous studies on haplogranitic (Bureau et al., 2010) and calc-alkaline magmas (Cadoux et
al., 2018; Cassidy et al., 2022) also suggest that Br fluid-melt partition coefficient (D<sub>Br</sub><sup>f/m</sup>) is
sensitive to pressure and temperature. Here we focus on alkali-rich evolved melts which are
known to have high concentrations of halogens (F, Cl, Br) (e.g., Kogarko 1974; Burnham 1979;
Bureau and Métrich 2003; Scaillet and Macdonald, 2003, 2006; Balcone-Boissard et al., 2010),
with up to 2 wt% F (e.g., Gold Flat Tuff, Nevada; MacDonald et al., 2019), more than 1 wt% Cl
(e.g., Green Tuff Pantelleria; Liszewska et al. 2018) and up to 30 ppm Br (Pantelleria; Bureau
and Métrich, 2003). In order to evaluate Br behaviour in alkali-rich felsic magmas, water-
saturated experiments were conducted on natural, Cl- and F-bearing phonolite and peralkaline
rhyolites, at *T* = 800–1100°C and *P* = 10–200 MPa. The experimental results provide insights
into the behaviour of Br during crustal storage and decompression of these magmas.

## 1.2. Background information: phase relations in halogen-H₂O fluids

Throughout this paper, fluid is a general term for an aqueous phase and does not refer to a
silicate melt. Vapor is a relatively low-density fluid, dominantly aqueous in composition.
Hydrosaline liquid, also referred to as a brine, is a higher-density, electrolyte-enriched aqueous
fluid.

The starting compositions studied here contain Cl (from 0.26 to 0.46 wt%, Table 1) and the
fluid equilibrated at run conditions is therefore likely to contain a variable amount of Cl,
depending on the *P*-*T*-*fO*₂ conditions. The NaCl-H₂O system shows non-ideal mixing behaviour
at magmatic-hydrothermal conditions, with vapor-liquid immiscibility for the aqueous solution under a wide range of *P-T* conditions encountered in the shallow crust (e.g., Bodnar et al., 1985; Anderko and Pitzer, 1993a; Driesner and Heinrich, 2007) (Fig. 1). Below the critical point, in the subcritical region, a vapor (low salinity aqueous fluid) coexists with a brine (Fig. 1). CI behaviour is therefore controlled by its partitioning between silicate melt, vapor and brine, in addition to CI-bearing crystals, if present. At subcritical conditions, the CI concentrations of the silicate melt and coexisting vapor increase with increasing CI in the system while the H₂O concentration of the melt remains relatively constant, until the system saturates in vapor + brine. In the NaCl-H₂O system, at fixed *P-T*, when vapor coexists with brine, the CI and H₂O concentrations of the silicate melt, vapor and brine are constant (e.g., Lowenstern, 1994). In other words, the concentration of CI in the silicate melt does not depend on the bulk CI concentration and remains constant. In contrast, if CI is added to a silicate melt saturated with a single volatile phase, the melt shows a progressive increase in Cl/H₂O, without a sharp break in slope, until it saturates with brine (see Webster, 2004). The CI concentration of a silicate melt that is saturated with brine only (whether or not it is subcritical) is constant.

While the H₂O-NaCl system is a useful end-member system, most natural magmatic fluids contain significant concentrations of other cations (e.g., K, Ca, Fe, Mg) that can affect the range of *P-T* conditions of fluid immiscibility (e.g., Anderko and Pitzer, 1993b; Liebscher, 2007, Steele-MacInnis et al., 2016; Lecumberri-Sanchez and Bodnar, 2018), but these effects are poorly constrained. Experiments with phonolitic (Alletti et al., 2014), andesitic (Botcharnikov et al., 2015), or basaltic (Beermann et al. 2015) magmas have shown a shrinkage of the immiscibility gap, compared to that for the NaCl-H₂O system (Fig. 1). The observed changes are likely related to the greater diversity of CI species in the fluid and melt (e.g., from (Na, K, Ca, ...)) *Clₓ* dominated to HCl-dominated, Botcharnikov et al., 2015).

The presence of other halogens in magmatic systems further complicates this picture. Alkaline silicic magmas can have very high concentrations of F, which may affect fluid immiscibility (e.g., Chevychelov et al., 2008). Br concentrations can reach ~100 to a few thousand ppm in metamorphic fluids or basinal brines (e.g., Yardley and Bodnar, 2014; Worden, 2018) although they are unlikely to occur at high enough levels in magmatic systems to significantly alter the phase behaviour of magmatic aqueous fluids. The present experimental study focuses on systems with bulk CI/Br ratio ranging from ~13.2 to ~0.4, and bulk Br concentration from ~300 to ~6000 ppm (Tables 2-4). Most bromides are salts with a continuous critical curve, similar to NaCl and therefore the topology of the phase diagrams for aqueous solutions of bromide salts is similar to that of the NaCl-H₂O system (Valyashko, 2004). Thus, bromide salts, like chlorides, enlarge the *P-T* field of fluid immiscibility relative to pure water (e.g., Webster et al. 2018). Ternary systems with two binary water-salt subsystems with immiscibility regions (e.g., H₂O-NaCl-KCl (Chou, 1987; Chou et al., 1992); H₂O-KCl-KBr (Ravich and Borovaya, 1949)) are characterised by eutonic equilibria, where two solid phases of components coexist with liquid and vapor solutions. In summary, the potentially complex cation assembly in the fluid may complicate the fluid phase relationships in natural magmas, when compared to the NaCl-H₂O system, and therefore requires special attention.

## 2. Experimental and analytical procedures

### 2.1. Rationale

We worked with natural, halogen-bearing magmas, as such an approach has the benefit of a direct applicability to natural systems. Experimental conditions were chosen to simulate pre-ruptive storage/decompression in crust (≤200 MPa, 800–1100°C). We explored redox conditions ranging from oxidising (high Fe³⁺/ΣFe) to slightly reducing (low Fe³⁺/ΣFe), to explore the potential effects of fO₂ on Br behaviour. A low (~0.1) initial fluid to melt mass ratio was used to minimize the modification of the melt composition by the dissolution of silicate in the fluid phase. At each *P-T-fO₂* condition and for each melt composition a series of capsules (up to 4)
with different Br concentrations allowing routine analysis of Br in the quenched melt were run (Tables 2-4). The small amount of fluid used in our experiments prohibited the analysis of run product fluids, and the composition of the bulk fluid at run conditions was obtained by mass balance. The experiments were not doped with other halogens, but, as stated above, natural starting glasses also contain Cl (up to ~0.46 wt%) and F (up to ~1.63 wt%) (Table 1), which play a role as shown below.

A total of 69 glass-bearing charges was produced, whose Br concentrations were measured by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), while F and Cl concentrations were determined by electron probe microanalysis.

## 2.2. Starting material

The starting materials are (Table 1): an aphyric pantellerite obsidian from the Eburru Rhyolitic Complex (Kenyan Rift Valley, Kenya), a comenditic obsidian from the Greater Olkaria Volcanic Complex (Naivasha area of the Kenyan Rift Valley, Kenya; Scaillet and Macdonald, 2001, 2003, 2006) and a slightly porphyritic ($< 1$ vol% crystals, mainly feldspars with maximum size of 1 mm) phonolitic pumice from the Roques Blancos eruption (Tenerife, Canary Islands, Spain; Andújar et al. 2013; Garcia et al. 2014). The samples were crushed and ground in an agate mortar. The pantelleritic and comenditic obsidian powders were used directly to fill the capsules, while the phonolite sample powder was melted twice in a platinum crucible at 1400°C for 5 hours in a 1-atm furnace in order to obtain a crystal-free, homogeneous glass. The resulting glass was then ground to a fine powder which was used as the starting material (Table 1).

## 2.3. Experimental techniques

The experiments consisted of preparing aqueous salt solutions with four different bromide concentrations, by dissolving from 0.5 to 8.4 wt% NaBr in distilled water (Tables S8-S10 Supplementary Material). This procedure limits the uncertainty of the Br content loaded in the capsules (about 5 % relative) and promotes homogeneity of the sample. Au ($T \le 1000°C$) or Au-Pd ($T > 1000°C$) capsules (length 15 to 30 mm, wall thickness 0.2 mm and outer diameter 2.9 mm) were filled with about 30 mg of glass powder and about 3 mg of the bromide solutions (9–11 wt% of NaBr-H₂O solution in the system), ensuring fluid-saturation at all pressures. The capsules were sealed by arc welding, immersed in a 120°C oil bath, stored in an oven at 120°C, and then weighed to check for volatile loss. Runs were performed with up to 9 capsules (charges with different glass compositions and variable Br concentrations) held together under the same P-T-fO₂ conditions and run duration.

The hydrothermal experiments were performed in internally heated pressure vessels (IHPV) equipped with a rapid quench device at the Institut des Sciences de la Terre d'Orléans (ISTO, Orléans, France). Twelve experiments were carried out under different P-T-fO₂ conditions, with a total of 69 successful charges (Tables 2-4). The experiments were carried out in the pressure range 10–200 MPa and between 800 and 1100°C. Temperature and pressure are accurate to within ± 10°C and 2 MPa, respectively (e.g., Andujar and Scaillet, 2012). The pressure medium was Ar or, for one run, an Ar-H₂ mixture, whose Ar/H₂ ratio is fixed by sequential loading of first H₂ and then Ar at room temperature to achieve the desired target fO₂ (Tables 2-4) (Scaillet et al., 1992). For the latter run, the oxygen fugacity is estimated to be around NNO-1 on the basis of the H₂ partial pressure imposed in the vessel (~10 bar) (e.g., Di Carlo et al., 2006), whereas for experiments pressurized with argon only, the fO₂ is estimated to be > NNO+2 (e.g., Prouteau and Scaillet, 2003; Pichavant et al., 2009). These experiments will be referred to as "reduced" and "oxidised", respectively. Run durations ranged from 23 to 196 h, decreasing with increasing pressure and temperature. The long run durations at the lowest P-T conditions are necessary to reach equilibrium, due to the slow diffusion rate of Br and, to a lesser extent, Cl and F at temperatures ≤ 1000°C (e.g., Balcone-Boissard et al., 2020). Br
partitioning experiments performed by Cadoux et al. (2018) with a silicic melt showed that less
than 100 h were sufficient to achieve equilibrium at 900°C and P> 100 MPa. Note that Br
diffusivities are enhanced in hydrous, Na-rich felsic melts (as used in this study) compared to
anhydrous, K-rich melts (Balcone-Boissard et al., 2020). The homogeneous distribution of Br
and Cl in the glass, confirmed by LA-ICPMS and electron probe analyses (Tables 2-4), attests
to the achievement of equilibrium in all experiments. The runs were terminated by isobaric drop
quench (Roux and Lefevre, 1992) with a cooling rate of ~100°C/s. After the experiment,
capsules were weighed to check for leaks and then pierced. During piercing, evidence of
excess fluid was observed in all charges. The capsules were then placed in a drying oven at
120°C for 30 minutes. Systematic weight loss after this step confirmed that vapor-saturated
conditions prevailed at high P-T conditions. Finally, the capsules were opened and glass chips
were embedded in epoxy resin and polished for analysis.

**2.4. Analytical methods**

The experimental glasses were characterised using scanning electron microscopy (SEM, ISTO, Orléans, France), electron microprobe (Cameca SX Five, ISTO/BRGM, Orléans, France), Raman spectroscopy (ISTO, Orléans, France) and LA-ICPMS (ISTO, Orléans, France).

**2.4.1. Electron probe microanalysis (EPMA): major elements, F and Cl**

The major elements of glasses were determined by electron microprobe with the following analytical conditions: accelerating voltage of 15 kV, sample current of 10 nA, counting time of 10 s on peak, with a defocused beam (10–20 µm diameter) to minimise alkali migration. Standards were albite (Si and Na), TiMnO₃ (Ti and Mn), Al₂O₃ (Al), Fe₂O₃ (Fe), MgO (Mg), andradite (Ca), orthoclase (K) and apatite (P). The detection limit of Na was below ~800 ppm. Alkalis migration under the probe beam was assessed by using a set of five hydrous phonolitic glass standards of known H₂O concentrations (from 2.11 to 6.4 wt% H₂O, as determined by FTIR, Supplementary Material) (Andújar & Scaillet, 2012). At the beginning of each analytical session the standards were analysed for retrieving the Na₂O correction factor (Supplementary Material). The water concentrations of the drop-quenched glasses were determined by using the by-difference method (Devine et al., 1995) and micro-Raman spectroscopy. The summation deficit to 100% of the electron microprobe analyses was calibrated with the five phonolitic standard glasses mentioned above (Andújar & Scaillet, 2012) (Fig. S1, Supplementary Material). The uncertainty of this method is estimated to be < 0.5 wt% (Supplementary Material).

Cl and F were measured last in the sequence, using a 10 nA sample current and counting times of 30 s for Cl and 60 s for F. Standards for Cl and F were vanadinite (2.62 wt% Cl) and topaz (20.65 wt% F), respectively. We analysed F using TAP diffraction crystals to correct for the problem of overlap of the FKα peak onto the shoulder of the FeLα1 peak that arises when using synthetic multi-layered diffraction crystals (e.g., Witter and Kuehner, 2004). The detection limit for Cl was ~250 ppm and that for F was ~2000 ppm. Repeat analyses of the pantelleritic glass Eburru-89, known to contain 0.40 wt% Cl and 1.10 wt% F (Scaillet and Macdonald, 2006), returned a mean concentration of 0.40 ± 0.03 (1σ) for Cl and 1.25 ± 0.29 (1σ) for F (Table 1).

The reported major elements and F and Cl concentrations are the average of 6–12 analyses of randomly selected spots. Multiple measurements are used to reduce potential analytical errors and to check homogeneity.

**2.4.2. Raman spectroscopy: water concentrations**
Raman spectroscopy analyses were carried out on 17 selected run product glasses to determine dissolved water concentration. The Raman spectra were recorded using a custom built micro-Raman apparatus equipped with a monochromatic 532 nm Coherent Genesis MX SLM laser as exciting source and an Andor Shamrock 500i spectrometer, equipped with an Edge filter to cancel intense Rayleigh elastic scattering and Peltier cooled back illuminated CCD Andor Newton detector. The system is coupled to a Nikon ECLIPSE Ni-U microscope. We used a Nikon objective with a long working distance (100×, N.A. 0.8) to focalise the laser on the sample surface, allowing micro-scale analysis with lateral resolution down to 1–2 µm². The laser power reaching the sample surface was a maximum of 30 mW and the acquisition time was 60 s. Prior to each session, the microspectrometer was calibrated based on the 520.8 cm⁻¹ peak of silicon. Spectra were acquired in two spectral regions: a low frequency window between ~100 and 1500 cm⁻¹ (alumino-silicate network domain) and a high frequency window between ~2900 and 3800 cm⁻¹ (OH stretching domain). The calibration was performed using hydrous standard glasses, compositionally similar to the analysed samples, with known water concentrations (2–6.4 wt%, determined by FTIR, Di Carlo et al., 2010; Andújar and Scaillet, 2012). Both standard glasses and samples were analysed during each analytical session. The principle of the calibration is based on the correlation between the water concentration of the standard glasses and the calculated R<sub>w/s</sub> ratios, between the areas of the water (A<sub>w</sub>) and silicate (A<sub>s</sub>) regions (Fig. S2 Supplementary Material) (Tables 2 and 4). An average of three spectra was used for each sample, and the repeatability of the obtained R<sub>w/s</sub> ratios (calculated as the standard deviation of the mean) was better than 93% relative, with the exception of two samples (>90% relative, R1–RB1 and RB4, Table 2). The baseline correction of spectra was done using OriginPro software. In some samples, typically those of comenditic composition, the spectra showed a high fluorescent background which prevented a clear baseline subtraction and thus an accurate determination of the water concentration.

### 2.4.3. Br analysis by LA-ICPMS

LA-ICPMS Br analyses were performed using a RESOlation ArF 193 nm Excimer laser ablation system (Resonetics) coupled to an Agilent 8900 Triple Quadrupole ICP-MS, available at the joint ISTO/IRAMAT-CNRS facility. The ablation spot size was set at 100 µm in diameter to maximise sensitivity, with a fluence of 6 J/cm² at the sample surface and a repetition rate of 10 Hz. The fluence used here provides a uniform rate of signal decay during ablation for all the glasses analysed. Routine laboratory gas flows were found to provide optimal signal/background ratios for Br (~0.9 l/min Ar; 400 ml/min He). The measurement cycle includes 30 s for baseline measurements, 50 s of acquisition time and a washout of 50 s. Dwell times were 40 ms for Br and 20 ms for all other isotopes (Table S1, Supplementary Material). The two isotopes of Br (<sup>79</sup>Br and <sup>81</sup>Br) were analysed, to better quantify the effect of interferences.

#### Reference materials, data reduction and limit of quantification

The time-resolved data were reduced using the GLITTER™ software package (Griffin et al., 2008). Peak heights were determined by background subtraction in the usual way. The synthetic glass NIST SRM 610 was used for external calibration (Jochum et al., 2005) for trace elements. The Br-doped basaltic glass B3000 was selected as the external standard for Br quantification. The recommended reference value for Br for this glass standard was established using SIMS and SR-XRF, as well as characterisation by LA-ICPMS (Br=2694 ± 134 ppm, Cadoux et al., 2017). We used the concentration of silicon (<sup>29</sup>Si) as an internal standard for data reduction (from EPM analysis; Table S7, Supplementary Material). BCR-2G glass (Jochum et al., 2005) was used as a secondary standard for trace elements and accuracy and precision on Br analysis were monitored in each analytical session with two secondary standards of basaltic composition (B1000 and B100, Cadoux et al. (2017); Table S2, Supplementary Material). Reference materials were analysed after every six samples during each analytical session. Most reference materials and run products provided measurable signals for both <sup>79</sup>Br and <sup>81</sup>Br, with a background of ~0.5 and ~1.5 kcps, respectively.
The analytical limit of quantification (LOQ), which represents the smallest amount of an analyte that can be measured with reasonable accuracy, is ~8 ppm for ⁸¹Br, identical to the LOQ reported by Caulfield et al. (2020). Details of the LOQ calculations are given in the Supplementary Material.

*Matrix and gas-based interferences and Br quantification*

Previous work has shown that multiple and significant polyatomic interferences are expected on both ⁷⁹Br and ⁸¹Br in an Ar-based plasma (e.g., [⁴⁰Ar³⁹K]⁺ or [³⁸Ar⁴⁰ArH]⁺ on ⁷⁹Br and [⁴¹K⁴⁰Ar]⁺ or [⁴⁰Ar⁴⁰ArH]⁺ on ⁸¹Br (Hammerli et al., 2013)). As potassium is a major element in our starting glasses (4.5 to 5.7 wt% K₂O, Table 1), K-based interferences can potentially lead to an overestimation of the Br concentration. Fusswinkel et al. (2018) have also shown that doubly charged rare earth elements (REE) interfere with the measurement of both ⁷⁹Br and ⁸¹Br. The main interferences are ¹⁵⁸Gd⁺⁺ and ¹⁵⁹Tb⁺⁺ at mass 79 and ¹⁶²Dy⁺⁺ at mass 81 (Fusswinkel et al., 2018; Kendrick et al., 2020; Caulfield et al. 2020). To quantify the effect of K- and REE-based interferences on Br quantification, we analysed well-characterised reference materials and in-house synthetic glasses using the analytical conditions described above. Details of the procedure and the correction factors are given in the Supplementary Material. These corrections can reasonably be neglected for the Br-doped experimental samples, but are significant for natural glasses (Table 1).

**2.5. Computing of the fluid phase composition and estimating the associated uncertainties**

The composition of the bulk fluid phase (i.e., a low-density vapor, higher density brine or coexisting vapor and brine, depending on the P-T-X conditions (see § 1.2)) at run conditions was determined by mass balance, based on the quantities of glass and solutions weighed prior to the experiments and the composition of quenched glass (cations, H₂O determined by the by-difference approach, and Br, Cl and F). The composition of the melt is affected by i) mineral crystallization, mainly Fe-Ti oxide, ii) dissolution of H₂O and Br (and possibly Na) from the added solutions and iii) fluid-melt interaction resulting in exchange of cations and volatiles (F, Cl). We have adapted the normalisation procedure of Botchanikov et al. (2015) in which the mass of SiO₂ in the melt is assumed to be constant and equal to that in the starting silicate glass loaded into the capsules, on the basis that quartz solubility is relatively low in the pressure range investigated (e.g., mole fraction of silica < 0.0020 in NaCl-H₂O fluids at 700°C and 200 MPa, Newton and Manning, 2000, 2010). The step-by-step calculation method and mass balance quantification are described in the Supplementary Material.

Following Alletti et al. (2014), we performed a propagation of the uncertainties assuming that they are independent, resulting in a large propagated error especially for Cl or F (Tables 2-4). However, previous studies have shown that these propagated errors often overestimate the absolute error and can therefore be considered as maximum values (e.g., Alletti et al. 2014; Cadoux et al. 2018).

**3. Results**

The concentrations of H₂O and halogens dissolved in the experimental glasses and the calculated compositions of the fluid phase under run conditions are given in Table 2 (phonolite), Table 3 (comendite) and Table 4 (pantellerite). The major element compositions of the run-product glasses are reported in Table S7 in Supplementary Material. These data are used to estimate partition coefficients of Br and Cl between fluid and melt (D<sub>Br, Cl</sub><sup>f/m</sup> according to Eq. (17) in Supplementary Material).

**3.1. Description of run products**
Experimental run products were carefully examined by microscopic observation and backscattered electron imaging (Fig. 2). Phonolite charges have vesicular glass with less than 1 wt% magnetite, at high fO₂ (Table S6, Supplementary material). Pantellerite or comendite charges mainly contain variably vesicular glass, with no or only trace magnetite crystals. Salt crystals (NaBr and/or NaCl) were observed in eight pantellerite and comendite charges as small (~10 µm) cubes embedded in the glass (Fig. 2b; Table S6, Supplementary Material), similar in shape and habit to those found in pantelleritic obsidian from Pantelleria (Lowenstern, 1994).

Glasses synthesized at 200 MPa (melt water concentration > 6%, Tables 2-4) are characterised by the presence of very small bubbles, generally less than 1 micron in diameter (Fig. 2a). They are heterogeneously distributed, producing “sponge-like” textures. These bubbles, observed only in the most hydrated glasses, are interpreted as quench products. In addition to these micron-sized vesicles, all the run-product glasses contain fluid inclusions (FI) ranging in size from ~5 µm to ~100 µm, with most falling in the 20–50 µm size range (Fig. 2c to h). The inclusions are mostly homogeneously distributed in the glass and their proportion ranges from 0.2 to 6 area % as determined by the analysis of BSE images using ImageJ software. Two types can be distinguished: the first type (hereafter type-1) contains a liquid phase wetting the walls plus a vapor bubble (essentially a vacuum) (see Fig. 2c, d, e). The second type (hereafter type-2) contains cubic daughter minerals together with a liquid and a vapor phase (Fig. 2f, g, h), indicating brine saturation. In the same charge, the fluid inclusion population(s) show uniform phase ratios at room temperature, indicating similar density and composition (Fig. 2b-h). This suggests that type-1 and type-2 fluid inclusions correspond to trapped phases that were present during the run (e.g., Shinohara et al., 1989). The run-product glasses contain type-1, type-2 or both inclusion types (Table S6, Supplementary Material). The distribution and relative abundance of type-1 or type-2 fluid inclusions are related to P-T conditions, starting glass composition and starting solution salinity. At 200 MPa and 1000°C at oxidising conditions, only type-1 fluid inclusions were found in the phonolitic system with a total Br concentration in the system ([Br]bulk) of about 300 ppm (Fig. 2c), while type-2 fluid inclusions were identified in the Br-richest charges of this run (Table S6, Supplementary Material). Type-2 fluid inclusions are common in comendite charges, which is the most Cl-rich composition, and their relative abundance increases with [Br]bulk (Fig. 2e, f). Type-2 inclusions are not observed at 1000°C and 50 MPa (regardless of the system; e.g., Fig. 2d) or 10 MPa (phonolitic system), but are predominant at 200 MPa and low temperatures (900 and 800°C; Fig. 2g, h and Table S6). Biphasic (type-1) inclusions may correspond to trapped supercritical or vapor phase, whereas triphasic (type-2) inclusions may be attributed to trapped hydrosaline liquid phase. The coexistence of two distinct populations of fluid inclusions with contrasting salinities in run products is first-order evidence that the system experienced vapor-brine immiscibility of the fluid phase at run conditions (e.g., Sterner and Bodnar, 1984; Shinohara et al., 1989; Signorelli and Carroll, 2000). Furthermore, in all experiments, the Cl and Br concentrations derived from mass balance for the bulk equilibrium fluid (Tables 2-4) are below salt saturation at room temperature (26.46 wt% NaCl at 25°C). Hence, the presence of type-2 fluid inclusions and/or salt crystals embedded in the glass is taken to indicate vapor-brine immiscibility at run conditions. The corresponding experiments are represented with black-filled symbols in Fig. 1 (and this colour code is used for all the figures). The second group consists of experiments with only type-1 fluid inclusions, which are likely to be in the vapor field (white-filled symbols in Fig. 1). The last group includes all other experiments, the status of which could not be clearly established from optical observation of the run product (grey-filled symbols in Fig. 1).

## 3.2. Halogens and H₂O dissolved in run-product glasses

### 3.2.1. Br concentration in the run-product glasses

The concentrations of Br dissolved in the quenched melts ([Br]melt) are listed in Tables 2-4 and plotted as a function of both *P* and *T* in Fig. 3. Br concentrations range from 148 ± 11 to 5489
± 417 ppm (Tables 2-4; Fig. 3), similar to previous studies (Bureau et al., 2000; Bureau & Metrich, 2003; Bureau et al., 2010; Cadoux et al., 2018). The low standard deviations show that the glasses are homogeneous in terms of Br concentration (Tables 2-4). The total error of the Br concentrations, which includes the analytical error and the statistical uncertainty calculated on the mean, was generally less than 16% relative.

At fixed experimental conditions and comparable $[Br]_{bulk}$, the highest $[Br]_{melt}$ are observed in Cl- and F-poor phonolite (204 ± 13 and 5489 ± 417 ppm, Table 2 and Fig. 2a, d), while Cl- and F-rich peralkaline comenditic glasses exhibit the lowest Br concentration range (148 ± 11 to 3181 ± 445 ppm, Table 3, Fig. 3b, e). For each melt composition, the main control parameter is $[Br]_{bulk}$. $[Br]_{melt}$ increases drastically with increasing $[Br]_{bulk}$, other parameters being equal. At fixed P and $[Br]_{bulk}$, $[Br]_{melt}$ increases with temperature (from 800–900° to 1000°C), for all the compositions, regardless of whether the charges are saturated in an immiscible fluid (Fig. 3e, $[Br]_{bulk}$ ~300 ppm) or lie in the vapor field (Fig. 3d). The effect of P is less straightforward. In the phonolite system and for $[Br]_{bulk} > 300$ ppm, $[Br]_{melt}$ decreases from 50 to 10 MPa, and this decrease is more pronounced for high $[Br]_{bulk}$ (Fig. 3a). In contrast, in pantellerite, $[Br]_{melt}$ increases from 50 to 25 MPa in experiments in the vapor field (Fig. 3c). From 200 to 50 MPa, the trends are difficult to assess due to the lack of data at low $[Br]_{bulk}$ and the uncertainty regarding brine saturation in some experiments. At 200 MPa/1000°C, a decrease in $fO_2$ leads to a small decrease in $[Br]_{melt}$ in the phonolite, while no clear effect is observed for peralkaline rhyolites (Fig. 2a, b, c).

### 3.2.2. Cl concentration in the run-product glasses

The concentrations of Cl dissolved in the quenched glasses ($[Cl]_{melt}$) are given in Tables 2-4 and plotted as a function of both P and T in Fig. 4. As charges were not doped with Cl, $[Cl]_{melt}$ is compared with the concentration of the starting glasses in Fig. 4. $[Cl]_{melt}$ ranges from 1045 ± 259 to 4227 ± 311 ppm. The total error, taking into account the detection limit of 250 ppm, is mostly less than 15% relative (Tables 2-4). All glasses (except R23-RB1, Table 2 and Fig. 4a) are depleted in $[Cl]_{melt}$ compared to their respective starting composition and this depletion becomes more pronounced with increasing $[Br]_{bulk}$ at fixed P-T-$fO_2$ (Fig. 4). This salting-out effect is systematically observed with increasing $[Br]_{bulk}$ which suggests that Cl and Br are competing with each other in the hydrous melt network (Bureau & Metrich, 2003; Cassidy et al., 2022).

As observed for $[Br]_{melt}$, at fixed pressure and $[Br]_{bulk}$, $[Cl]_{melt}$ increases with increasing temperature in all compositions (Fig. 4d, e, f). The effect of P is again difficult to fully assess. In vapor-saturated phonolite melts, $[Cl]_{melt}$ increases from 200 to 50 MPa and then decreases from 50 MPa to 10 MPa (except for $[Br]_{bulk}$ ~300 ppm), as observed for Br (Fig. 4a). A slight decrease in $[Cl]_{melt}$ is also observed in vapor-saturated pantelleritic charges from 50 MPa to 25 MPa (Fig. 4c). For brine-saturated comendite charges (with $[Br]_{bulk}$ ~6000 ppm), variations in $[Cl]_{melt}$ are limited in the 200–25 MPa pressure range at 1000°C (Fig. 4b). At 200 MPa/1000°C, a decrease in $fO_2$ leads to a limited decrease in $[Cl]_{melt}$ in phonolite and comendite, but no clear effect is observed for the pantellerite (Fig. 4a, b, c).

### 3.2.3. F concentration in the run-product glasses

F concentrations in experimental glasses are reported in Tables 2-4, being close to or below the detection limit (0.2 wt%) for most phonolitic glasses which, along with high uncertainties (> 100% relative), hampers the interpretation of the data. More significant variations in F concentration are observed for comenditic and, to a lesser extent, pantelleritic glasses (Tables 3 and 4). Some glasses are significantly depleted compared to their starting composition, indicating F loss towards the fluid in hydrous peralkaline rhyolitic systems. However, the imprecision of the F analysis hinders a thorough assessment of the effect of experimental parameters on the behaviour of F in these systems.
### 3.2.4. Water dissolved in glasses

The concentration of H₂O dissolved in the run-product glasses ([H₂O]melt) is given in Tables 2-4, either measured by Raman spectroscopy and/or estimated by the by-difference method, both methods agreeing within 1 wt%, except for 6 charges, for unclear reasons. The low standard deviation of the by-difference data demonstrates the homogeneity of the glasses (Tables 2-4). Standard deviations are generally higher for the Raman data, owing to the fluorescence background affecting some spectra (Tables 2 and 4).

The [H₂O]melt ranges from 0.7 to 7.1 wt% (Tables 2-4), with a similar concentration range for all three compositions. For fixed P-T-fO₂, [H₂O]melt remains roughly constant with increasing [Br]bulk, within uncertainty (Tables 2-4; Fig. S9, Supplementary Material). However, we note that at 1000°C and 10 MPa, the [H₂O]melt of phonolite glasses increases slightly with increasing [Br]bulk (Table 2). [H₂O]melt increases with pressure from 10 to 200 MPa with values similar to the H₂O solubility for each composition, as calculated with the model of Moore et al. (1998) (Tables 2-4, Fig. S8, Supplementary Material).

## 3.3. Major element composition of the glasses

The glass compositions recalculated to 100 on a volatile-free basis, are given in Table S7 in the Supplementary Material. In addition, Fig. S10 (Supplementary Material) shows these compositions as a function of the Br concentration in the melt and compares them with the corresponding starting glass compositions, showing no significant differences in oxide contents, except for FeO and Na₂O. FeO is slightly depleted in phonolite charges at 900°C, owing to magnetite crystallization. Na₂O may show gain or loss relative to starting material as illustrated in Fig. 5. Loss of Na is observed in all experiments, with the exception of the phonolite at 10 MPa (Fig. 5a). At 1000°C and oxidising conditions, increasing P enhances Na loss for all compositions (Fig. 5a, c, e). A limited Na gain is observed at 10 MPa for the phonolitic composition (Fig 5a). At fixed P-T-X, decreasing fO₂ limits Na loss (Fig. 5a, c, e). At fixed P-T-fO₂, an increase in NaBr in the starting solution - reflected by an increase in the Br concentration of the melt in Fig. 5 - generally leads to a decrease in Na extraction (or an increase in Na gain at 10 MPa for phonolite). This trend is observed for experimental series in the vapor field (phonolite at 50 MPa, Fig. 5a) or for experiments saturated in a brine (e.g., pantellerite at 800°C, Fig. 5f). The effect of temperature at 200 MPa is not obvious, although Na loss clearly further increases as the temperature decreases to 800°C for the pantelleritic composition (Fig. 5f). These compositional changes are therefore indicative of interaction between the melt and halogen-bearing fluids, resulting in cation (particularly sodium) and volatiles exchange.

## 3.4. Composition of the fluid phase and partitioning of Br and Cl between phases

### 3.4.1. Calculated composition of the fluid phase

The calculated volatile and cation concentrations in the bulk fluid are given in Tables 2-4 and detailed in Supplementary Material (Fig. S11). The relationships between calculated compositions of the fluid and measured concentrations of Br and Br+Cl in the quenched melts are shown in Fig. 6. In general, the breaks in the slope observed in Fig. 6 point to non-ideal behaviour of the halogen-H₂O system under the conditions investigated. However, the experiments carried out at 50 and 10 MPa with phonolite show very distinct trends with a steep and continuous increase in the Cl+Br concentration of the melt with increasing bulk Br, while the total salinity of the fluid (NaCl+NaBr equivalent concentration in the bulk fluid, $eqC_{NaCl+NaBr}^f$) does not exceed ~3 wt% (Fig. 6b). All experiments at 50 MPa are in the vapor field (having type-1 inclusions only). Given the similar trends, low fluid salinities, and absence of detected or suspected salts, it is likely that all experiments at 10 MPa are also in the vapor field, although lying well within the subcritical region of the NaCl-H₂O system (Fig. 1). The status of
experiments at 1100°C is more difficult to establish (because of the lack of textural arguments in particular), but it is likely that the three experiments with the lowest total salinities are also in the vapor field.

### 3.4.2. Fluid-melt Br and Cl partitioning

The data in Figure 6 and Tables 2-4 can be used to estimate the partition coefficients of Br and Cl (wt% ratios) between bulk fluid and melt. The $D_{Br,Cl}^{f/m}$ are reported in Tables 2-4, with 1 $\sigma$ propagated error from the variables considered in the mass balance (Table S5, Supplementary Material). For experiments in which immiscible vapor and brine coexist with melt, $D_{Br,Cl}^{f/m}$ are apparent partition coefficients, with reference to the bulk fluid. $D_{Br}^{f/m}$ are highest for the halogen-rich comenditic rhyolite in the vapor + brine field at 200 MPa and low T (Fig. 7c), and lowest for the phonolite and pantellerite in the vapor field at low P (Fig. 7a, e). All runs show that Br partitions predominantly into the fluid phase over the melt phase, with the exception of one pantellerite experiment with $[Br]_{bulk} \sim 300$ ppm at 25 MPa/1000°C ($D_{Br}^{f/m} \sim 0.8$; Table 4; Fig. 7e). For all compositions, $D_{Br}^{f/m}$ shows broad positive correlations with the bulk fluid salinity, independently of P, T or $fO_2$, consistent with previous studies (Fig. 7, Tables 2-4). Specifically, $D_{Br}^{f/m}$ increases with both the Br and Cl concentration of the bulk fluid phase, both of which in turn also increase as Br is added to the system. This behaviour is observed in experiments saturated in vapor only or in vapor + brine. Thus, the total halogen concentration is one of the primary controls on $D_{Br}^{f/m}$. At fixed bulk fluid salinity and melt composition, $D_{Br}^{f/m}$ is sensitive to T and, to a lesser extent, to P and, in the phonolitic system, to $fO_2$. The same behaviour is observed for Cl (Tables 2-4; Fig. S12, Supplementary Material). $D_{Cl}^{f/m}$ increases with $[Br]_{bulk}$ in response to the Cl salting-out effect mentioned previously. Overall, $D_{Cl}^{f/m}$ is higher for Br than for Cl (Tables 2-4). A similar relative fractionation of heavy halogens has previously been demonstrated for synthetic systems (albite-water-alkali halogenide, Bureau et al., 2000) or for intermediate to silicic natural calc-alkaline melts (Cassidy et al., 2022) and is attributed to increasing ionic radii of halogenide ions from Cl to Br.

## 4. Discussion

### 4.1. Phase relations in the fluid phase and the effect of Br

At low Br concentrations (and high Cl/Br ratios), Cl (± F) can be expected to control the fluid phase relationships, which affects Br behaviour (e.g., Cassidy et al., 2022). However, at high Br concentrations (and low Cl/Br ratios) in the system, it is possible that Br may affect the volatile phase assemblage. All experiments carried out at 200 MPa and 900 or 800°C in the pantellerite or comendite systems show unambiguous evidence of immiscibility, whereas these conditions are in the supercritical region of the NaCl-H₂O phase diagram (Fig. 1). The critical pressure in the systems of our study therefore extends to $P \ge 200$ MPa at 900–800°C. Conversely, all the experiments performed at 25 MPa in the pantellerite system fall well within the immiscibility region in Fig. 1 (note that this is also the case if only the NaCl equivalent concentration in the bulk fluid ($^{eq}C_{NaCl}^{f}$) is considered). However, examination of the run products shows that the lowest-halogen fluids in this series are in the vapor field (Table S6, Supplementary Material), which suggests a shrinkage of the immiscibility gap (see also Alletti et al., 2014, Botcharnikov et al., 2015). The melt compositions studied here are rich in K, which is known to affect immiscibility at low temperature (e.g., Anderko and Pitzer, 1993b). To assess the potential effect of Br on fluid phase relationships, Fig. 8 compares the relationships between the Cl concentration of the fluid and the melt phases produced in experiments performed in the Br-free NaCl-H₂O-phonolite system (Alletti et al., 2014; Signorelli and Carroll, 2000) with the results of selected experiments in the F-poor phonolitic system of this study. The trend observed at 50 MPa in our experiments saturated in a vapor phase (very low in halogens) is very similar to those observed in the 880°C experiments of Signorelli and Carroll
(2000), which used the same starting phonolite, or in the 1000°C experiments of Alletti et al. (2014), with a metaluminous phonolite (Fig. 8a). Note that Signorelli and Carroll (2000) reported fluid immiscibility at 50 MPa for ~2 wt% bulk Cl concentration, whereas the maximum (Cl+Br) bulk concentration investigated in our 50 MPa experiments is ~0.8 wt% (Table 2). Alletti et al. (2014) found non-ideal behaviour in the system at 50 or 10 MPa for high bulk Cl concentration, but did however not report fluid phase immiscibility. At 200 MPa and ~900°C, the trends observed in our experiments and in the Br-free vapor-saturated experiments by Signorelli and Carroll (2000) are, again, almost superimposable (i.e., the sum of the halogen concentration of our samples falls within the range of the Cl concentration of the Signorelli and Carroll (2000) experiments), suggesting similar mixing properties of the fluid phase (Fig. 8b). At 1000°C and oxidising conditions, we observe immiscibility in our experiments for (Br+Cl) in the melt at ~0.5 wt%, and the behaviour in the chloride-H₂O system in the experiments of Alletti et al. (2014) shifts from Henrian to highly non-ideal for Cl concentration in the melt between ~0.35 and 0.6 wt% (Fig. 8b). Taken together, these observations suggest that Br does not drastically affect the mixing properties of the fluid phase in the Br concentration range investigated, at least for the least Br-doped charges, and also seem to confirm that Br and Cl dissolution mechanisms in silicate melt are similar, as suggested by Bureau and Metrich (2003).

## 4.2. Br partitioning in silicic alkaline magmas

As the salinity of the fluid is controlled by the mass halogen/H₂O ratio at fluid saturation, our experiments produced a wide range of fluid salinities ($eqC_{NaCl+NaBr}^f$ up to ~25 wt%, e.g., Fig. 6f). Br shows a Henrian behaviour in the phonolitic system at 1000°C/50 and 10 MPa, which produced a low salinity vapor phase at run conditions (Fig. 6b). In all other series, we observe the non-Henrian dependence of $D_{Br}^{f/m}$ on fluid salinity. To correctly assess the effect of P, T or fO₂ on the behaviour of Br in natural magmas, we focus the discussion below on experiments doped with ~300 ppm Br (with the exception of phonolite at 10 and 50 MPa, see below), which is closer to natural Br concentrations in magmas (< 20 ppm for intermediate and felsic melts, e.g., Webster et al., 2018) (Fig. 9). For clarity, $D_{Br,Cl}^{v/m}$ and $D_{Br,Cl}^{bf/m}$ in Figures 9-11 below refer to partition coefficients in melt + vapor and melt + vapor + brine experiments respectively.

### 4.2.1. Vapor-melt partitioning of Br ($D_{Br}^{v/m}$)

As all experiments at 10 and 50 MPa in the phonolitic system show Henrian behaviour, the $D_{Br}^{v/m}$ reported in Fig. 10a are calculated by linear regression (considering all experiments at each pressure, Fig. S13 in Supplementary Material). $D_{Br}^{v/m}$ range from 0.8 ± 0.7 (pantellerite, 1000°C, 25 MPa) to 18.5 ± 3.6 (phonolite, 900°C, 200 MPa) being within the range determined for calc-alkaline andesite to rhyolite in the studies of Cadoux et al. (2018) (6.4 ± 3.5 to 20.2 ± 1.2) and Cassidy et al. (2022) (1.85 ± 0.80 to 36.40 ± 8.40 in CO₂-free experiments). At 200 MPa, 1000-1060°C and oxidising conditions, $D_{Br}^{v/m}$ increases significantly from the phonolite to the calc-alkaline andesite (both with ~60 wt% SiO₂), indicating that $D_{Br}^{v/m}$ decreases with increasing alkalinity in silicic magmas (Fig 10a). However, at 50-100 MPa, 1000°C and oxidising conditions, $D_{Br}^{v/m}$ values determined by Cassidy et al. (2022) for Br-undoped dacite melt is of the same order of magnitude as determined in the phonolite in this study (Fig. 10a). This low $D_{Br}^{v/m}$ in calc-alkaline composition is perhaps partly related to the lower halogen concentration in the Cassidy et al. study (~1000-1500 ppm Cl and ~4-5 ppm Br in the equilibrium fluid phase) compared to our study. At fixed bulk halogen concentration and melt composition, Fig. 9a shows that $D_{Br}^{v/m}$ increases with bulk fluid salinity (Cl dominated). $D_{Br}^{v/m}$ shows a weak dependence on P, whereas bulk salinity decreases strongly with decreasing P. $D_{Br}^{v/m}$ varies little between 1100 and 1000°C but increases markedly (from 3.8 ± 1.5 to 18.5 ± 3.6) between 1000 and 900°C at 200 MPa, for a limited increase in salinity (Fig. 9a). The
decrease in fO₂ induces an increase in D<sub>Br</sub><sup>v/m</sup>, which is less pronounced than for the decrease in T.

The effect of P on D<sub>Br</sub><sup>v/m</sup> at 1000°C

Our results show that at 1000°C under oxidising conditions, the D<sub>Br</sub><sup>v/m</sup> are low (<4) over the P range studied, but exhibit significant variations in behaviour. At 1000°C and oxidising conditions, D<sub>Br</sub><sup>v/m</sup> in the phonolite system decreases from 3.8 ± 1.5 at 200 MPa to 1.8 ± 0.9 at 50 MPa and then increases to 4.8 ± 0.9 at 10 MPa. For the pantelleritic composition, D<sub>Br</sub><sup>v/m</sup> decreases from 3.0 ± 2.1 at 50 MPa to less than unity at 25 MPa. The total error in D<sub>Cl</sub><sup>v/m</sup> is very large (Tables 2-4). However, <sup>eq</sup>C<sub>NaCl</sub><sup>f</sup> decreases dramatically in the phonolitic system from 200 MPa (~1.55 wt%, Table 2) to 50 MPa (<0.1 wt%, Table 2), suggesting a strong positive dependence of Cl partitioning on P within this interval.

No previous experimental study has systematically investigated the effect of pressure on fluid-
melt Br partitioning at crustal pressures and fixed T-fO₂-X conditions. Cassidy et al. (2022)
explored halogens behaviour in the hydrous dacitic system (0.7 ppm Br in bulk rock, 6.1 ppm
in starting matrix glass) and found very little variation in D<sub>Br</sub><sup>v/m</sup> between 100 and 50 MPa at
1000°C (Fig. 10a), but because the pressure of the experiments also affects melt composition
through the crystallization or melting of mineral phases, it is difficult to disentangle these two
factors. Cl solubility/partitioning with pressure have been extensively studied, but the observed
effects are in some cases contradictory. In general, in experiments saturated in vapor phase
only, D<sub>Cl</sub><sup>v/m</sup> is found to decrease with decreasing P for a wide range of melt compositions, from
basalt to andesite, phonolite or rhyolite (Shinohara et al. 1989; Webster, 1992a, b; Signorelli
and Carroll, 2000; Alletti et al., 2009, 2014; Botcharnikov et al., 2015; Hsu et al., 2019; Tattitch
et al., 2021). It is worth noting, however, that the lowest pressure range (< 100 MPa) has
received relatively little attention. Alletti et al. (2014) reported a pronounced minimum in D<sub>Cl</sub><sup>v/m</sup>
at 50 MPa and 1000°C in metaluminous phonolite. Below 50 MPa, D<sub>Cl</sub><sup>v/m</sup> shows an inverse
relationship with P, i.e., decreasing P promotes Cl exsolution. Such a minimum value has also
been identified in the andesitic system at 200 MPa and 1200°C (Botcharnikov et al., 2015). In
contrast, however, the recent study by Tattitch et al. (2021) reported no minimum D<sub>Cl</sub><sup>v/m</sup> for
rhyodacitic or rhyolitic systems in the range of 800–50 MPa and 800–850°C, leaving open the
possibility that D<sub>Cl</sub><sup>v/m</sup> increases at P < 50 MPa. Our data show such a minimum in D<sub>Br</sub><sup>f/m</sup> for the
peralkaline phonolite at P < 50 MPa, but not for the most peralkaline rhyolite in the pressure
range studied (Fig. 10a).

Cl exists primarily as metal-chloride species (mainly NaCl, KCl, CaCl₂, FeCl₂) and HCl species in aqueous fluids, and both experimental and theoretical constraints have consistently shown that their relative proportions are controlled by silicate melt composition, *P* and total dissolved chloride concentration (Shinohara et al., 1989; Candela, 1990; Williams et al., 1997; Frank et al., 2003; Shinohara, 2009; Zajacz et al., 2012; Dalou et al., 2015). Spectroscopic studies have shown general similarities between Cl and Br speciations in aqueous solutions and silicate glasses (e.g., Evans et al., 2008; Dalou et al., 2015; Louvel et al., 2020a, b). The strong positive dependence of D<sub>Cl</sub><sup>v/m</sup> on *P* has been attributed to the thermodynamic properties of NaCl (or KCl) in the fluid phase (Shinohara et al., 1989; Signorelli and Carroll, 2000; Dolejš and Zajacz, 2018, Tattitch et al. 2021). Tattitch et al. (2021) recently calculated the apparent partial molar volume of NaCl in aqueous fluids in the 50–800 MPa range and found a strong similarity in behaviour to D<sub>Cl</sub><sup>v/m</sup> (D<sub>Cl</sub><sup>f/v/m</sup> increases with pressure up to 200–300 MPa, then begins to plateau) in calc-alkaline metaluminous andesitic to rhyolitic systems. This suggests that the minimum D<sub>Cl</sub><sup>v/m</sup> observed in certain experimental studies is related to the change in Cl speciation (i.e., from metal-chloride species to HCl, Shinohara, 2009) and to the mixing properties of the fluid phase (Botcharnikov et al., 2015). This can be expressed in terms of the equilibrium reaction, which goes rightward with decreasing pressure:
$$2\text{NaCl}_{\text{(aq)}} + \text{H}_2\text{O}_{\text{(m)}} \leftrightarrow \text{Na}_2\text{O}_{\text{(m)}} + 2\text{HCl}_{\text{(aq)}} \quad (1)$$

Where m is the melt and aq is the aqueous fluid.

Fig. 5 shows a general decrease in Na loss from melts with decreasing P in our experiments and the mass balance shows a clear decrease in the total cation concentration of the fluid as P decreases (Tables 2-4). In detail, Fig. 5a shows that Na loss is very limited or absent in phonolite at 10 MPa, supporting the idea that the minimum $D_{\text{Br}}^{\text{v/m}}$ value corresponds to a change in Br speciation in the fluid (from NaBr to HBr), similar to Cl in andesite (Botcharnikov et al., 2015). It is noteworthy that the minimum $D_{\text{Br}}^{\text{v/m}}$ occurs at a pressure similar to the minimum $D_{\text{Cl}}^{\text{v/m}}$ observed by Alletti et al. (2014) in phonolite, but at a much lower pressure than in andesite (Botcharnikov et al., 2015). Furthermore, the absence of a minimum $D_{\text{Br}}^{\text{v/m}}$ for pantellerite suggests that speciation in the fluid phase depends on melt composition (A/CNK or Al+Fe$^{3+}$/(Na+K)) as previous studies have shown (Urabe, 1985; Candela, 1990; Williams et al., 1997; Frank et al., 2003; Shinohara, 2009; Zajacz et al., 2012). Altogether, the dependence of HCl exsolution on pressure, together with the observation that the pressure of the minimum $D_{\text{Cl}}^{\text{v/m}}$ and $D_{\text{Br}}^{\text{v/m}}$ appears to increase with increasing A/CNK in felsic systems supports the hypothesis that halogen speciation has a strong effect on $D_{\text{halogen}}^{\text{v/m}}$, but more work is needed clearly in this field.

### Effect of T on $D_{\text{Br}}^{\text{v/m}}$

The $D_{\text{Br}}^{\text{v/m}}$ in the phonolitic system at 200 MPa and oxidising conditions shows a 4-fold increase between 1000°C and 900°C (Fig. 9a and 10b). A smaller and less significant decrease is observed for $D_{\text{Cl}}^{\text{v/m}}$ (Table 2), indicating that the Cl/Br ratio in the vapor phase decreases during isobaric cooling and degassing. These results are consistent with the experimental results of Cadoux et al. (2018), who showed a moderate (30% rel.) increase in $D_{\text{Br}}^{\text{v/m}}$ in the rhyodacite system from 1060 to 900°C and of Cassidy et al. (2022), who observed a strong increase in $D_{\text{Br}}^{\text{v/m}}$ for Br-undoped rhyolite composition from 900 to 800°C at 120 MPa (Fig. 11b). As in our phonolite experiments, Cassidy et al. (2022) found a larger effect of T on $D_{\text{Br}}^{\text{f/m}}$ compared to Cl in calc-alkaline systems.

It appears consistent that high temperature, more depolymerized melts dissolve higher halogen concentrations than melts at lower T but available data are still somewhat contradictory. Webster et al. (2015) have shown that Cl solubility increases with T in brine saturated melts. Hsu et al. (2019) reported an increase in vapor-melt Cl partition coefficients from 15.1 ± 0.2 at 1000°C to 26.7 ± 0.9 at 850°C in the haplogranitic system at 200 MPa. In contrast, Iveson et al. (2019) reported no detectable T effect (925–1050°C) on $D_{\text{Cl}}^{\text{v/m}}$ in dacite experiments at 200 MPa, while in a similar system, Cassidy et al. (2022) found such an effect, yet at lower T (<900°C). Thomas and Wood (2021) performed experiments on anhydrous basaltic compositions at HP-HT (0.5–2 GPa, 1200–1500°C) and found that Cl solubility increases with increasing T at fixed Cl$_2$ and O$_2$ fugacities, while increasing P has the reverse effect. The larger effect of T on $D_{\text{Br}}^{\text{v/m}}$ compared to Cl suspected in our study in peralkaline system and evidenced in Cassidy et al. (2022) study in calc-alkaline system could be explained by their respective chemical characteristics. Br would form relatively weaker metal-ligand bonds in the silicate melt compared to Cl due to its lower electronegativity and charge density (e.g., Cassidy et al., 2022). Moreover, Br could also be more affected by temperature-induced structural changes in the melt. In addition, it is possible that the decreasing activity coefficients of NaBr and NaCl in the fluid phase (expected to be dominated by metal-halide species at 200 MPa) contribute, at least in part, to the increase in halogen fluid-melt partitioning with decreasing T. Such a behaviour is expected because, as T decreases, the density of the aqueous solution increases significantly (e.g., Lvov and Wood, 1990; Driesner, 2007) and therefore the ability of water to form hydration shells around associated NaBr and NaCl ions
pairs is greater due to the increased dielectric constant of water at higher fluid densities (e.g., Helgeson, 1981; Bowers and Helgeson, 1983; Driesner et al., 1998).

### $fO_2$ effect on $D_{Br}^{v/m}$

In the phonolite system at 200 MPa and 1000°C, as $fO_2$ decreases, $D_{Br}^{v/m}$ increases from 3.8 ± 1.5 to 9.9 ± 3.3 (Fig. 9a and 10a). $D_{Cl}^{v/m}$ is less well quantified, but is also observed to increase slightly as $fO_2$ decreases (Table 2). Alletti et al. (2009) and Zajacz et al. (2012) found a negligible effect of $fO_2$ on $D_{Cl}^{v/m}$ at 200 MPa in basaltic (NNO to NNO+2) and andesitic (NNO-1.2 to NNO+1.8) systems, respectively. In contrast, the results from previous studies on phonolitic melts (Webster et al. 2009, Chevychelov et al., 2008; Alletti et al., 2014) and pantellerite magmas (Di Carlo et al., 2010) suggest that reduced conditions may favour the dissolution of Cl in the melt and hence lower $D_{Cl}^{v/m}$. This hypothesis is, however, based on the comparison of several experimental studies, not on a systematic investigation of the effect of $fO_2$. Iveson et al. (2017) and Bell and Webster (2015) suggested that a greater proportion of Cl could be accommodated in more reduced melts through complexation with the greater abundances of $Fe^{2+}$, and hence $D_{Cl}^{v/m}$ values should decrease. Indeed, studies of the compositional dependence of Cl solubility in silicate melts have shown that Ca and Fe have a major effect on Cl solubility, suggesting that Cl bonds with the $2^{+}$ cations present in the melt (e.g., Thomas and Wood, 2023; Thomas et al., 2023). Br, like Cl, favours bonding with network-modifier cations in sites involving either $Na^+$, $K^+$ or $Ca^{2+}$, although bonding with $Fe^{2+}$ has not been reported (Cochain et al., 2015; Louvel et al., 2020a, b).

Although we have no constraints on the $Fe^{2+}/Fe^{3+}$ ratios of our glasses, higher $Fe^{2+}/Fe^{3+}$ ratio in the melt phase is expected at low $fO_2$ (<NNO), favouring Cl, and probably Br, bonding with Fe. Such changes in Br or Cl speciation are not detected in our experiments since $D_{Br,Cl}^{v/m}$ is higher in reduced conditions. This suggests in turn that Br-Fe (and Cl-Fe) bonding does not dominantly control the dissolution of Br and Cl in the peralkaline felsic melt under the conditions investigated. Since Fe is expected to be predominantly present in the ferrous form in the fluid phase (see Dolejš and Zajacz, 2018), changes in Br (and Cl) speciation in the fluid phase could also contribute to the higher $D_{Br,Cl}^{v/m}$ observed at reduced conditions. While further work is required, our results highlight the need to better account for $fO_2$ in volcanic Br (and probably other halogens) budgets. If the trend observed here is confirmed by future work, it means that alkali-rich magmas, which are often halogen-rich and reduced (e.g., Scaillet and MacDonald, 2006), could potentially release at least as much Br as their calc-alkaline counterparts, which are generally oxidised. This is particularly evident in Fig. 10a, which shows that $D_{Br}^{v/m}$ for an alkaline magma under reduced conditions is close to the $D_{Br}^{v/m}$ for a calc-alkaline magma under oxidising conditions, for the same P-T conditions and comparable $SiO_2$ concentrations (~60 wt%).

### 4.2.2. Behaviour of Br in peralkaline rhyolites in the immiscibility field at 200 MPa

At 200 MPa, oxidising conditions and $T<1100°C$, all pantelleritic and comenditic charges were brine saturated (Fig. 9b) despite being above the critical point of the NaCl-H₂O system (~950°C at 200 MPa, Fig. 1) (see also Métrich and Rutherford, 1992; Signorelli and Carroll, 2000; Chevychelov et al., 2008). Métrich and Rutherford (1992) found that the Cl saturation limit in a pantelleritic melt saturated with an immiscible fluid at 200 MPa and 830°C is 5040 ppm. According to the model by Webster et al. (2015), the calculated Cl solubility at brine (± vapor) saturation in pantelleritic water-saturated melts exceeds 10000 ppm at 200 MPa within the temperature range 900–800°C. In this study, the pantelleritic melt saturated in vapor and brine at 200 MPa and 800°C contains ~2900 ppm Cl and 170 ppm Br (R22-EB1, Table 4). This suggests that our system composition and/or experimental conditions promote vapor + brine exsolution from the melt. For charges with ~300 ppm bulk Br, Cl largely dominates the heavy halogen budget (e.g., Cl/Br ratio of the melt range from ~11 to 18, Tables 3-4), and, as
discussed earlier, it is unlikely that the Br present in the system affects drastically the phase
relationships of the fluid phase. It is possible that the oxidising conditions of our experiments
promoted the exsolution of brines, as suggested by the increase in cation concentration of the
bulk fluids with decreasing fO₂. However, the data base of Webster et al. (2015) model also
uses oxidised experiments (NNO+2.5). Also, our two rhyolites are very rich in F (1.25 to 1.63
wt%, Table 1) and our results suggest that the fluids may contain several wt% of F at run
conditions (Tables 3-4). Chevychelov et al. (2008) have shown that unmixing of the Cl-bearing
fluid phase coexisting with phonolite melt is promoted by high F concentration of the system
and may begin at 845°C at 200 MPa. NMR and spectroscopic measurements have shown that
in peraluminous glasses, F only bonds to the network-building cations Si and Al, whereas in
peralkaline glasses F additionally bonds to the network-modifying cations Na and Ca by
forming salt-like environments (e.g., Baasner et al., 2014), strengthening the idea that F
competes with Cl and Br in the melt structure. This would explain the difference between our
experimental results and those of Metrich and Rutherford (1992), who used a pantellerite
(Kenya) with a composition very similar to ours, but with a lower F concentration (~4000 ppm
for the bulk rock).

In the vapor + brine region, the apparent partition coefficient of Br (Br bulk fluid-melt partition coefficient D<sub>Br</sub><sup>bf/m</sup>) varies from 6.8 ± 2.1 to 34.7 ± 7.3 (Fig. 9b). We cannot estimate the composition of the vapor and brine phases due to the lack of relevant phase equilibrium data for our systems. Therefore, D<sub>Br</sub><sup>bf/m</sup> is only a crude estimate of the behaviour of Br in the bulk fluid extracted from the magma. D<sub>Br</sub><sup>bf/m</sup> increases with the total salinity of the bulk fluid (largely dominated by NaCl) and with T. D<sub>Br</sub><sup>bf/m</sup> is systematically greater than D<sub>Cl</sub><sup>bf/m</sup> and the ratio between D<sub>Br</sub><sup>bf/m</sup> and D<sub>Cl</sub><sup>bf/m</sup> increases as T decreases (Tables 3-4). In the comendite, as T decreases from 1000 to 900°C, the salinity of the bulk fluid increases from ~4 wt% to ~8 wt%, while D<sub>Br</sub><sup>bf/m</sup> increases from 6.8 ± 2.1 to 34.7 ± 7.3. The total salinity of the bulk fluid phase in the pantellerite system varies little between 900 and 800°C (eqC<sub>NaCl+NaBr</sub><sup>f</sup> ~5 wt%), but D<sub>Br</sub><sup>bf/m</sup> increases drastically from 13.1 ± 4.7 to 29.6 ± 6.4 in response to the decrease in T and perhaps the relative proportions of vapor and brine in the expelled bulk fluid (Fig. 9b).

These results suggest that the exsolution of immiscible vapor + hydrosaline liquid enhances the removal of Br from peralkaline silicic melts during cooling accompanied by degassing. Few studies have addressed the partitioning of Br between vapor and brine. Experiments performed by Seo and Zajacz (2016) at 900°C, 90–150 MPa indicate that the vapor phase is characterised by higher Br/Cl ratios relative to brine in alkali halide-H₂O systems. Similarly, hydrothermal experiments conducted in the NaCl-H₂O system (388–550°C, 25–35 MPa) indicate preferential partitioning of Br (trace amount) into the low-salinity vapor fluids (Foustoukos and Seyfried, 2007). However, as the vapor/brine mass ratio is unknown, it is not yet possible to estimate the amount of Br that remains sequestered in subvolcanic brine lenses.

**4.3. Implications on halogens degassing in peralkaline silicic magmas**

In addition to decompression, crystallization-induced degassing (i.e., “second boiling”) during
storage in crustal reservoirs results in the exsolution of a fluid phase, whose contribution to the
release of volcanogenic volatiles into the atmosphere is increasingly recognised as being of
paramount importance (e.g., Westrich and Gerlach, 1992; Wallace et al., 1995, 1999; Wallace,
2001; Scaillet et al., 2003; Kutterolf et al. 2013, 2015; Iacovino et al., 2016; Vidal et al., 2016;
Iddon and Edmonds, 2020; Waelkens et al., 2021; Scaillet and Oppenheimer, 2023) with pre-
eruptive amounts of exsolved fluid up to 5–10 wt%. We illustrate this with a simple model of
closed system degassing, using the results gathered from our phonolite experiments in the
two-phase field (melt + vapor). We simulate the decompression of a magma having 6.15 wt%
H₂O, 3000 ppm Cl and 30 ppm Br in the melt, and coexisting with an exsolved fluid in amounts
of 0.1, 2 and 5 wt% (see Supplementary Material for further details).
The behaviour of Br and Cl during decompression is shown in Fig. 11. Both elements show little variations during decrompression until 50 MPa, beyond which their concentrations start to decrease (Fig. 11a, b). Increasing the amount of exsolved fluid only modestly affects those patterns, illustrating the considerable propensity of Cl and Br to remain in the melt. The ratio Cl/Br of the melt increases regularly with decreasing pressure (Fig. 11c). The corresponding Cl/Br ratio of the fluid exhibits a more complex pattern with pressure, with a first pronounced decrease down to 50 MPa, followed by a moderate increase down to near atmospheric pressure (Fig. 11d). This shows that an ascending phonolitic magma may release a diversity of fluid compositions, depending on the pressure at which the fluid escapes the rising magma body.

Altogether these calculations show that phonolite magmas begin to degas their Br and Cl in the uppermost parts of the plumbing system, probably as HBr and HCl species, as proposed for Cl in pantelleritic magmas (e.g., Nicholls and Carmichael, 1969; Lowenstern, 1994). According to our results, peralkaline rhyolites are expected to begin to degas their Br at even lower pressures, below 25 MPa. Melt inclusion studies of peralkaline felsic volcanic rocks (Kovalenko et al., 1993; Webster et al., 1993; Barclay et al., 1996; Balcone-Boissard et al., 2010; Gioncada and Landi, 2010; Macdonald et al., 2012; Neave et al., 2012; Lanzo et al., 2013, Iacovino et al., 2016; Iddon and Edmonds, 2020) indeed show that Cl or F concentrations of melt inclusions are generally similar to those of the outgassed matrix, suggesting little halogen degassing during eruption. The case for Br is unknown but is expected to be comparable to that of Cl. Finally, the observed overlap in halogen concentrations in alkaline systems contrasts with metaluminous calc-alkaline felsic magmas, where matrix glasses typically have lower Br and Cl concentrations than silicate melt inclusions, even in highly explosive contexts (e.g., Westrich et al., 1991; Kutterolf et al., 2013, 2015; Cadoux et al., 2015). Kutterolf et al. (2015) quantified Br and Cl emissions from Plinian eruptions along the Central American Volcanic Arc. They measured Br and Cl in matrix glasses and melt inclusions and showed that ~32 % of the initial Br and ~6 % of the initial Cl were degassed during the eruption. The more efficient extraction of Br compared to Cl is also consistent with the higher $D_{Br}^{v/m}$ values compared to $D_{Cl}^{v/m}$ determined experimentally in natural felsic calc-alkaline systems (Cassidy et al., 2022). Therefore, although syn-eruptive halogen extraction is undoubtedly strongly dependent upon the degassing regime in silicic systems (e.g., dome-forming vs. Plinian eruptions), it also appears to be controlled by magma composition, even in highly explosive eruptions, consistent with the results of our partitioning experiments and previous experimental work.

## 5. Concluding remarks

We have investigated the fluid-melt partitioning of Br in vapor-saturated alkali-rich silicic magmas at crustal pressure (800–1100°C, 10–200 MPa, oxidising and reducing conditions). Our results show that Br partitions increasingly into the vapor phase as the bulk halogen concentration increases. Furthermore, Br partitioning in the vapor phase increases with decreasing alkalinity in silicic magmas. The pressure dependence of $D_{Br}^{v/m}$ is complex, but clearly shows that syneruptive degassing of peralkaline felsic magmas is limited to the shallowest parts of the plumbing system, and occurs at lower pressures than in the case of their metaluminous counterparts. An important finding is that temperature has a significant effect on the fluid-melt partitioning of Br and Cl, at least for the studied melt compositions. In phonolitic magmas saturated in vapor phase, $D_{Br}^{v/m}$ increase drastically on cooling. Furthermore, the ratio between Br and Cl partition coefficients is not conservative and the Br/Cl ratio in the vapor phase is likely to increase during isobaric cooling and degassing. The vapor-melt Br and, to a lesser extent, Cl partition coefficients also appear to be significantly sensitive to $fO_2$ and for comparable P-T conditions and $SiO_2$ concentration, $D_{Br}^{v/m}$ for a reduced phonolitic magma is close to the $D_{Br}^{v/m}$ of an oxidised calc-alkaline magma. Our results also show that
during upper crustal storage, alkali- and F-rich rhyolites coexist with vapor and brines. This suggests that high F concentration promotes unmixing of the halogen-bearing phase coexisting with such melts. The exsolution of immiscible vapor and hydrosaline liquid efficiently removes Br from the peralkaline magma and probably limits the flux of volcanic Br to the atmosphere if high-density brines accumulate at depth, as suggested by fluid flow models (e.g., Afanasyev et al. 2018) or electrical conductivity measurements under active volcanoes (e.g., Laumonier et al., 2017; Comeau et al., 2015).

## Acknowledgments

This work was funded by LabEx VOLTAIRE (LABX-100-01), EquipEx PLANEX (ANR-11-EQPX-0036) projects and by the ANR VOLC-HAL-CLIM (ANR-18-CE01-0018).

We thank Saskia Erdman for assistance with LA-ICPMS, Ida Di Carlo and Patricia Benoist for assistance with EPMA and SEM, and Aneta Slodczyk for assistance with Raman spectroscopy.

We thank Ying-Jui Hsu, Brian Tattich, an anonymous reviewer and the Associate Editor Zoltan Zajacz for their very constructive comments that greatly helped improve this manuscript.

## Data Availability

Data are available through Zenodo at https://doi.org/10.5281/zenodo.13859895

## Appendix A. Supplementary Material:

**Supplementary data 1:** Alkali migration and calibration of glass H₂O concentration by the by-difference method (Fig. S1); Raman spectroscopy method (Fig. S2); LA-ICPMS method (Tables S1, S2, S3, S4 and Figs. S3, S4 and S5); mass balance calculations (Figs. S6, S7 and Table S5); water concentration (Figs. S8 and S9) and major elements in run product glasses (Fig S10); composition of the fluid phase (Fig. S11); CI fluid-melt partitioning (Fig. S12) and Br vapor-melt partition coefficients (50 and 10 MPa, phonolite) by regression (Fig. S13); degassing model.

**Supplementary data 2:** Description of the run products (Table S6); major element composition of the run-product glasses (Table S7); mass balance calculations (Table S8 (phonolite), Table S9 (comendite), Table S10 (pantellerite)), closed system degassing model (Table S11).

## References

Afanasyev A., Blundy J., Melnik O., Sparks S., 2018. Formation of magmatic brine lenses via focussed fluid-flow beneath volcanoes. Earth Planet. Sc. Lett. 486, 119–128.

Aiuppa, A., Baker, D.R., Webster, J.D., 2009. Halogens in volcanic systems. Chem. Geol. 263, 1–18.
Alletti, M., Baker, D.R., Scaillet, B., Aiuppa, A., Moretti, R., Ottolini, L., 2009. Chlorine partitioning between a basaltic melt and H₂O–CO₂ fluids at Mount Etna. Chem. Geol. 263, 37–50.

Alletti, M., Burgisser, A., Scaillet, B., Oppenheimer, C., 2014. Chloride partitioning and solubility in hydrous phonolites from Erebus volcano: A contribution towards a multi-component degassing model. GeoResJ 3–4, 27–45.

Anderko, A., Pitzer, K. S., 1993a. Equation-of-state representation of phase equilibria and volumetric properties of the system NaCl-H₂O above 573 K. Geochim. Cosmochim. Acta 57, 1657–1680.

Anderko, A., Pitzer, K. S., 1993b. Phase equilibria and volumetric properties of the systems KCl-H₂O and NaCl-KCl-H₂O above 573 K: Equation of state representation. Geochim. Cosmochim. Acta 57, 4885–4897.

Andújar, J., Costa, F., Scaillet, B., 2013. Storage conditions and eruptive dynamics of central versus flank eruptions in volcanic islands: The case of Tenerife (Canary Islands, Spain). J. Volcanol. Geotherm. Res. 260, 62–79.

Andújar, J., Scaillet, B., 2012. Experimental Constraints on Parameters Controlling the Difference in the Eruptive Dynamics of Phonolitic Magmas: the Case of Tenerife (Canary Islands). J. Petrol. 53, 1777–1806.

Ayalew, D., Barbey, P., Marty, B., Reisberg, L., Yirgu, G., Pik, R., 2002. Source, genesis, and timing of giant ignimbrite deposits associated with Ethiopian continental flood basalts. Geochim. Cosmochim. Acta 66, 1429–1448.

Baasner, A., Schmidt, B.C., Dupree, R., Webb, S.L., 2014. Fluorine speciation as a function of composition in peralkaline and peraluminous Na₂O–CaO–Al₂O₃–SiO₂ glasses: A multinuclear NMR study. Geochim. Cosmochim. Acta 132, 151–169.

Balcone-Boissard, H., Baker, D.R., Villemant, B., Cauzid, J., Boudon, G., Deloule, E., 2020. Br diffusion in phonolitic melts: Comparison with fluorine and chlorine diffusion. Am. Mineral. 105, 1639–1646.

Balcone-Boissard, H., Villemant, B., Boudon, G., 2010. Behavior of halogens during the degassing of felsic magmas. Geochem. Geophys. Geosystems 11, Q09005.

Barclay, J., Carroll, M.R., Houghton, B.F., Wilson, C.J.N., 1996. Pre-eruptive volatile content and degassing history of an evolving peralkaline volcano. J. Volcanol. Geotherm. Res. 74, 75–87.

Beermann, O., Botcharnikov, R.E., Nowak, M., 2015. Partitioning of sulfur and chlorine between aqueous fluid and basaltic melt at 1050°C, 100 and 200MPa. Chem. Geol. 418, 132–157.

Bell, A., Webster, J., 2015. Dissolved Cl, oxygen fugacity, and their effects on Fe behavior in a hydrous rhyodacitic melt. Am. Mineral. 100, 1595–1599.

Bluth, G. J. S., Doiron, S. D., Schnetzler, C. C., Krueger, A. J., Walter, L. S., 1992. Global tracking of the SO₂ clouds from the June, 1991 Mount Pinatubo eruptions. Geophys. Res. Lett. 19, 151–154.
Bodnar, R. J., Burnham, C. W., Sterner, S. M., 1985. Synthetic fluid inclusions in natural quartz. III. Determination of phase equilibrium properties in the system H₂O-NaCl to 1000°C and 1500 bars. Geochim. Cosmochim. Acta 49, 1861–1873.

Botcharnikov, R.E., Behrens, H., Holtz, F., Koepke, J., Sato, H., 2004. Sulfur and chlorine solubility in Mt. Unzen rhyodacitic melt at 850 °C and 200 MPa. Chem. Geol. 213, 207–225.

Botcharnikov, R.E., Holtz, F., Behrens, H., 2015. Solubility and fluid–melt partitioning of H₂O and Cl in andesitic magmas as a function of pressure between 50 and 500MPa. Chem. Geol. 418, 117–131.

Bowers, T. S., Helgeson, H. C., 1983. Calculation of the thermodynamic and geochemical consequences of nonideal mixing in the system H₂O-CO₂-NaCl on phase relations in geologic systems: Equation of state for H₂O-CO₂-NaCl fluids at high pressures and temperatures. Geochim. Cosmochim. Acta 47, 1247–1275.

Brenna, H., Kutterolf, S., Mills, M.J., Krüger, K., 2020. The potential impacts of a sulfur- and halogen-rich supereruption such as Los Chocoyos on the atmosphere and climate. Atmospheric Chem. Phys. 20, 6521–6539.

Bureau, H., Foy, E., Raepsaet, C., Somogyi, A., Munsch, P., Simon, G., Kubsky, S., 2010. Bromine cycle in subduction zones through in situ Br monitoring in diamond anvil cells. Geochim. Cosmochim. Acta 74, 3839–3850.

Bureau, H., Keppler, H., Métrich, N., 2000. Volcanic degassing of bromine and iodine: experimental fluid/melt partitioning data and applications to stratospheric chemistry. Earth Planet. Sci. Lett. 183, 51–60.

Bureau, H., Métrich, N., 2003. An experimental study of bromine behaviour in water-saturated silicic melts. Geochim. Cosmochim. Acta 67, 1689–1697.

Burnham, C.W., 1979. Magmas and hydrothermal fluids. In: Barnes, H.L. (Ed.), Geochemistry of Hydrothermal Ore Deposits, second ed. John Wiley and Sons, New York, 71–136.

Cadoux, A., Iacono-Marziano, G., Paonita, A., Deloule, E., Aiuppa, A., Nelson Eby, G., Costa, M., Brusca, L., Berlo, K., Geraki, K., Mather, T.A., Pyle, D.M., Di Carlo, I., 2017. 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. Chem. Geol. 452, 60–70.

Cadoux, A., Iacono-Marziano, G., Scaillet, B., Aiuppa, A., Mather, T.A., Pyle, D.M., Deloule, E., Gennaro, E., Paonita, A., 2018. The role of melt composition on aqueous fluid vs. silicate melt partitioning of bromine in magmas. Earth Planet. Sci. Lett. 498, 450–463.

Cadoux, A., Scaillet, B., Bekki, S., Oppenheimer, C., Druitt, T.H., 2015. Stratospheric Ozone destruction by the Bronze-Age Minoan eruption (Santorini Volcano, Greece). Sci. Rep 5, 12243.

Candela, P.A., 1990. Theoretical constraints on the chemistry of the magmatic aqueous phase. In: Stein, H.J., Hannah, J.L. (Eds.), Ore-Bearing Granite Systems; Petrogenesis and Mineralizing Processes. Geol. Soc. Am. https://doi.org/10.1130/SPE246-p11.

Cassidy, M., Iveson, A.A., Humphreys, M.C.S., Mather, T.A., Helo, C., Castro, J.M., Ruprecht, P., Pyle, D.M., EIMF, 2022. Experimentally derived F, Cl, and Br fluid/melt partitioning of intermediate to silicic melts in shallow magmatic systems. Am. Mineral. 107, 1825–1839.
Caulfield, J.T., Tomlinson, E.L., Chew, D.M., Marks, M.A.W., McKenna, C.A., Ubide, T., Smith, V.C., 2020. Microanalysis of Cl, Br and I in apatite, scapolite and silicate glass by LA-ICP-MS. Chem. Geol. 557, 119854.

Chevychelov, V.Y., Botcharnikov, R.E., Holtz, F., 2008. Partitioning of Cl and F between fluid and hydrous phonolitic melt of Mt. Vesuvius at 850–1000 °C and 200 MPa. Chem. Geol. 256, 172–184.

Chevychelov, V.Y., Zaraisky, G.P., Borisovskiy, S., Borkov, D.A., 2005. Effect of melt composition and temperature on the partitioning of Ta, Nb, Mn, and F between granitic (alkaline) melt and fluorine-bearing aqueous fluid: Fractionation of Ta and Nb and conditions of ore formation in rare-metal granites. Petrology 13, 305–321.

Chou, I.-M., 1987. Phase relations in the system NaCl-KCl-H₂O. III: Solubilities of halite in vapor-saturated liquids above 445°C and redetermination of phase equilibrium properties in the system NaCl-H2O to 1000°C and 1500 bars. Geochim. Cosmochim. Acta 51, 1965–1975.

Chou, I.-M., Sterner, S. M., Pitzer, K. S., 1992. Phase relations in the system NaCl-KCl-H2O: IV. Differential thermal analysis of the sylvite liquidus in the KCl-H2O binary, the liquidus in the NaCl-KCl-H2O ternary, and the solidus in the NaCl-KCl binary to 2 kb pressure, and a summary of experimental data for thermodynamic-PTX analysis of solid-liquid equilibria at elevated P-T conditions. Geochim. Cosmochim. Acta 56, 2281–2293.

Cochain, B., Sanloup, C., de Grouchy, C., Crépisson, C., Bureau, H., Leroy, C., Kantor, I., Irifune, T., 2015. Bromine speciation in hydrous silicate melts at high pressure. Chem. Geol. 404, 18–26.

Comeau M. J., Unsworth M. J., Ticona F., Sunagua M., 2015. Magnetotelluric images of magma distribution beneath Volcán Uturuncu, Bolivia: Implications for magma dynamics. Geology 43, 243–246.

Dalou, C., Mysen, B.O., 2015. The effect of H₂O on F and Cl solubility and solution mechanisms of siluminosilicate melts at high pressure and high temperature. Am. Mineral. 100, 633–643.

Devine, J.D., Gardner, J.E., Brack, H.P., Layne, G.D., Rutherford, M.J., 1995. Comparison of microanalytical methods for estimating H2O contents of silicic volcanic glasses. Am. Mineral. 80, 319–328.

Di Carlo, I., Pichavant, M., Rotolo, S.G., Scaillet, B., 2006. Experimental Crystallization of a High-K Arc Basalt: the Golden Pumice, Stromboli Volcano (Italy). J. Petrol. 47, 1317–1343.

Di Carlo, I., Rotolo, S.G., Scaillet, B., Buccheri, V., Pichavant, M., 2010. Phase Equilibrium Constraints on Pre-eruptive Conditions of Recent Felsic Explosive Volcanism at Pantelleria Island, Italy. J. Petrol. 51, 2245–2276.

Dolejš, D., Baker, D.R., 2007. Liquidus Equilibria in the System K₂O-Na₂O-Al₂O₃-SiO₂-F₂O₁-H₂O to 100 MPa: I. Silicate-Fluoride Liquid Immiscibility in Anhydrous Systems. J. Petrol. 48, 785–806.

Dolejš, D., Zajacz, Z., 2018. Halogens in Silicic Magmas and Their Hydrothermal Systems. In: Harlov, D.E., Aranovich, L. (Eds.), The Role of Halogens in Terrestrial and Extraterrestrial Geochemical Processes. Springer Geochemistry. Springer, Cham. 431–543.
Driesner, T., 2007. The system H₂O-NaCl. Part II: Correlations for molar volume, enthalpy, and isobaric heat capacity from 0 to 1000°C, 1 to 5000bar, and 0 to 1 XNaCl. Geochim. Cosmochim. Acta 71, 4902–4919.

Driesner, T., Heinrich, C.A., 2007. The system H₂O-NaCl. Part I: Correlation formulae for phase relations in temperature–pressure–composition space from 0 to 1000°C, 0 to 5000bar, and 0 to 1 XNaCl. Geochim. Cosmochim. Acta 71, 4880–4901.

Driesner, T., Seward, T. M., Tironi, I. G., 1998. Molecular dynamics simulation study of ionic hydration and ion association in dilute and 1 molal aqueous sodium chloride solutions from ambient to supercritical conditions. Geochim. Cosmochim. Acta 62, 3095–3107.

Evans, K.A., Mavrogenes, J.A., O'Neill, H.S., Keller, N.S., Jang, L.-Y., 2008. A preliminary investigation of chlorine XANES in silicate glasses. Geochem. Geophys. Geosyst. 9, Q10003.

Foustoukos, D. I., Seyfried, W. E. Jr., 2007. Fluid Phase Separation Processes in Submarine Hydrothermal Systems. Rev. Mineral. Geochem. 65, 213–239.

Frank, M.R., Candela, P.A., Piccoli, P.M., 2003. Alkali exchange equilibria between a silicate melt and coexisting magmatic volatile phase: an experimental study at 800°C and 100 MPa. Geochim. Cosmochim. Acta 67, 1415–1427.

Fusswinkel, T., Giehl, C., Beermann, O., Fredriksson, J.R., Garbe-Schönberg, D., Scholten, L.C., Wagner, T., 2018. Combined LA-ICP-MS microanalysis of iodine, bromine and chlorine in fluid inclusions. J. Anal. At. Spectrom., 33, 768–783.

García, O., Guzmán, S., Martí, J., 2014. Stratigraphic correlation of Holocene phonolitic explosive episodes of the Teide-Pico Viejo Volcanic Complex, Tenerife. J. Geol. Soc. 171, 375–387.

Gioncada, A., Landi, P., 2010. The pre-eruptive volatile contents of recent basaltic and pantelleritic magmas at Pantelleria (Italy). J. Volcanol. Geotherm. Res. 189, 191–201.

Griffin, W.L., Powell, W.J., Pearson, N.J., O'Reilly, S.Y., 2008. GLITTER: data reduction software for laser ablation ICP-MS. In: Sylvester, P. (Ed.), Laser Ablation-ICP-MS in the Earth Sciences. Mineralogical Association of Canada of Canada Short Course Series 40, 204-207.

Hammerli, J., Rusk, B., Spandler, C., Emsbo, P., Oliver, N.H.S., 2013. In situ quantification of Br and Cl in minerals and fluid inclusions by LA-ICP-MS: A powerful tool to identify fluid sources. Chem. Geol. 337–338, 75–87.

Helgeson, H. C., 1981. Prediction of the thermodynamic properties of electrolytes at high pressures and temperatures. Phys. Chem. Earth 13–14, 133–177.

Holland, H. D., 1972. Granites, Solutions, and Base Metal Deposits. Econ. Geol. 67, 281–301.

Hsu, Y.-J., Zajacz, Z., Ulmer, P., Heinrich, C.A., 2019. Chlorine partitioning between granitic melt and H₂O-CO₂-NaCl fluids in the Earth's upper crust and implications for magmatic-hydrothermal ore genesis. Geochim. Cosmochim. Acta 261, 171-190.

Iacovino, K., Oppenheimer, C., Scaillet, B., Kyle, P., 2016. Storage and Evolution of Mafic and Intermediate Alkaline Magmas beneath Ross Island, Antarctica. J. Petrol. 57, 93–118.

Iddon, F., Edmonds, M., 2020. Volatile-Rich Magmas Distributed Through the Upper Crust in the Main Ethiopian Rift. Geochem. Geophys. Geosyst. 21 (6), e2019GC008904.
Iveson, A.A., Webster, J.D., Rowe, M.C., Neill, O.K., 2017. Major Element and Halogen (F, Cl) Mineral–Melt–Fluid Partitioning in Hydrous Rhyodacitic Melts at Shallow Crustal Conditions. J. Petrol. 58, 2465–2492.

Iveson, A.A., Webster, J.D., Rowe, M.C., Neill, O.K., 2019. Fluid-melt trace-element partitioning behaviour between evolved melts and aqueous fluids: Experimental constraints on the magmatic-hydrothermal transport of metals. Chem. Geol. 516, 18–41.

Jochum, K.P., Willbold, M., Raczek, I., Stoll, B., Herwig, K., 2005. Chemical Characterisation of the USGS Reference Glasses GSA-1G, GSC-1G, GSD-1G, GSE-1G, BCR-2G, BHVO-2G and BIR-1G Using EPMA, ID-TIMS, ID-ICP-MS and LA-ICP-MS. Geostand. Geoanal. Res. 29, 285–302.

Kendrick, M.A., Caulfield, J.T., Nguyen, A.D., Zhao, J., Blakey, I., 2020. Halogen and trace element analysis of carbonate-veins and Fe-oxyhydroxide by LA-ICPMS: Implications for seafloor alteration, Atlantis Bank, SW Indian Ridge. Chem. Geol. 547, 119668.

Kogarko, L. N., 1974. Role of volatiles. In: Sorensen H (ed.), The alkaline rocks, Wiley and Sons, London, 474–487.

Kovalenko, V.I., Naumov, V.B., Solovova, I.P., 1993. Behavior of Cl during differentiation and eruption of magmas at Pantelleria. Geochem. Int. 30, 105-108.

Kravchuk, I.F., Keppler, H., 1994. Distribution of chloride between aqueous fluids and felsic melts at 2 kbar and 800 degrees C. Eur. J. Mineral. 6, 913–923.

Kutterolf, S., Hansteen, T.H., Appel, K., Freundt, A., Krüger, K., Pérez, W., Wehrmann, H., 2013. Combined bromine and chlorine release from large explosive volcanic eruptions: A threat to stratospheric ozone? Geology 41, 707–710.

Kutterolf, S., Hansteen, T.H., Freundt, A., Wehrmann, H.A., Appel, K.W., Krüger, K., Pérez, W., 2015. Bromine and chlorine emissions from Plinian eruptions along the Central American Volcanic Arc: From source to atmosphere. Earth Planet. Sci. Lett. 429, 234–246.

Lanzo, G., Landi, P., Rotolo, S.G., 2013. Volatiles in pantellerite magmas: A case study of the Green Tuff Plinian eruption (Island of Pantelleria, Italy). J. Volcanol. Geotherm. Res. 262, 153–163.

Laumonier M., Gaillard F., Muir D., Blundy J., Unsworth M., 2017. Giant magmatic water reservoirs at mid-crustal depth inferred from electrical conductivity and the growth of the continental crust. Earth Planet. Sc. Lett. 457, 173–180.

Lecumberri-Sanchez, P., Bodnar, R. J., 2018. Halogen Geochemistry of Ore Deposits: Contributions Towards Understanding Sources and Processes. In The Role of Halogens in Terrestrial and Extraterrestrial Geochemical Processes: Surface, Crust, and Mantle (eds. D. E. Harlov and L. Aranovich). Springer International Publishing, Cham. pp. 261–305.

Liebscher, A., 2007. Experimental Studies in Model Fluid Systems. Rev. Mineral. Geochem. 65, 15–47.

Liszewska, K.M., White, J.C., Macdonald, R., Bagiński, B., 2018. Compositional and thermodynamic variability in a stratified magma chamber: Evidence from the Green Tuff Ignimbrite (Pantelleria, Italy). J. Petrol. 59, 2245–2272.
Louvel, M., Cadoux, A., Brooker, R.A., Proux, O., Hazemann, J.-L., 2020a. New insights on Br speciation in volcanic glasses and structural controls on halogen degassing. Am. Mineral. 105, 795-802.

Louvel, M., Sanchez-Valle, C., Malfait, W.J., Pokrovski, G.S., Borca, C.N., Grolimund, D., 2020b. Bromine speciation and partitioning in slab-derived aqueous fluids and silicate melts and implications for halogen transfer in subduction zones. Solid Earth 11, 1145-1161.

Lowenstern, J.B., 1994. Chlorine, fluid immiscibility, and degassing in peralkaline magmas from Pantelleria, Italy. Am. Mineral. 79, 353-369.

Lvov, S.N., Wood, R.H., 1990. Equation of state of aqueous NaCl solutions over a wide range of temperatures, pressures and concentrations. Fluid Phase Equilib. 60, 273-287.

Macdonald, R., 2012. Evolution of peralkaline silicic complexes: Lessons from the extrusive rocks. Lithos 152, 11-22.

Macdonald, R., Bagiński, B., Belkin, H.E., White, J.C., Noble, D.C., 2019. The Gold Flat Tuff, Nevada: Insights into the evolution of peralkaline silicic magmas. Lithos 328-329, 1-13.

Metrich, N., Rutherford, M.J., 1992. Experimental study of chlorine behavior in hydrous silicic melts. Geochim. Cosmochim. Acta 56, 607-616.

Moore, G.M., Vennemann, T.W., Carmichael, I., 1998. An empirical model for the solubility of H₂O in magmas to 3 kilobars. Am. Mineral. 83, 36-42.

Neave, D.A., Fabbro, G., Herd, R.A., Petrone, C.M., Edmonds, M., 2012. Melting, Differentiation and Degassing at the Pantelleria Volcano, Italy. J. Petrol. 53, 637-663.

Newton, R.C., Manning, C.E., 2000. Quartz solubility in H₂O-NaCl and H₂O-CO₂ solutions at deep crust-upper mantle pressures and temperatures: 2-15 kbar and 500-900°C. Geochim. Cosmochim. Acta 64, 2993-3005.

Newton, R.C., Manning, C.E., 2010. Role of saline fluids in deep-crustal and upper-mantle metasomatism: insights from experimental studies. Geofluids 10, 58-72.

Nicholls, J., Carmichael, J., 1969. Peralkaline acid liquids: a petrological study. Contrib. to Mineral. Petrol. 20, 268-294.

Oppenheimer, C., Tsanev, V. I., Braban, C. F., Cox, R. A., Adams, J. W., Aiuppa, A., Bobrowski, N., Delmelle, P., Barclay, J., McGonigle, A. J., 2006. BrO formation in volcanic plumes. Geochim. Cosmochim. Acta 70, 2935-2941.

Pichavant, M., Di Carlo, I., Le Gac, Y., Rotolo, S. G., Scaillet, B. (2009). Experimental constraints on the deep magma feeding system at Stromboli volcano, Italy. J. Petrol., 50, 601-624.

Prouteau, G., Scaillet, B., 2003. Experimental Constraints on the Origin of the 1991 Pinatubo Dacite. J. Petrol. 44, 2203-2241.

Ravich, M.I., Borovaya, F.E., 1949. Phase equilibria in ternary water-salt systems at elevated temperatures. (in Russian). Akad. Nauk. SSSR Izv. Sektora Fiz. Khim. Anal. 19, 69-81.
Roberts T. J., Braban C. F., Martin R. S., Oppenheimer C., Adams J. W., Cox R. A., Jones R. L., Griffiths P. T., 2009. Modelling reactive halogen formation and ozone depletion in volcanic plumes. Chem. Geol. 263, 151–163.

Roberts T. J., Martin R. S., Jourdain L., 2014. Reactive bromine chemistry in Mount Etna's volcanic plume: the influence of total Br, high-temperature processing, aerosol loading and plume-air mixing. Atmos. Chem. and Phys. 14, 11201–11219.

Roux, J., Lefevre, A., 1992. A fast-quench device for internally heated pressure vessels. Eur. J. Mineral. 4, 279–281.

Scaillet, B., Luhr, J., Carroll, M., 2003. Petrological and Volcanological Constraints on Volcanic Sulfur Emissions to the Atmosphere. In: Volcanism and the Earth's Atmosphere 139, 11–40.

Scaillet, B., Macdonald, R., 2001. Phase Relations of Peralkaline Silicic Magmas and Petrogenetic Implications. J. Petrol. 42, 825–845.

Scaillet, B., Macdonald, R., 2003. Experimental Constraints on the Relationships between Peralkaline Rhyolites of the Kenya Rift Valley. J. Petrol. 44, 1867–1894.

Scaillet, B., Macdonald, R., 2006. Experimental constraints on pre-eruption conditions of pantelleritic magmas: Evidence from the Eburru complex, Kenya Rift. Lithos 91, 95–108.

Scaillet, B., Oppenheimer, C., 2023. Reassessment of the sulfur and halogen emissions from the Millennium Eruption of Changbaishan (Paektu) volcano. J. Volcanol. Geotherm. Res. 442, 107909.

Scaillet, B., Pichavant, M., Roux, J., Humbert, G., Lefevre, A., 1992. Improvements of the shaw membrane technique for measurement and control of fH2 at high temperatures and pressures. Am. Mineral. 77, 647–655.

Seo, J. H., Zajacz, Z., 2016. Fractionation of Cl/Br during fluid phase separation in magmatic-hydrothermal fluids. Geochim. Cosmochim. Acta 183, 125-137.

Seward, T. M., Williams-Jones, A. E., Migdisov, A. A., 2014. The Chemistry of Metal Transport and Deposition by Ore-Forming Hydrothermal Fluids. In Treatise on Geochemistry, Elsevier Oxford, 29-57.

Shinohara, H., 2009. A missing link between volcanic degassing and experimental studies on chloride partitioning. Chem. Geol. 263, 51–59.

Shinohara, H., Iiyama, J.T., Matsuo, S., 1989. Partition of chlorine compounds between silicate melt and hydrothermal solutions: I. Partition of NaCl-KCl. Geochim. Cosmochim. Acta 53, 2617–2630.

Signorelli, S., Carroll, M., 2000. Solubility and fluid-melt partitioning of Cl in hydrous phonolitic melts. Geochim. Cosmochim. Acta 64, 2851–2862.

Signorelli, S., Carroll, M., 2002. Experimental study of Cl solubility in hydrous alkaline melts: constraints on the theoretical maximum amount of Cl in trachytic and phonolitic melts. Contrib. to Mineral. Petrol., 143, 209–218.

Simon, A. C., Pettke, T., Candela, P. A., Piccoli, P. M., Heinrich, C. A., 2006. Copper partitioning in a melt-vapor-brine-magnetite-pyrrhotite assemblage. Geochim. Cosmochim. Acta 70, 5583–5600.
Steele-MacInnis, M., Ridley, J., Lecumberri-Sanchez, P., Schlegel, T. U., Heinrich, C. A., 2016. Application of low-temperature microthermometric data for interpreting multicomponent fluid inclusion compositions. Earth Sci. Rev. 159, 14–35.

Stelling, J., Botcharnikov, R.E., Beermann, O., Nowak, M., 2008. Solubility of H₂O- and chlorine-bearing fluids in basaltic melt of Mount Etna at T=1050–1250 °C and P=200 MPa. Chem. Geol. 256, 102–110.

Sterner, S.M., Bodnar, R.J., 1984. Synthetic fluid inclusions in natural quartz I. Compositional types synthesized and applications to experimental geochemistry. Geochim. Cosmochim. Acta 48, 2659–2668.

Tattitch, B.C., Blundy, J.D., 2017. Cu–Mo partitioning between felsic melts and saline-aqueous fluids as a function of X<sub>NaCleq</sub>, fO<sub>2</sub>, and fS<sub>2</sub>. Am. Mineral. 102, 1987–2006

Tattitch, B., Chelle-Michou, C., Blundy, J., Loucks, R.R., 2021. Chemical feedbacks during magma degassing control chlorine partitioning and metal extraction in volcanic arcs. Nat. Commun. 12, 1774.

Thomas R. W., Wade J., Wood B. J., 2023. The bonding environment of chlorine in silicate melts. Chem. Geol. 617, 121269.

Thomas, R.W., Wood, B.J., 2021. The chemical behaviour of chlorine in silicate melts. Geochim. Cosmochim. Acta 294, 28–42.

Thomas, R.W., Wood, B.J., 2023. The effect of composition on chlorine solubility and behavior in silicate melts. Am. Mineral. 108, 814-825.

Urabe, T., 1985. Aluminous granite as a source magma of hydrothermal ore deposits; an experimental study. Econ. Geol. 80, 148–157.

Valyashko, V.M., 2004. Chapter 15 - Phase equilibria of water-salt systems at high temperatures and pressures, in: Palmer, D.A., Fernández-Prini, R., Harvey, A.H. (Eds.), Aqueous Systems at Elevated Temperatures and Pressures. Academic Press, London 597–641.

Vidal, C.M., Métrich, N., Komorowski, J.-C., Pratomo, I., Michel, A., Kartadinata, N., Robert, V., Lavigne, F., 2016. The 1257 Samalas eruption (Lombok, Indonesia): the single greatest stratospheric gas release of the Common Era. Sci. Rep., 6, 34868.

von Glasow, R., Bobrowski, N., Kern, C., 2009. The effects of volcanic eruptions on atmospheric chemistry. Chem. Geol. 263, 131–142.

Waelkens, C.M., Stix, J., Monteleone, B.D., Burckel, P., 2021. Efficient release of bromine by super-eruptions. Geology 49, 1416-1420.

Wallace, P.J., 2001. Volcanic SO₂ emissions and the abundance and distribution of exsolved gas in magma bodies. J. Volcanol. Geotherm. Res. 108, 85–106.

Wallace, P.J., Anderson, A.T., Davis, A.M., 1995. Quantification of pre-eruptive exsolved gas contents in silicic magmas. Nature 377, 612–616.

Wallace, P. J., Anderson, A. T., Davis, A.M., 1999. Gradients in H₂O, CO₂, and exsolved gas in a large-volume silicic magma system: Interpreting the record preserved in melt inclusions from the Bishop Tuff. J. Geophys. Res., 104 (B9), 20097–20122, doi:10.1029/1999JB900207.
Webster, J.D., 1992a. Fluid-melt interactions involving Cl-rich granites: Experimental study from 2 to 8 kbar. *Geochim. Cosmochim. Acta* 56, 659–678.

Webster, J.D., 1992b. Water solubility and chlorine partitioning in Cl-rich granitic systems: Effects of melt composition at 2 kbar and 800°C. *Geochim. Cosmochim. Acta* 56, 679–687.

Webster, J. D., 2004. The exsolution of magmatic hydrosaline chloride liquids. *Chem. Geol.* 210, 33–48.

Webster, J.D., Baker, D.R., Aiuppa, A., 2018. Halogens in Mafic and Intermediate-Silica Content Magmas. In: Harlov, D.E., Aranovich, L. (Eds.), The Role of Halogens in Terrestrial and Extraterrestrial Geochemical Processes. Springer Geochemistry. Springer, Cham. 307–430.

Webster, J.D., Goldoff, B., Sintoni, M.F., Shimizu, N., De Vivo, B., 2014. C–O–H–Cl–S–F Volatile Solubilities, Partitioning, and Mixing in Phonolitic–Trachytic Melts and Aqueous–Carbonic Vapor ± Saline Liquid at 200 MPa. *J. Petrol.* 55, 2217–2248.

Webster, J.D., Holloway, J.R., 1990. Partitioning of F and Cl between magmatic hydrothermal fluids and highly evolved granitic magmas, in: Stein, H.J., Hannah, J.L. (Eds.), Ore-Bearing Granite Systems; Petrogenesis and Mineralizing Processes. Geol. Soc. America. https://doi.org/10.1130/SPE246-p21

Webster, J.D., Kinzler, R.J., Mathez, E.A., 1999. Chloride and water solubility in basalt and andesite melts and implications for magmatic degassing. *Geochim. Cosmochim. Acta* 63, 729–738.

Webster, J.D., Sintoni, M.F., De Vivo, B., 2009. The partitioning behavior of Cl, S, and H₂O in aqueous vapor- ± saline-liquid saturated phonolitic and trachytic melts at 200 MPa. *Chem. Geol.* 263, 19–36.

Webster, J.D., Taylor, R.P., Bean, C., 1993. Pre-eruptive melt composition and constraints on degassing of a water-rich pantellerite magma, Fantale volcano, Ethiopia. *Contributions to Mineralogy and Petrology* 114, 53–62.

Webster, J.D., Vetere, F., Botcharnikov, R.E., Goldoff, B., McBirney, A., Doherty, A.L., 2015. 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. *Am. Mineral.* 100, 522–535. Westrich, H.R., Eichelberger, J.C., Hervig, R.L., 1991. Degassing of the 1912 Katmai magmas. *Geophys. Res. Lett.* 18, 1561–1564.

Westrich, H.R., Gerlach, T.M., 1992. Magmatic gas source for the stratospheric SO₂ cloud from the June 15, 1991, eruption of Mount Pinatubo. *Geology* 20, 867–870.

Williams, T.J., Candela, P.A., Piccoli, P.M., 1997. Hydrogen-alkali exchange between silicate melts and two-phase aqueous mixtures: an experimental investigation. *Contrib. to Mineral. Petrol.* 128, 114–126.

Williams-Jones, A.E., Heinrich, C.A., 2005. Vapor Transport of Metals and the Formation of Magmatic-Hydrothermal Ore Deposits. *Econ. Geol.* 100, 1287–1312.

Witter, J.B., Kuehner, S.M., 2004. A simple empirical method for high-quality electron microprobe analysis of fluorine at trace levels in Fe-bearing minerals and glasses. *Am. Mineral.* 89, 57–63.
Worden, R. H., 2018. Halogen Elements in Sedimentary Systems and Their Evolution During Diagenesis. In *The Role of Halogens in Terrestrial and Extraterrestrial Geochemical Processes* (eds. D. E. Harlov and L. Aranovich). Springer Geochemistry. Springer International Publishing, Cham. pp. 185–260.

Yardley, B. W. D., Bodnar, R. J., 2014. Fluids in the Continental Crust. Geochem. Perspect. 3, 1–2.

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.

Zajacz, Z., Seo, J. H., Candela, P. A., Piccoli, P. M., Tossell J. A., 2011. The solubility of copper in high-temperature magmatic vapors: A quest for the significance of various chloride and sulfide complexes. Geochim. Cosmochim. Acta 75, 2811–2827.
Tables

<table><caption>Table 1. Starting compositions</caption><thead><tr><th></th><th>Pantellerite Eburru-89</th><th>Comendite</th><th>SMN-49</th><th>Phonolite</th><th>RB-Roques Blancos</th></tr><tr><th></th><th>sd</th><th>sd</th><th>sd</th><th>sd</th><th>sd</th></tr></thead><tbody><tr><td>Oxides</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>wt%<sup>(1)</sup></td><td>(n=8)</td><td>(n=8)</td><td>(n=8)</td><td>(n=8)</td><td>(n=8)</td></tr><tr><td>SiO<sub>2</sub></td><td>72.57</td><td>0.29</td><td>74.90</td><td>0.29</td><td>59.99</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>8.34</td><td>0.13</td><td>10.59</td><td>0.18</td><td>19.73</td></tr><tr><td>FeO<sub>tot</sub></td><td>7.22</td><td>0.24</td><td>3.96</td><td>0.21</td><td>3.35</td></tr><tr><td>MgO</td><td>0.01</td><td>0.01</td><td>0.02</td><td>0.01</td><td>0.39</td></tr><tr><td>MnO</td><td>0.20</td><td>0.06</td><td>0.06</td><td>0.06</td><td>0.19</td></tr><tr><td>CaO</td><td>0.25</td><td>0.02</td><td>0.05</td><td>0.02</td><td>0.78</td></tr><tr><td>Na<sub>2</sub>O</td><td>6.62</td><td>0.14</td><td>5.69</td><td>0.26</td><td>9.19</td></tr><tr><td>K<sub>2</sub>O</td><td>4.52</td><td>0.08</td><td>4.56</td><td>0.16</td><td>5.66</td></tr><tr><td>TiO<sub>2</sub></td><td>0.26</td><td>0.03</td><td>0.17</td><td>0.02</td><td>0.64</td></tr><tr><td>P<sub>2</sub>O<sub>5</sub></td><td>0.01</td><td>0.01</td><td></td><td></td><td>0.09</td></tr><tr><td>Halogens</td><td></td><td>1σ</td><td></td><td>1σ</td><td>1σ</td></tr><tr><td>F (wt%)<sup>2</sup></td><td>1.25</td><td>0.29</td><td>1.63</td><td>0.30</td><td>0.20</td></tr></tbody></table>
<table><tr><td>Cl (wt%)<sup>2</sup></td><td>0.40</td><td>0.03</td><td>0.46</td><td>0.03</td><td>0.26</td><td>0.03</td></tr><tr><td>Br (ppm)<sup>3</sup></td><td>11.11</td><td>0.55</td><td>9.23</td><td>0.52</td><td>8.66</td><td>0.96</td></tr></table>

Trace elements

ppm<sup>(4)</sup>

<table><tr><td>Gd</td><td>54.9</td><td>36.1</td><td>6.9</td></tr><tr><td>Tb</td><td>9.3</td><td>7.1</td><td>1.1</td></tr><tr><td>Dy</td><td>60.4</td><td>49.4</td><td>6.6</td></tr></table>

Molar ratios

<table><tr><td>NK/A</td><td>2.0</td><td>1.4</td><td>1.1</td></tr><tr><td>A/CNK</td><td>0.49</td><td>0.69</td><td>0.9</td></tr></table>

<footnote>(1): EPMA, volatile-free, normalized to 100%; (2): EPMA; (3): LA-ICPMS, values corrected for K- and REE-based interferences (see text); (4): ICP-MS, CRPG, Nancy, France. Uncertainties are between 5 and 10 % for Gd and Dy and ~15 % for Tb. sd: 1 sigma standard deviation on multiple analyses (number of analyses given in brackets).</footnote>

<footnote>1σ: total error (analytical and statistical uncertainties).</footnote>

<footnote>NK/A= (Na+K)/Al; A/CNK= Al/ (Na+K+2Ca).</footnote>
<figure><img src="image_3.png" /></figure>
<table><caption>**Table 2.** Experimental conditions, concentrations of volatiles in the melt and in the fluid phase and f/m partition coefficients for the phonolitic composition</caption><thead><tr><th rowspan="2">Sample</th><th rowspan="2">NK/A<sup>a</sup></th><th rowspan="2">bulk<br>pp<br>m<sup>b</sup></th><th colspan="2">[Br]</th><th colspan="2">Br melt<sup>c</sup></th><th colspan="2">CI melt<sup>d</sup></th><th colspan="2">F melt<sup>d</sup></th><th colspan="2">H<sub>2</sub>O melt (wt%)</th><th colspan="2">Br<br>fluid<sup>h</sup></th><th colspan="2">H<sub>2</sub>O<br>fluid<sup>h</sup></th><th colspan="2">Σcations<br>fluid<sup>h</sup></th><th rowspan="2">D<sub>Br</sub><sup>i</sup></th><th rowspan="2">D<sub>Cl</sub><sup>i</sup></th></tr><tr><th>LA-ICP-MS</th><th>EPMA</th><th>LA-ICP-MS</th><th>EPMA</th><th>LA-ICP-MS</th><th>EPMA</th><th>LA-ICP-MS</th><th>EPMA</th><th>LA-ICP-MS</th><th>EPMA</th><th>LA-ICP-MS</th><th>EPMA</th><th>LA-ICP-MS</th><th>EPMA</th><th>LA-ICP-MS</th><th>EPMA</th></tr></thead><tbody><tr><td colspan="20" style="text-align: left; font-weight: bold;">#1; 1000 °C, 200MPa, oxidising conditions, 24h</td></tr><tr><td>R1-<br>RB1</td><td>1.08</td><td>336</td><td>325</td><td>4</td><td>4</td><td>14</td><td>2175</td><td>1</td><td>10</td><td>272</td><td>0.15</td><td>8</td><td>0.0</td><td>0.21</td><td>5.7 (0.1)</td><td>5.8</td><td>9</td><td>0.3</td><td>5.4</td><td>1.3</td><td>0.12 (0.05)</td><td>0.94 (0.91)</td><td>81.1 (11.3)</td><td>16.7 (7.0)</td><td>3.8* (1.5)</td><td>4.3* (4.2)</td></tr><tr><td>R1-<br>RB3</td><td>1.08</td><td>2816</td><td>259</td><td>4</td><td>15</td><td>109</td><td>211</td><td>8</td><td>10</td><td>270</td><td>0.22</td><td>6</td><td>0.0</td><td>0.21</td><td>5.7 (0.1)</td><td>5.5</td><td>9</td><td>0.4</td><td>n.<br>a.</td><td></td><td>1.38 (0.4)</td><td>1.36 (1.15)</td><td>82.3 (14.4)</td><td>15.6 (8.9)</td><td>5.3 (2.1)</td><td>6.4 (5.5)</td></tr><tr><td>R1-<br>RB4</td><td>1.08</td><td>6055</td><td>410</td><td>5</td><td>82</td><td>191</td><td>1863</td><td>8</td><td>63</td><td>258</td><td>0.14</td><td>6</td><td>0.0</td><td>0.21</td><td>5.7 (0.1)</td><td>5.6</td><td>9</td><td>0.4</td><td>6.1</td><td>1.2</td><td>5.52 (1.10)</td><td>1.52 (0.85)</td><td>80.8 (10.6)</td><td>10.9 (6.9)</td><td>13.5 (2.7)</td><td>8.2 (4.7)</td></tr><tr><td colspan="20" style="text-align: left; font-weight: bold;">#3; 1000 °C, 200MPa, reducing conditions, 38h</td></tr><tr><td>R3-<br>RB1</td><td>1.06</td><td>360</td><td>278</td><td>7</td><td>7</td><td>30</td><td>1979</td><td>8</td><td>13</td><td>284</td><td>0.14</td><td>7</td><td>0.1</td><td>0.25</td><td>5.7 (0.1)</td><td>5.9</td><td>7</td><td>0.3</td><td>n.<br>a.</td><td></td><td>0.28 (0.09)</td><td>1.31 (0.89)</td><td>89.4 (12.4)</td><td>7.8 (7.2)</td><td>9.9* (3.3)</td><td>6.6* (4.6)</td></tr><tr><td>R3-<br>RB3</td><td>1.07</td><td>3214</td><td>218</td><td>6</td><td>28</td><td>231</td><td>1886</td><td>8</td><td>11</td><td>275</td><td>0.24</td><td>4</td><td>0.0</td><td>0.21</td><td>5.7 (0.1)</td><td>5.8</td><td>7</td><td>0.3</td><td>6.9</td><td>0.2</td><td>3.16 (0.73)</td><td>1.66 (0.96)</td><td>90.6 (7.9)</td><td>4.6 (8.1)</td><td>14.5 (3.7)</td><td>8.8 (5.2)</td></tr></tbody></table>
<table><tr><td>R3-<br/>RB4</td><td>1.08</td><td>6014</td><td>375<br/>0</td><td>5</td><td>65</td><td>416</td><td>1766</td><td>8</td><td>86</td><td>264</td><td>0.18</td><td>7</td><td>0.1<br/>8</td><td>0.27</td><td>5.7 (0.1)</td><td>5.7</td><td>7</td><td>0.<br/>4</td><td>5.<br/>6</td><td>0.<br/>1</td><td>6.91<br/>(1.98)</td><td>2.27<br/>(1.18)</td><td>85.4<br/>(15.6)</td><td>4.8 (9.5)</td><td>18.4<br/>(5.7)</td><td>12.8 (7.0)</td></tr></table>

#4; 1000 °C, 100MPa, oxidising conditions, 45h

<table><tr><td>R4-<br/>RB3</td><td>1.10</td><td>3167</td><td>237<br/>7</td><td>6</td><td>22</td><td>252</td><td>2216</td><td>8</td><td>73</td><td>260</td><td>0.22</td><td>7</td><td>0.1<br/>0</td><td>0.22</td><td>3.7 (0.1)</td><td>3.3</td><td>7</td><td>0.<br/>3</td><td>n.<br/>a.</td><td>1.63<br/>(0.43)</td><td>0.55<br/>(0.58)</td><td>90.9 (5.0)</td><td>6.9 (5.0)</td><td>6.8 (1.9)</td><td>2.5 (2.6)</td></tr></table>

<table><tr><td>R4-<br/>RB4</td><td>1.12</td><td>5789</td><td>397<br/>6</td><td>6</td><td>66</td><td>430</td><td>2171</td><td>9</td><td>89</td><td>265</td><td>0.13</td><td>6</td><td>0.1<br/>2</td><td>0.23</td><td>3.7 (0.1)</td><td>3.6</td><td>6</td><td>0.<br/>4</td><td>n.<br/>a.</td><td>4.08<br/>(0.96)</td><td>0.71<br/>(0.67)</td><td>90.7 (9.7)</td><td>3.4 (5.9)</td><td>10.3<br/>(2.7)</td><td>3.3 (3.1)</td></tr></table>

#25; 1000 °C, 50MPa, oxidising conditions, 93h

<table><tr><td>R25-<br/>RB1</td><td>1.10</td><td>348</td><td>315</td><td>4</td><td>9</td><td>21</td><td>2566</td><td>9</td><td>14<br/>5</td><td>289</td><td>0.20</td><td>9</td><td>0.1<br/>6</td><td>0.25</td><td>2.4 (0.1)</td><td>2.7</td><td>9</td><td>0.<br/>2</td><td>3.<br/>2</td><td>0.<br/>8</td><td>0.09<br/>(0.04)</td><td>0.04<br/>(0.59)</td><td>95.9 (5.0)</td><td>4.0 (5.0)</td><td>3.0* (1.2)</td><td>0.2* (2.3)</td></tr></table>

<table><tr><td>R25-<br/>RB2</td><td>1.10</td><td>1620</td><td>162<br/>4</td><td>4</td><td>47</td><td>111</td><td>2592</td><td>9</td><td>97</td><td>268</td><td>0.18</td><td>8</td><td>0.1<br/>2</td><td>0.23</td><td>2.4 (0.1)</td><td>3.0</td><td>8</td><td>0.<br/>2</td><td>n.<br/>a.</td><td>0.22<br/>(0.17)</td><td>0.01<br/>(0.52)</td><td>95.3 (8.0)</td><td>4.1 (4.7)</td><td>1.4* (1.0)</td><td>-</td></tr></table>

<table><tr><td>R25-<br/>RB3</td><td>1.09</td><td>3142</td><td>288<br/>9</td><td>5</td><td>25<br/>8</td><td>307</td><td>2494</td><td>9</td><td>10<br/>2</td><td>270</td><td>0.25</td><td>8</td><td>0.1<br/>7</td><td>0.26</td><td>2.4 (0.1)</td><td>2.8</td><td>8</td><td>0.<br/>2</td><td>n.<br/>a.</td><td>0.82<br/>(0.49)</td><td>0.15<br/>(0.59)</td><td>99.0 (0.8)</td><td>tr?</td><td>2.8* (1.7)</td><td>0.6* (2.4)</td></tr></table>

<table><tr><td>R25-<br/>RB4</td><td>1.11</td><td>5673</td><td>548<br/>9</td><td>5</td><td>29<br/>4</td><td>417</td><td>2320</td><td>9</td><td>15<br/>8</td><td>296</td><td>0.21</td><td>7</td><td>0.1<br/>3</td><td>0.24</td><td>2.4 (0.1)</td><td>3.0</td><td>7</td><td>0.<br/>3</td><td>2.<br/>9</td><td>0.<br/>9</td><td>1.23<br/>(0.84)</td><td>0.47<br/>(0.71)</td><td>98.3 (1.1)</td><td>tr?</td><td>2.2* (1.5)</td><td>2.0* (3.1)</td></tr></table>

#23; 1000 °C, 10MPa, oxidising conditions, 196h

<table><tr><td>R23-<br/>RB1</td><td>1.11</td><td>383</td><td>368</td><td>5</td><td>8</td><td>40</td><td>2612</td><td>8</td><td>22<br/>7</td><td>338</td><td>0.14</td><td>5</td><td>0.1<br/>0</td><td>0.22</td><td>0.9 (0.1)</td><td>0.7</td><td>9</td><td>0.<br/>3</td><td>n.<br/>a.</td><td>0.06<br/>(0.04)</td><td>tr?</td><td>98.1 (.2)</td><td>1.3 (3.7)</td><td>1.5* (1.2)</td><td>-</td></tr></table>

<table><tr><td>R23-<br/>RB2</td><td>1.12</td><td>1496</td><td>111<br/>5</td><td>4</td><td>57</td><td>100</td><td>2119</td><td>1<br/>1</td><td>83</td><td>263</td><td>0.16</td><td>8</td><td>0.1<br/>7</td><td>0.26</td><td>0.9 (0.1)</td><td>0.9</td><td>8</td><td>0.<br/>3</td><td>n.<br/>a.</td><td>0.61<br/>(0.14)</td><td>0.55<br/>(0.46)</td><td>97.78<br/>(7.4)</td><td>0.5 (4.1)</td><td>5.4* (1.4)</td><td>2.6* (2.2)</td></tr></table>

<table><tr><td>R23-<br/>RB3</td><td>1.12</td><td>3022</td><td>254<br/>1</td><td>4</td><td>11<br/>0</td><td>194</td><td>2275</td><td>8</td><td>17<br/>2</td><td>303</td><td>0.13</td><td>1<br/>0</td><td>0.1<br/>2</td><td>0.23</td><td>0.9 (0.1)</td><td>1.1</td><td>1<br/>0</td><td>0.<br/>3</td><td>n.<br/>a.</td><td>0.94<br/>(0.29)</td><td>0.39<br/>(0.52)</td><td>96.9 (7.5)</td><td>0.9 (4.4)</td><td>3.7* (1.2)</td><td>1.7* (2.3)</td></tr></table>
<table><tr><td>R23-<br/>RB4</td><td>1.12</td><td>5359</td><td>454</td><td>4</td><td>18</td><td>328</td><td>2066</td><td>8</td><td>72</td><td>260</td><td>0.16</td><td>1</td><td>0.1</td><td>0.23</td><td>0.9 (0.1)</td><td>1.3</td><td>1</td><td>0</td><td>0.4</td><td>n.<br/>a.</td><td>1.83<br/>(0.57)</td><td>0.74<br/>(0.56)</td><td>96.9 (7.0)</td><td>tr?</td><td>4.0* (1.3)</td><td>3.6* (2.7)</td></tr></table>

#13; 1100 °C, 200MPa, oxidising conditions, 23h

<table><tr><td>R13-<br/>RB1</td><td>1.08</td><td>369</td><td>345</td><td>3</td><td>5</td><td>12</td><td>2430</td><td>7</td><td>13</td><td>8</td><td>285</td><td>0.22</td><td>8</td><td>0.1</td><td>0.25</td><td>5.4 (0.1)</td><td>5.5</td><td>8</td><td>0.3</td><td>3.7</td><td>0.5</td><td>0.12<br/>(0.04)</td><td>0.31<br/>(0.77)</td><td>95.5 (6.6)</td><td>4.1 (6.5)</td><td>3.3* (1.0)</td><td>1.3* (3.2)</td></tr></table>

<table><tr><td>R13-<br/>RB2</td><td>1.09</td><td>1697</td><td>145</td><td>8</td><td>6</td><td>30</td><td>161</td><td>2268</td><td>7</td><td>10</td><td>7</td><td>272</td><td>0.17</td><td>6</td><td>0.1</td><td>0.23</td><td>5.4 (0.1)</td><td>5.3</td><td>6</td><td>0.6</td><td>n.<br/>a.</td><td>0.78<br/>(0.35)</td><td>0.60<br/>(0.74)</td><td>94.2<br/>(11.0)</td><td>4.0 (6.5)</td><td>5.4 (2.5)</td><td>2.6 (3.3)</td></tr></table>

<table><tr><td>R13-<br/>RB3</td><td>1.09</td><td>3305</td><td>267</td><td>0</td><td>5</td><td>50</td><td>309</td><td>2358</td><td>7</td><td>11</td><td>2</td><td>274</td><td>0.17</td><td>7</td><td>0.1</td><td>0.24</td><td>5.4 (0.1)</td><td>5.5</td><td>7</td><td>0.2</td><td>n.<br/>a.</td><td>1.99<br/>(0.74)</td><td>0.47<br/>(0.81)</td><td>94.5<br/>(12.1)</td><td>2.5 (7.2)</td><td>7.5 (2.9)</td><td>2.0 (3.4)</td></tr></table>

<table><tr><td>R13-<br/>RB4</td><td>1.09</td><td>6026</td><td>382</td><td>8</td><td>5</td><td>67</td><td>451</td><td>2255</td><td>7</td><td>12</td><td>2</td><td>278</td><td>0.21</td><td>6</td><td>0.1</td><td>0.22</td><td>5.4 (0.1)</td><td>5.0</td><td>6</td><td>0.1</td><td>4.0</td><td>0.8</td><td>5.94<br/>(1.12)</td><td>0.72<br/>(0.86)</td><td>93.3 (1.4)</td><td>tr?</td><td>15.5<br/>(3.4)</td><td>3.2 (3.8)</td></tr></table>

#11; 900 °C, 200MPa, oxidising conditions, 141h

<table><tr><td>R11-<br/>RB1</td><td>1.05</td><td>359</td><td>204</td><td>6</td><td>4</td><td>13</td><td>2035</td><td>9</td><td>15</td><td>8</td><td>296</td><td>0.14</td><td>7</td><td>0.0</td><td>0.22</td><td>5.9 (0.2)</td><td>6.0</td><td>7</td><td>0.3</td><td>4.7</td><td>1.0</td><td>0.38<br/>(0.07)</td><td>1.11<br/>(0.84)</td><td>85.3<br/>(10.7)</td><td>12.0<br/>(6.6)</td><td>18.5*<br/>(3.6)</td><td>5.5* (4.2)</td></tr></table>

<table><tr><td>R11-<br/>RB2</td><td>1.06</td><td>1610</td><td>861</td><td>6</td><td>19</td><td>58</td><td>1987</td><td>9</td><td>29</td><td>6</td><td>387</td><td>0.20</td><td>6</td><td>0.0</td><td>0.20</td><td>6.0 (0.2)</td><td>6.1</td><td>6</td><td>0.5</td><td>n.<br/>a.</td><td>2.12<br/>(0.46)</td><td>1.40<br/>(1.15)</td><td>87.6<br/>(12.6)</td><td>8.8 (7.8)</td><td>24.6<br/>(5.5)</td><td>7.1 (5.9)</td></tr></table>

<table><tr><td>R11-<br/>RB3</td><td>1.06</td><td>2932</td><td>163</td><td>2</td><td>5</td><td>35</td><td>114</td><td>1855</td><td>8</td><td>87</td><td>265</td><td>0.21</td><td>5</td><td>0.1</td><td>0.25</td><td>6.0 (0.2)</td><td>6.1</td><td>5</td><td>0.2</td><td>n.<br/>a.</td><td>4.70<br/>(0.92)</td><td>2.18<br/>(1.21)</td><td>84.1 (9.7)</td><td>9.0<br/>(10.2)</td><td>28.8<br/>(6.0)</td><td>11.8 (6.7)</td></tr></table>

<table><tr><td>R11-<br/>RB4</td><td>1.08</td><td>6013</td><td>362</td><td>1</td><td>3</td><td>20</td><td>341</td><td>1045</td><td>8</td><td>69</td><td>259</td><td>0.17</td><td>6</td><td>0.1</td><td>0.27</td><td>6.0 (0.2)</td><td>6.0</td><td>6</td><td>0.1</td><td>n.<br/>a.</td><td>6.73<br/>(1.58)</td><td>3.41<br/>(1.08)</td><td>78.1<br/>(11.9)</td><td>11.1<br/>(7.5)</td><td>18.6<br/>(4.7)</td><td>32.7 (13.1)</td></tr></table>

n is the number of analyses per sample. "n.a." = not analysed. tr? = probably present in the considered phase but not determined.

<footnote>ª NK/A = (Na+K)/Al in run-product glasses.</footnote>
<sup>b</sup> Total Br concentration in the system.

<sup>c</sup> Measured by LA-ICPMS in run-product glasses; **sd**: 1 sigma standard deviation (statistical uncertainty calculated on the mean value); **total error 1σ** of the LA-ICP-MS analysis (taking into account the statistical uncertainty (sd) and the mean analytical uncertainty as given by Glitter software.

<sup>d</sup> Cl and F concentrations in run-product glasses determined by EPM analyses; **sd**: 1 sigma standard deviation; **total error 1σ**: 1sigma calculated taking into account the statistical uncertainty (sd) plus the EPMA detection limit for the specific element (250 ppm for Cl and 2000 ppm for F).

<sup>e</sup> H₂O concentration determined by the empirical solubility model of Moore et al., 1998; **1σ**: 1 sigma error obtained by propagating the uncertainties on the parameters of the model.

<sup>f</sup> H₂O concentration estimated by the "by-difference" method (Devine et al., 1995); **sd**: 1 sigma standard deviation on multiple analyses.

<sup>g</sup> H₂O concentration determined by Raman spectroscopy; **sd**: 1 sigma standard deviation on multiple analyses (n=3).

<sup>h</sup> Calculated by mass balance; the values in parentheses correspond to the 1 sigma propagated error from the variables considered in the mass balance.

<sup>i</sup> Fluid/melt partition coefficients calculated after the mass balance. **1σ**: 1 sigma propagated error from the variables considered in the mass balance. Note that for experiments with vapor + brine, D<sub>Br</sub> and D<sub>Cl</sub> are apparent partition coefficients that would apply to a single-phase-fluid. Experiments marked with (*) are in the vapor field, D<sub>Br</sub> and D<sub>Cl</sub> are the vapor-melt Br and Cl partition coefficients, respectively (see text).

<table><caption>**Table 3.** Experimental conditions, concentrations of volatiles in the melt and in the fluid phase and f/m partition coefficients for the comenditic composition</caption><thead><tr><th rowspan="2">Sample</th><th rowspan="2">NK/A<sup>a</sup></th><th rowspan="2">[Br]<sub>bul</sub><br>k<sub>1</sub><br>ppm<sup>b</sup></th><th colspan="2">Br melt<sup>c</sup></th><th colspan="2">CI melt<sup>d</sup></th><th colspan="2">F melt<sup>d</sup></th><th colspan="2">H<sub>2</sub>O melt (wt%)</th><th colspan="2">Br fluid<sup>g</sup></th><th colspan="2">CI fluid<sup>g</sup></th><th colspan="2">F fluid<sup>g</sup></th><th colspan="2">H<sub>2</sub>O<br>fluid<sup>g</sup></th><th colspan="2">Σcatio<br>ns<br>fluid<sup>g</sup></th><th rowspan="2">D<sub>Br</sub><sup>h</sup></th><th rowspan="2">D<sub>Cl</sub><sup>h</sup></th></tr><tr><th>LA-ICP-MS</th><th>EPMA</th><th>EPMA</th><th>EPMA</th><th>solubili<br>ty<br>model<sup>e</sup></th><th>Difference<br>method<sup>f</sup></th><th>Mass Balance</th><th>wt<br>%</th><th>n</th><th>s<br>d</th><th>wt%</th><th>wt%</th><th>wt%</th><th>wt%</th><th>wt%</th><th>wt%</th></tr></thead><tbody><tr><td></td><td></td><td></td><td>ppm</td><td>n</td><td>sd</td><td>pp<br>m</td><td>n</td><td>sd</td><td>wt<br>%</td><td>n</td><td>sd</td><td>wt<br>%</td><td>n</td><td>s<br>d</td><td>wt%</td><td>wt%</td><td>wt%</td><td>wt%</td><td>wt%</td><td>wt%</td><td></td></tr><tr><td></td><td></td><td></td><td>total</td><td></td><td></td><td>total</td><td></td><td></td><td>total</td><td></td><td></td><td>total</td><td></td><td></td><td>total</td><td></td><td>total</td><td></td><td>total</td><td></td><td></td></tr><tr><td></td><td></td><td></td><td>error</td><td></td><td></td><td>error</td><td></td><td></td><td>error</td><td></td><td></td><td>error</td><td></td><td></td><td>error</td><td></td><td>error</td><td></td><td>error</td><td></td><td></td></tr></tbody></table>
<table><tbody><tr><td>R9-SMN4</td><td>1.34</td><td>5678</td><td>2622</td><td>3</td><td>63</td><td>404</td><td>288</td><td>8</td><td>17</td><td>304</td><td>1.55</td><td>8</td><td>0.2</td><td>0.30</td><td>2.5 (0.1)</td><td>2.4</td><td>8</td><td>0.3</td><td>4.97 (0.81)</td><td>2.31 (0.62)</td><td>1.55 (8.36)</td><td>91.2 (7.8)</td><td>tr?</td><td>18.9 (4.2)</td><td>8.0 (2.3)</td></tr><tr><td colspan="24">#18-19; 1000 °C, 25MPa, oxidising conditions, 165h</td></tr><tr><td>R18-SMN2</td><td>1.34</td><td>1723</td><td>1398</td><td>4</td><td>68</td><td>103</td><td>333</td><td>1</td><td>9</td><td>11</td><td>277</td><td>1.65</td><td>6</td><td>0.2</td><td>0.33</td><td>1.6 (0.1)</td><td>1.6</td><td>6</td><td>0.2</td><td>0.56 (0.14)</td><td>1.32 (0.44)</td><td>0.15 (6.78)</td><td>96.8 (7.6)</td><td>1.2 (3.8)</td><td>4.0 (1.0)</td><td>4.0 (1.4)</td></tr><tr><td>R19-SMN3</td><td>1.35</td><td>3136</td><td>2246</td><td>3</td><td>35</td><td>131</td><td>294</td><td>1</td><td>9</td><td>18</td><td>314</td><td>1.50</td><td>7</td><td>0.1</td><td>0.22</td><td>1.6 (0.1)</td><td>1.6</td><td>7</td><td>0.2</td><td>1.42 (0.23)</td><td>1.88 (0.52)</td><td>1.87 (6.26)</td><td>93.7 (7.2)</td><td>1.1 (4.1)</td><td>6.3 (1.1)</td><td>6.4 (1.9)</td></tr><tr><td>R19-SMN4</td><td>1.34</td><td>5776</td><td>2696</td><td>4</td><td>64</td><td>166</td><td>251</td><td>1</td><td>1</td><td>93</td><td>267</td><td>1.65</td><td>9</td><td>0.2</td><td>0.30</td><td>1.6 (0.1)</td><td>1.6</td><td>9</td><td>0.1</td><td>4.54 (0.53)</td><td>2.54 (0.54)</td><td>0.18 (7.62)</td><td>92.7 (7.1)</td><td>tr?</td><td>16.8 (2.2)</td><td>10.1 (2.4)</td></tr><tr><td colspan="24">#12-13; 1100 °C, 200MPa, oxidising conditions, 23h</td></tr><tr><td>R12-SMN1</td><td>1.35</td><td>395</td><td>306</td><td>4</td><td>7</td><td>30</td><td>360</td><td>4</td><td>8</td><td>14</td><td>287</td><td>1.30</td><td>6</td><td>0.1</td><td>0.23</td><td>5.5 (0.1)</td><td>6.0</td><td>6</td><td>0.5</td><td>0.19 (0.06)</td><td>1.61 (0.70)</td><td>5.95 (8.70)</td><td>91.1 (10.1)</td><td>1.1 (6.0)</td><td>6.3* (2.1)</td><td>4.5* (2.0)</td></tr><tr><td>R12-SMN2</td><td>1.33</td><td>1760</td><td>1226</td><td>4</td><td>59</td><td>133</td><td>336</td><td>5</td><td>8</td><td>14</td><td>288</td><td>1.11</td><td>6</td><td>0.1</td><td>0.23</td><td>5.5 (0.1)</td><td>6.5</td><td>6</td><td>0.1</td><td>1.26 (0.33)</td><td>2.25 (0.81)</td><td>10.06 (9.39)</td><td>84.3 (10.5)</td><td>2.1 (6.6)</td><td>10.2 (2.9)</td><td>6.7 (2.5)</td></tr><tr><td>R13-SMN3</td><td>1.36</td><td>2963</td><td>1867</td><td>4</td><td>79</td><td>204</td><td>343</td><td>6</td><td>8</td><td>20</td><td>320</td><td>1.45</td><td>6</td><td>0.1</td><td>0.24</td><td>5.5 (0.1)</td><td>6.1</td><td>6</td><td>0.2</td><td>3.69 (0.98)</td><td>3.23 (1.35)</td><td>5.91 (15.18)</td><td>87.2 (14.2)</td><td>tr?</td><td>19.8 (5.7)</td><td>9.4 (4.0)</td></tr><tr><td>R13-SMN4</td><td>1.35</td><td>5904</td><td>2620</td><td>4</td><td>15</td><td>309</td><td>274</td><td>2</td><td>8</td><td>76</td><td>261</td><td>1.23</td><td>7</td><td>0.4</td><td>0.49</td><td>5.5 (0.1)</td><td>6.1</td><td>7</td><td>0.5</td><td>8.84 (2.13)</td><td>4.27 (1.30)</td><td>9.99 (15.89)</td><td>76.9 (13.9)</td><td>tr?</td><td>33.7 (9.1)</td><td>15.6 (5.0)</td></tr><tr><td colspan="24">#10-11; 900 °C, 200MPa, oxidising conditions, 141h</td></tr><tr><td>R10-SMN1</td><td>1.30</td><td>344</td><td>148</td><td>5</td><td>2</td><td>11</td><td>267</td><td>8</td><td>8</td><td>64</td><td>258</td><td>1.42</td><td>5</td><td>0.1</td><td>0.27</td><td>6.0 (0.2)</td><td>6.3</td><td>5</td><td>0.1</td><td>0.51 (0.10)</td><td>4.40 (1.19)</td><td>5.64 (13.06)</td><td>85.5 (13.9)</td><td>4.0 (8.1)</td><td>34.7 (7.3)</td><td>16.4 (4.7)</td></tr><tr><td>R10-SMN2</td><td>1.31</td><td>1448</td><td>591</td><td>3</td><td>12</td><td>44</td><td>270</td><td>9</td><td>9</td><td>13</td><td>282</td><td>1.35</td><td>7</td><td>0.3</td><td>0.40</td><td>6.0 (0.2)</td><td>6.3</td><td>7</td><td>0.5</td><td>2.81 (0.75)</td><td>5.40 (1.78)</td><td>9.04 (18.34)</td><td>77.5 (17.9)</td><td>5.2 (10.2)</td><td>47.6 (13.2)</td><td>19.9 (6.9)</td></tr></tbody></table>
<table><tr><td>R11-SMN3</td><td>1.32</td><td>2822</td><td>1289</td><td>4</td><td>72</td><td>121</td><td>247</td><td>7</td><td>13</td><td>286</td><td>1.28</td><td>6</td><td>0.2</td><td>0.28</td><td>6.1 (0.2)</td><td>6.3</td><td>6</td><td>0.3</td><td>5.49 (1.40)</td><td>6.60 (1.98)</td><td>11.98 (16.85)</td><td>75.9 (14.9)</td><td>tr?</td><td>42.6 (11.5)</td><td>26.7 (8.6)</td></tr><tr><td>R11-SMN4</td><td>1.31</td><td>5985</td><td>1609</td><td>4</td><td>14</td><td>170</td><td>197</td><td>7</td><td>12</td><td>277</td><td>1.55</td><td>6</td><td>0.2</td><td>0.28</td><td>6.1 (0.2)</td><td>6.3</td><td>6</td><td>0.2</td><td>11.60 (2.34)</td><td>6.07 (1.48)</td><td>2.66 (13.76)</td><td>76.9 (13.1)</td><td>2.8 (8.6)</td><td>72.1 (16.4)</td><td>30.8 (8.7)</td></tr></table>

n is the number of analyses per sample. "n.a." = not analysed. tr? = probably present in the considered phase but not determined.

a NK/A = (Na+K)/Al in run-product glasses.

b Total Br concentration in the system.

c Measured by LA-ICPMS in run-product glasses; sd: 1 sigma standard deviation (statistical uncertainty calculated on the mean value); total error 1σ of the LA-ICP-MS analysis (taking into account the statistical uncertainty (sd) and the mean analytical uncertainty as given by Glitter software.

d Cl and F concentrations in run-product glasses determined by EPM analyses; sd: 1 sigma standard deviation; total error 1σ: 1sigma calculated taking into account the statistical uncertainty (sd) plus the EPMA detection limit for the specific element (250 ppm for Cl and 2000 ppm for F).

e H₂O concentration determined by the empirical solubility model of Moore et al., 1998; 1σ: 1 sigma error obtained by propagating the uncertainties on the parameters of the model.

f H₂O concentration estimated by the "by-difference" method (Devine et al., 1995); sd: 1 sigma standard deviation on multiple analyses.

g Calculated by mass balance; the values in parentheses correspond to the 1 sigma propagated error from the variables considered in the mass balance.

h Fluid/melt partition coefficients calculated after the mass balance. 1σ: 1 sigma propagated error from the variables considered in the mass balance. Note that for experiments with vapor + brine, D<sub>Br</sub> and D<sub>Cl</sub> are apparent partition coefficients that would apply to a single-phase-fluid. Experiments marked with (*) are in the vapor field, D<sub>Br</sub> and D<sub>Cl</sub> are the vapor-melt Br and Cl partition coefficients, respectively (see text).
<table><caption>**Table 4.** Experimental conditions, concentrations of volatiles in the melt and in the fluid phase and f/m partition coefficients for the pantelleritic composition</caption><thead><tr><th rowspan="2">Sample</th><th rowspan="2">NK/A<sup>a</sup></th><th rowspan="2">[Br]<sub>bul</sub><br>k<sub>s</sub><br>ppm<sup>b</sup></th><th colspan="3">LA-ICP-MS</th><th colspan="3">EPMA</th><th colspan="3">EPMA</th><th colspan="3">Solubility model<sup>e</sup></th><th colspan="3">Difference method<sup>f</sup></th><th colspan="3">RAMAN spectra<sup>g</sup></th><th colspan="3">Mass Balance</th><th colspan="3">Σcatio<br>ns<br>fluid<sup>h</sup></th><th rowspan="2">D<sub>Br</sub><sup>i</sup></th><th rowspan="2">D<sub>Cl</sub><sup>i</sup></th></tr><tr><th>pp<br>m</th><th>n</th><th>sd</th><th>total<br>error<br>1σ</th><th>pp<br>m</th><th>n</th><th>sd</th><th>total<br>error<br>1σ</th><th>wt.%</th><th>n</th><th>sd</th><th>total<br>error<br>1σ</th><th>wt<br>%</th><th>n</th><th>sd</th><th>sd</th><th>wt%</th><th>wt%</th><th>wt%</th><th>wt%</th><th>wt%</th><th>wt%</th><th>wt%</th><th>wt%</th></tr></thead><tbody><tr><td colspan="30">#2; 1000 °C, 200MPa, oxidising conditions, 92h</td></tr><tr><td>R2-EB3</td><td>1.82</td><td>3282</td><td>23<br>87</td><td>6</td><td>2</td><td>106</td><td>33<br>49</td><td>6</td><td>10<br>9</td><td>273</td><td>n.a.</td><td></td><td></td><td>5.9 (0.1)</td><td>6.1</td><td>11</td><td>0.3</td><td>n.a.</td><td>2.38<br>(0.45)</td><td>1.60<br>(0.87)</td><td>-</td><td>83.0<br>(7.1)</td><td>13.0<br>(7.3)</td><td>10.0<br>(1.9)</td><td>4.8<br>(2.6)</td></tr><tr><td>R2-EB4</td><td>1.80</td><td>5789</td><td>37<br>05</td><td>6</td><td>50</td><td>173</td><td>30<br>72</td><td>7</td><td>19<br>9</td><td>320</td><td>n.a.</td><td></td><td></td><td>5.9 (0.1)</td><td>6.1</td><td>11</td><td>0.4</td><td>6.1<br>0.6</td><td>6.34<br>(1.19)</td><td>2.55<br>(1.33)</td><td>-</td><td>76.3<br>(7.8)</td><td>14.9<br>(8.2)</td><td>17.1<br>(3.3)</td><td>8.3<br>(3.8)</td></tr><tr><td colspan="30">#3; 1000 °C, 200MPa, reducing conditions, 38h</td></tr><tr><td>R3-EB3</td><td>1.77</td><td>3432</td><td>23<br>63</td><td>6</td><td>71</td><td>255</td><td>30<br>19</td><td>8</td><td>35<br>2</td><td>432</td><td>1.34</td><td>8</td><td>0.2<br>6</td><td>0.31</td><td>5.9 (0.1)</td><td>6.5</td><td>8</td><td>0.2</td><td>n.a.</td><td>2.83<br>(0.72)</td><td>2.25<br>(1.16)</td><td>tr?</td><td>86.8<br>(7.4)</td><td>8.2 (7.7)</td><td>12.0<br>(3.3)</td><td>7.5<br>(4.0)</td></tr><tr><td>R3-EB4</td><td>1.85</td><td>5842</td><td>40<br>32</td><td>6</td><td>20<br>6</td><td>473</td><td>32<br>00</td><td>8</td><td>35<br>7</td><td>436</td><td>1.36</td><td>9</td><td>0.2<br>1</td><td>0.31</td><td>5.9 (0.1)</td><td>5.8</td><td>9</td><td>0.4</td><td>n.a.</td><td>5.80<br>(1.69)</td><td>2.22<br>(1.41)</td><td>tr?</td><td>86.7<br>(9.0)</td><td>5.3 (9.4)</td><td>14.4<br>(4.5)</td><td>6.9<br>(4.5)</td></tr><tr><td colspan="30">#4; 1000 °C, 100MPa, oxidising conditions, 45h</td></tr><tr><td>R4-EB3</td><td>1.94</td><td>3157</td><td>22<br>16</td><td>4</td><td>45</td><td>308</td><td>36<br>82</td><td>9</td><td>14<br>7</td><td>290</td><td>1.26</td><td>1<br>1</td><td>0.2<br>7</td><td>0.34</td><td>3.8 (0.1)</td><td>3.9</td><td>11</td><td>0.2</td><td>n.a.</td><td>1.99<br>(0.61)</td><td>0.62<br>(0.72)</td><td>tr?</td><td>95.8<br>(6.4)</td><td>1.6 (6.5)</td><td>9.0<br>(3.0)</td><td>1.7<br>(1.9)</td></tr></tbody></table>
<table><tr><td>R4-EB4</td><td>1.93</td><td>6243</td><td>3473</td><td>465</td><td>365</td><td>2835</td><td>8121</td><td>278</td><td>0.94</td><td>12</td><td>0.20</td><td>0.28</td><td>3.8 (0.1)</td><td>4.112</td><td>0.2</td><td>n.a.</td><td>5.02 (0.87)</td><td>1.79 (0.64)</td><td>4.82 (8.62)</td><td>85.8 (9.2)</td><td>2.6 (5.6)</td><td>14.5 (2.9)</td><td>6.3 (2.3)</td></tr></table>

#8; 1000 °C, 50MPa, oxidising conditions, 93h

<table><tr><td>R8-EB1</td><td>1.89</td><td>313</td><td>276</td><td>46</td><td>34</td><td>3806</td><td>9178</td><td>307</td><td>0.98</td><td>63</td><td>0.13</td><td>0.24</td><td>2.5 (0.1)</td><td>2.76</td><td>0.2</td><td>3.80.8</td><td>0.08 (0.06)</td><td>0.60 (0.65)</td><td>4.33 (8.21)</td><td>93.1 (9.8)</td><td>1.9 (5.9)</td><td>3.0* (2.1)</td><td>1.6* (1.7)</td></tr><tr><td>R8-EB2</td><td>1.84</td><td>1445</td><td>1189</td><td>532</td><td>153</td><td>3688</td><td>1014</td><td>289</td><td>1.04</td><td>60</td><td>0.20</td><td>0.28</td><td>2.5 (0.1)</td><td>2.98</td><td>0.1</td><td>n.a.</td><td>0.56 (0.25)</td><td>0.79 (0.65)</td><td>3.48 (9.06)</td><td>92.2 (10.3)</td><td>3.0 (5.8)</td><td>4.7* (2.2)</td><td>2.2* (1.8)</td></tr><tr><td>R8-EB3</td><td>1.86</td><td>2856</td><td>2419</td><td>572</td><td>322</td><td>3853</td><td>9158</td><td>296</td><td>1.00</td><td>56</td><td>0.26</td><td>0.33</td><td>2.5 (0.1)</td><td>2.98</td><td>0.1</td><td>n.a.</td><td>1.02 (0.58)</td><td>0.56 (0.68)</td><td>4.26 (9.96)</td><td>94.2 (9.8)</td><td>tr?</td><td>4.2* (2.5)</td><td>1.5* (1.8)</td></tr><tr><td>R8-EB4</td><td>1.82</td><td>5322</td><td>3977</td><td>5130</td><td>560</td><td>3087</td><td>8235</td><td>343</td><td>1.02</td><td>62</td><td>0.12</td><td>0.23</td><td>2.5 (0.1)</td><td>2.97</td><td>0.1</td><td>3.20.1</td><td>2.84 (1.04)</td><td>1.80 (0.76)</td><td>3.99 (8.88)</td><td>90.0 (10.2)</td><td>1.4 (6.3)</td><td>7.1* (2.8)</td><td>5.8* (2.6)</td></tr></table>

#18; 1000 °C, 25MPa, oxidising conditions, 165h

<table><tr><td>R18-EB1</td><td>1.86</td><td>394</td><td>383</td><td>424</td><td>24</td><td>3770</td><td>1104</td><td>271</td><td>0.93</td><td>68</td><td>0.18</td><td>0.27</td><td>1.7 (0.1)</td><td>1.78</td><td>0.1</td><td>n.a.</td><td>0.03 (0.03)</td><td>0.43 (0.41)</td><td>3.42 (5.82)</td><td>93.9 (6.8)</td><td>2.2 (3.9)</td><td>0.8* (0.7)</td><td>1.2* (1.1)</td></tr><tr><td>R18-EB2</td><td>1.83</td><td>1874</td><td>1843</td><td>540</td><td>106</td><td>3510</td><td>102</td><td>270</td><td>1.02</td><td>68</td><td>0.18</td><td>0.27</td><td>1.6 (0.1)</td><td>1.78</td><td>0.1</td><td>n.a.</td><td>0.22 (0.12)</td><td>0.66 (0.39)</td><td>2.38 (5.62)</td><td>96.1 (6.6)</td><td>0.7 (3.7)</td><td>1.2* (0.7)</td><td>1.9* (1.1)</td></tr><tr><td>R18-EB3</td><td>1.83</td><td>3231</td><td>2947</td><td>566</td><td>170</td><td>3058</td><td>107</td><td>272</td><td>1.06</td><td>62</td><td>0.12</td><td>0.24</td><td>1.6 (0.1)</td><td>1.87</td><td>0.1</td><td>n.a.</td><td>0.67 (0.24)</td><td>1.27 (0.46)</td><td>2.33 (6.22)</td><td>92.7 (7.1)</td><td>3.0 (4.2)</td><td>2.3 (0.8)</td><td>4.2 (1.6)</td></tr><tr><td>R18-EB4</td><td>1.80</td><td>5459</td><td>3890</td><td>571</td><td>220</td><td>2514</td><td>9108</td><td>272</td><td>0.96</td><td>66</td><td>0.16</td><td>0.26</td><td>1.6 (0.1)</td><td>1.88</td><td>0.2</td><td>n.a.</td><td>2.64 (0.48)</td><td>2.16 (0.56)</td><td>3.94 (7.27)</td><td>87.0 (7.9)</td><td>4.2 (4.8)</td><td>6.8 (1.3)</td><td>8.6 (2.4)</td></tr></table>

#12; 1100 °C, 200MPa, oxidising conditions, 23h

<table><tr><td>R12-EB1</td><td>1.85</td><td>344</td><td>287</td><td>55</td><td>23</td><td>3560</td><td>8178</td><td>307</td><td>0.98</td><td>78</td><td>0.18</td><td>0.27</td><td>5.6 (0.1)</td><td>6.07</td><td>0.4</td><td>n.a.</td><td>0.17 (0.07)</td><td>1.18 (1.05)</td><td>6.98 (13.45)</td><td>91.7 (13.3)</td><td>tr?</td><td>5.8* (2.6)</td><td>3.3* (3.0)</td></tr></table>
<table><tbody><tr><td>R12-EB2</td><td>1.83</td><td>1710</td><td>1349</td><td>6</td><td>16</td><td>123</td><td>3556</td><td>9</td><td>83</td><td>263</td><td>1.10</td><td>5</td><td>0.14</td><td>0.25</td><td>5.6 (0.1)</td><td>6.3</td><td>5</td><td>0.2</td><td>n.a.</td><td>1.01 (0.36)</td><td>1.07 (0.89)</td><td>3.67 (12.24)</td><td>93.5 (14.1)</td><td>0.8 (8.0)</td><td>7.5 (2.7)</td><td>3.0 (2.5)</td></tr><tr><td>R12-EB4</td><td>1.86</td><td>5831</td><td>4873</td><td>5</td><td>143</td><td>484</td><td>3376</td><td>8</td><td>88</td><td>265</td><td>1.03</td><td>6</td><td>0.22</td><td>0.30</td><td>5.7 (0.1)</td><td>6.4</td><td>6</td><td>0.2</td><td>5.1</td><td>1.0</td><td>3.79 (1.90)</td><td>2.08 (1.31)</td><td>7.46 (17.95)</td><td>86.7 (17.0)</td><td>tr?</td><td>7.8 (4.0)</td><td>6.2 (3.9)</td></tr><tr><td colspan="22">#10; 900 °C, 200MPa, oxidising conditions, 141h</td></tr><tr><td>R10-EB1</td><td>1.82</td><td>333</td><td>239</td><td>6</td><td>5</td><td>16</td><td>3183</td><td>10</td><td>138</td><td>286</td><td>0.97</td><td>6</td><td>0.40</td><td>0.45</td><td>6.2 (0.2)</td><td>6.5</td><td>6</td><td>0.3</td><td>7.1</td><td>0.5</td><td>0.31 (0.11)</td><td>2.71 (1.41)</td><td>8.85 (19.58)</td><td>87.4 (21.5)</td><td>0.8 (11.8)</td><td>13.1 (4.7)</td><td>8.5 (4.5)</td></tr><tr><td>R10-EB2</td><td>1.78</td><td>1629</td><td>933</td><td>6</td><td>17</td><td>62</td><td>2815</td><td>10</td><td>84</td><td>264</td><td>1.15</td><td>6</td><td>0.28</td><td>0.34</td><td>6.2 (0.2)</td><td>6.5</td><td>6</td><td>0.2</td><td>n.a.</td><td>1.99 (0.47)</td><td>3.17 (1.17)</td><td>2.84 (15.71)</td><td>86.5 (16.3)</td><td>5.5 (9.1)</td><td>21.4 (5.3)</td><td>11.3 (4.3)</td></tr><tr><td>R10-EB3</td><td>1.77</td><td>2936</td><td>1676</td><td>6</td><td>37</td><td>115</td><td>2748</td><td>8</td><td>100</td><td>320</td><td>1.08</td><td>6</td><td>0.35</td><td>0.40</td><td>6.2 (0.2)</td><td>6.5</td><td>6</td><td>0.2</td><td>n.a.</td><td>4.58 (1.39)</td><td>4.30 (1.80)</td><td>5.86 (20.86)</td><td>79.1 (20.2)</td><td>6.2 (11.6)</td><td>27.3 (8.5)</td><td>15.6 (6.8)</td></tr><tr><td>R10-EB4</td><td>1.76</td><td>5414</td><td>2546</td><td>4</td><td>25</td><td>170</td><td>2280</td><td>9</td><td>125</td><td>305</td><td>1.20</td><td>6</td><td>0.20</td><td>0.28</td><td>6.2 (0.2)</td><td>6.3</td><td>6</td><td>0.2</td><td>6.6</td><td>0.1</td><td>11.35 (3.48)</td><td>6.40 (2.36)</td><td>2.57 (20.86)</td><td>75.8 (19.6)</td><td>3.9 (13.1)</td><td>44.6 (14.0)</td><td>28.1 (11.0)</td></tr><tr><td colspan="22">#22; 800 °C, 200MPa, oxidising conditions, 145h</td></tr><tr><td>R22-EB1</td><td>1.71</td><td>382</td><td>170</td><td>6</td><td>3</td><td>14</td><td>2903</td><td>9</td><td>100</td><td>269</td><td>1.27</td><td>8</td><td>0.28</td><td>0.34</td><td>6.6 (0.2)</td><td>6.4</td><td>8</td><td>0.5</td><td>7.2</td><td>0.6</td><td>0.50 (0.10)</td><td>2.67 (0.96)</td><td>tr?</td><td>86.9 (13.6)</td><td>9.9 (7.1)</td><td>29.6 (6.4)</td><td>9.2 (3.4)</td></tr><tr><td>R22-EB3</td><td>1.69</td><td>2914</td><td>1573</td><td>6</td><td>84</td><td>139</td><td>2819</td><td>9</td><td>99</td><td>269</td><td>1.33</td><td>8</td><td>0.41</td><td>0.46</td><td>6.6 (0.2)</td><td>6.3</td><td>8</td><td>0.4</td><td>n.a.</td><td>4.51 (0.93)</td><td>4.08 (1.37)</td><td>tr?</td><td>75.1 (9.2)</td><td>16.3 (9.7)</td><td>28.7 (6.4)</td><td>14.5 (5.1)</td></tr><tr><td>R22-EB4</td><td>1.72</td><td>5347</td><td>2675</td><td>6</td><td>321</td><td>346</td><td>2587</td><td>9</td><td>111</td><td>274</td><td>1.44</td><td>9</td><td>0.30</td><td>0.36</td><td>6.6 (0.2)</td><td>6.0</td><td>9</td><td>0.2</td><td>6.7</td><td>0.2</td><td>9.57 (1.97)</td><td>5.09 (1.51)</td><td>tr?</td><td>71.8 (9.3)</td><td>13.6 (10.4)</td><td>35.8 (8.7)</td><td>19.7 (6.2)</td></tr></tbody></table>

n is the number of analyses per sample. "n.a." = not analysed. tr? = probably present in the considered phase but not determined.

<footnote>a NK/A = (Na+K)/Al in run-product glasses.</footnote>

<footnote>b Total Br concentration in the system.</footnote>
<footnote><sup>c</sup> Measured by LA-ICPMS in run-product glasses; **sd**: 1 sigma standard deviation (statistical uncertainty calculated on the mean value); **total error 1σ**: of the LA-ICP-MS analysis (taking into account the statistical uncertainty (sd) and the mean analytical uncertainty as given by Glitter software.</footnote>

<footnote><sup>d</sup> Cl and F concentrations in run-product glasses determined by EPM analyses; **sd**: 1 sigma standard deviation; **total error 1σ**: 1sigma calculated taking into account the statistical uncertainty (sd) plus the EPMA detection limit for the specific element (250 ppm for Cl and 2000 ppm for F).</footnote>

<footnote><sup>e</sup> H<sub>2</sub>O concentration determined by the empirical solubility model of Moore et al., 1998; **1σ**: 1 sigma error obtained by propagating the uncertainties on the parameters of the model.</footnote>

<footnote><sup>f</sup> H<sub>2</sub>O concentration estimated by the "by-difference" method (Devine et al., 1995); **sd**: 1 sigma standard deviation on multiple analyses.</footnote>

<footnote><sup>g</sup> H<sub>2</sub>O concentration determined by Raman spectroscopy; **sd**: 1 sigma standard deviation on multiple analyses (n=3).</footnote>

<footnote><sup>h</sup> Calculated by mass balance; the values in parentheses correspond to the 1 sigma propagated error from the variables considered in the mass balance.</footnote>

<footnote><sup>i</sup> Fluid/melt partition coefficients calculated after the mass balance. **1σ**: 1 sigma propagated error from the variables considered in the mass balance. Note that for experiments with vapor + brine, D<sub>Br</sub> and D<sub>Cl</sub> are apparent partition coefficients that would apply to a single-phase-fluid. Experiments marked with (*) are in the vapor field, D<sub>Br</sub> and D<sub>Cl</sub> are the vapor-melt Br and Cl partition coefficients, respectively (see text).</footnote>
Figure Captions

**Figure 1.** Phase diagram of the H₂O-NaCl system delimiting the vapor and vapor + liquid fields. Vapor-liquid equilibrium curves at 1000, 900 and 800°C were calculated using the model of Driesner and Heinrich (2007). The symbols represent the wt% NaCl+NaBr equivalent calculated concentration of the bulk fluid phase coexisting at run conditions with **a)** phonolitic, **b)** comenditic and **c)** pantelleritic melts. White-filled symbols correspond to experiments in the two-phase field (melt + vapor) as determined by the examination of fluid inclusions (type-1 fluid inclusions only) and/or with Henrian behaviour of Br (10 and 50 MPa in **a**)), black-filled symbols correspond to experiments in the vapor + brine field (type-1 fluid inclusion + type-2 fluid inclusions and/or salt crystal in glass) and grey-filled symbols correspond to experiments with uncertain status (see text and Table S6, Supplementary Material).

**a)** also show the data of Alletti et al. (2014) in the phonolite system (down-pointing triangles; experiments at 250 MPa are supercritical; white-filled symbols correspond to experiments in the two-phase field and with Henrian behaviour, black-filled symbols to experiments in the immiscibility field and grey-filled symbols correspond to experiments with strongly non-ideal behaviour).

**Figure 2.** Photomicrographs of experimental run product glasses and representative fluid inclusions. Experiment #, total bromine concentration of the system [Br]~bulk~ and melt composition are given for each image.

**a)** Back-scattered electron (BSE) image of the run product glass from experiment #R3 showing the “sponge-like texture” (see text).

**b)** Transmitted light image of an NaCl or NaBr cube embedded in glass quenched from experiment at 200 MPa and 900°C with pantellerite.

Transmitted light image of type-1 fluid inclusions in glasses quenched from experiments at 1000°C and 200 MPa (**c**) and 50 MPa (**d**) with phonolite. The type-1 fluid inclusions consist of vapor bubbles (V) and liquid phases (L).

**e)** Type-1 fluid inclusions in glass from experiments at 1000°C and 200 MPa with comendite. As the bulk bromine concentration increases at fixed P-T-fO₂, the proportion of type-2 fluid inclusions (vapor, liquid and cubic daughter minerals) increases as shown in **f**). The same is observed in the pantellerite experiments at 200 MPa and 800°C: type-1 fluid inclusions are largely predominant for low [Br]~bulk~ (**g**), whereas type-2 inclusions are in the majority for high [Br]~bulk~ (**h**).

**Figure 3.** Br concentration in the experimental glass products as a function of pressure (top panels) and temperature (bottom panels) for phonolitic (**a**, **d**), comenditic (**b**, **e**) and pantelleritic (**c**, **f**) compositions. Error bars on [Br]~melt~ are the 1σ total error of the measured Br concentration in glasses by LA-ICPMS (Tables 2-4). Coloured solid lines indicate the different bulk bromine concentrations of the experimental charges. White-filled symbols correspond to experiments in the two-phase field (melt + vapor), black-filled symbols correspond to experiments in the vapor + brine field and grey-filled symbols correspond to experiments with uncertain status (see text and Table S6, Supplementary Material).
**Figure 4.** Cl concentration in the experimental glass products as a function of pressure (top panels) and temperature (bottom panels) for phonolitic (a, d), comenditic (b, e) and pantelleritic (c, f) compositions. Error bars on [Cl]~melt~ are the 1σ total error of the measured Cl concentration in glasses by EPMA (Tables 2-4). Coloured solid lines indicate the different bulk bromine concentrations of the experimental charges. The dashed lines show the Cl concentration in the starting glasses, and the grey rectangles indicate the uncertainties in Cl concentration. Note the change in scale for the y-axis between the phonolite (a, d) and the two rhyolites (b, c, e, f).

Same symbol legend as in Figure 3.

**Figure 5.** Relationship between sodium molar balance and [Br]~melt~, for phonolitic (a, b), comenditic (c, d) and pantelleritic (e, f) compositions, with *P* fixed (right panels) and *T* fixed (left panels).

$n\text{Na}_{\text{initial glass}} = n\text{Na}$ in starting glasses; $n\text{Na}_{\text{quenched melt}} = n\text{Na}$ in run-product glasses.

The masses of the final glass components are calculated following the normalisation procedure of Botcharnikov et al. (2015), assuming that the mass of SiO~2~ in the melt is constant and equal to that in the starting silicate glass loaded into the capsules (see §2.5. and Supplementary Material). The samples falling in the positive region represent the melts depleted in Na at run conditions. Points falling in the negative region represent those glasses that incorporated the Na previously added as bromide solutions.

Same symbol legend as in Figure 3.

**Figure 6.** Br concentration in melt versus calculated Br concentration in bulk fluid for phonolite (a), comendite (c) and pantellerite (e) experiments.

Total heavy halogen concentration (Br+Cl) in phonolitic (b), comenditic (d) and pantelleritic (e) melts versus (NaCl+NaBr) equivalent concentration in the bulk fluid. The error bars on the x-axis represent the total propagated error on the fluid composition (1σ), while the error bars on the y-axis represent the 1σ total error of the Cl and Br concentrations measured by EPMA and LA-ICPMS, respectively (Tables 2-4). Same symbol legend as in Figure 3.

**Figure 7.** Bromine fluid-melt partition coefficients against NaBr equivalent concentration (left) or (NaBr+NaCl) equivalent concentration (right) of the bulk fluid for phonolitic (a,b), comenditic (c,d) and pantelleritic (e,f) compositions. Note that for experiments in which immiscible vapor and brine coexist with melt, $D_{\text{Br}}^{f/m}$ are apparent partition coefficients that would apply to a single phase fluid. Same symbol legend as in Figure 3.

**Figure 8.** Relationships between melt Cl or (Cl+Br) concentration and bulk fluid phase Cl or (Cl+Br) concentration in selected phonolite experiments from this study (coloured symbols) and in experiments performed in the Br-free, F-poor, NaCl-H~2~O-phonolite system (Alletti et al., 2014 (A14; down-pointing triangles); Signorelli and Carroll, 2000 (SC00; squares)) at a) 1000°C in the 10–50 MPa pressure range and b) 200 MPa in the 900–1000°C temperature range. The experiments of Alletti et al. (2014) were performed with a metaluminous phonolite (Erebus) with fO~2~ ~QFM-QFM+1. Signorelli and Carroll (2000) used the same phonolite as in
this study (MB: Montaña Blanca) and performed their experiments at oxidising conditions (NNO+1). For our data and that of Signorelli and Carroll (2000), white-filled symbols correspond to experiments in the two-phase field (melt + vapor), black-filled symbols to experiments in the immiscibility field and grey-filled symbols correspond to experiments with uncertain status. For the experiments of Alletti et al. (2014), white-filled symbols correspond to experiments in the two-phase field and with Henrian behaviour, while grey-filled symbols correspond to experiments with strongly non-ideal behaviour. Note that Alletti et al. (2014) and Signorelli and Carroll (2000) performed experiments with up to more than 2 wt% bulk Cl in the system, while the maximum bulk Cl+Br concentration in our experiments (regardless of P and T) did not exceed ~0.8 wt%.

Ox: oxidising conditions; Red: reducing conditions.

**Figure 9.** Br vapor-melt ($D_{Br}^{v/m}$) or bulk fluid-melt ($D_{Br}^{bf/m}$) partition coefficients of all the experiments of this study doped with ~300 ppm Br, as a function of the calculated total salinity of the bulk fluid phase at run conditions. **a)** Experiments performed in the phonolitic system (2600 ppm bulk Cl). All the experiments are in the vapor field ($D_{Br}^{v/m}$). **b)** Experiments performed in the halogen-rich comenditic (4600 ppm bulk Cl) and pantelleritic (4000 ppm bulk Cl) systems. The comendite and pantellerite also contain > 1 wt% F. Only the low pressure experiments at 1000°C and (possibly, see text) the experiments at 1100°C are in the vapor field (($D_{Br}^{v/m}$); blue rectangle). The experiments performed at 200 MPa and ≤ 1000°C are all saturated with an immiscible fluid ($D_{Br}^{bf/m}$; yellow rectangle).

All experiments were performed under oxidising conditions, except for one phonolite experiment at 1000°C/200 MPa (reducing (Red)) conditions. Same symbol legend as in Figure 3.

**Figure 10.** **a)** Bromine vapor-melt partition coefficients versus pressure. For the phonolite (Pho), pantellerite (Pan) and comendite (Com) of this study, only experiments in the vapor-field, doped with ~300 ppm Br are considered, except for the phonolite at 10 and 50 MPa: all experiments in the phonolite composition at 10 and 50 MPa are in the vapor field, with fluid and melt bromine concentrations being proportional, so the reported $D_{Br}^{v/m}$ values are calculated by linear regression through the origin (see text and Figure S13). Vapor-melt partition coefficients for silicic calc-alkaline magmas are shown for comparison (andesite (And), dacite (Dac) and rhyodacite (RD); Cadoux et al., 2018 (Cad18); Cassidy et al., 2022 (Cas22)). All experiments illustrated were performed under oxidising (Ox) conditions, except for one experiment of this study at 1000°C/200 MPa in the phonolitic system (reducing (Red) conditions).

**b)** Dependence of $D_{Br}^{v/m}$ on temperature at 120–200 MPa for phonolite, comendite and pantellerite (this study), calc-alkaline rhyodacite (Cadoux et al. 2018) and rhyolite (Rhyo; Cassidy et al., 2022). Only experiments from this study doped with ~300 ppm Br are considered. Note that the status of experiments performed at 1100°C in pantellerite and comendite systems is uncertain (see text). All experiments were performed under oxidising conditions (see text).

Note the change in scale for $D_{Br}^{v/m}$ between Figures 10 **a)** and **b)**.

**Figure 11.** Evolution of Cl and Br during the decompression of a phonolite magma. The initial H₂O, Cl and Br melt concentrations are 6.15 wt%, 3000 ppm and 30 ppm, respectively. The
evolution of Cl melt concentration (a), Br melt concentration (b), Cl/Br melt mass ratio (c) and Cl/Br fluid mass ratio (d) are shown for three different initial fluid contents of the reservoir: 0.1, 2 and 5 wt%. All calculations are carried out at 1000°C.
<figure><img src="image_4.png" /><figcaption>Figure</figcaption></figure>

Click here to access/download:Figure 1 revised.pdf
<figure><img src="image_5.png" /><figcaption>Figure 2</figcaption></figure>

Click here to access/download Figure 2 revised.pdf
<figure><img src="image_6.png" /><figcaption>Figure 3</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Figure 4</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Figure 6</figcaption></figure>

Click here to access/download:Figure:Figure 6 revised.pdf
Phonolite (a-b)

P= 10-200 MPa
T= 900-1100 °C

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

Comendite (c-d)

P= 25-200 MPa
T= 900-1100 °C

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

Pantellerite (e-f)

P= 25-200 MPa
T= 800-1100 °C

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>
<caption>Figure 7</caption>

Click here to access/download:Figure 7 revised.pdf

<figure><img src="image_18.png" /></figure>
<figure><img src="image_19.png" /><figcaption>Figure 8</figcaption></figure>

<figure><img src="image_20.png" /></figure>

Click here to access/download:Figure 8 revised.pdf
<figure><img src="image_21.png" /></figure>

<figure><img src="image_22.png" /></figure>
<figure><img src="image_23.png" /></figure>

<figure><img src="image_24.png" /></figure>
<figure><img src="image_25.png" /><figcaption>Figure 11</figcaption></figure>

Click here to access/download;Figure;Figure 11 revised.pdf