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

Highlights

• First bromine partition experiments performed with natural silicate glasses.

• First Br fluid/melt partition coefficients on mafic and intermediate compositions.

• D_Br^f/m increases with SiO₂ content of the melt, from basalt to rhyodacite compositions.

• First Br data in melt inclusions from Etna, Stromboli, Merapi and Santorini volcanoes.

• First model describing Br degassing behaviour in mafic volcanic systems.

Abstract

Volcanogenic halogens, in particular bromine, potentially play an important role in the ozone depletion of the atmosphere. Understanding bromine behaviour in magmas is therefore crucial to properly evaluate the contribution of volcanic eruptions to atmospheric chemistry and their environmental impact. To date, bromine partitioning between silicate melts and the gas phase is very poorly constrained, with the only relevant experimental studies limited to investigation of synthetic melt with silicic compositions. In this study, fluid/melt partitioning experiments were performed using natural silicate glasses with mafic, intermediate and silicic compositions. For each composition, experiments were run with various Br contents in the initial fluid (H₂O–NaBr), at T–P conditions representative of shallow magmatic reservoirs in volcanic arc contexts (100–200 MPa, 900–1200 °C). The resulting fluid/melt partition coefficients (D_Br^f/m) are: 5.0 ± 0.3 at 1200 °C–100 MPa for the basalt, 9.1 ± 0.6 at 1060 °C–200 MPa for the andesite and 20.2 ± 1.2 at 900 °C–200 MPa for the rhyodacite. Our experiments show that D_Br^f/m increases with increasing SiO₂ content of the melt (as for chlorine) and suggest that it is also sensitive to melt temperature (increase of D_Br^f/m with decreasing temperature). We develop a simple model to predict the S–Cl–Br degassing behaviour in mafic systems, which accounts for the variability of S–Cl–Br compositions of volcanic gases from Etna and other mafic systems, and shows that coexisting magmatic gas and melt evolve from S-rich to Cl–Br enriched (relative to S) upon increasing degree of degassing. We also report first Br contents for melt inclusions from Etna, Stromboli, Merapi and Santorini eruptions and calculate the mass of bromine available in the magma reservoir prior to the eruptions under consideration. The discrepancy that we highlight between the mass of Br in the co-existing melt and fluid prior to the Merapi 2010 eruption (433 and 73 tons, respectively) and the lack of observed BrO (from space) hints at the need to investigate further Br speciation in ‘ash-rich’ volcanic plumes. Overall, our results suggest that the Br yield into the atmosphere of cold and silicic magmas will be much larger than that from hotter and more mafic magmas.

Introduction

Volcanic degassing is an important process in sustaining the composition of Earth's atmosphere (e.g., Gaillard and Scaillet, 2014, Mather, 2015). Whilst much progress has been made constraining global volcanic fluxes, uncertainties remain regarding the emissions of the key halogen species, especially the trace Br- and I-bearing species (Pyle and Mather, 2009). However, improvements in remote sensing techniques and analytical techniques, and their application to an increasing number of active volcanoes, have provided new data on the concentrations of these minor components in volcanic gases (e.g., Gerlach, 2004, Aiuppa et al., 2005, Aiuppa, 2009, Bobrowski et al., 2015), which in turn can be used to better constrain their global fluxes to the atmosphere (Pyle and Mather, 2009). Bromine has received particular attention over the last decade, owing to its important role in atmospheric chemistry in general (e.g., Oppenheimer et al., 2006; Roberts et al., 2009, Roberts et al., 2014) and ozone depletion in the troposphere and stratosphere in particular (von Glasow et al., 2009, Kutterolf et al., 2013, Cadoux et al., 2015). Global compilations show that Br sources (emissions to the atmosphere) and sinks (removal routes from the atmosphere) are not strictly balanced, hinting at a missing natural source of Br (Montzka et al., 2011). The direct detection of HBr and BrO in volcanic plumes (Bobrowski et al., 2003, Aiuppa et al., 2005) suggests that volcanic activity may be one such a source.

The correct evaluation of the contribution of past volcanic eruptions to atmospheric chemistry depends on our ability to evaluate Br behaviour in magmas, in particular its partitioning between silicate melt and gas phases. So far, only a few experimental studies have been performed on this topic, and have investigated Br behaviour in synthetic albite to rhyolite melt compositions (Bureau et al., 2000, Bureau and Métrich, 2003). However, natural silicate melt compositions can depart significantly from such model systems, in particular by having elevated contents of Fe, Mg or Ca, which (as Na) can complex with halogens thereby enhancing their solubility in silicate melts (Cochain et al., 2015). The relationship between halogen solubility and their complexation with cations has been shown for Cl; chlorine solubility in most silicate melts is dominantly controlled by the abundances of $\text{Mg}\sim \text{Ca}>\text{Fe}>\text{Na}>\mathrm{K}>\text{network-forming}$ $\text{Al}>\text{Li}\sim \text{Rb}\sim \text{Cs}$, but Ti, F, and P also have strong influences (e.g., Webster et al., 1999; Webster and De Vivo, 2002). There is thus a need to evaluate the role of melt composition on Br behaviour in magmas, which is the main motivation of the present study. To that end, we have performed fluid/melt partitioning experiments on natural basalt, andesite and rhyodacite compositions under P–T–H₂O–redox storage conditions relevant to shallow arc magmas. Combining our Br partition coefficient for the basaltic composition, with other experimental data on S and Cl behaviour, and volcanic gas compositions from the literature, we develop a simple first-order model to predict the S–Cl–Br degassing behaviour in mafic systems. We also measure Br contents of melt inclusions from Etna, Stromboli, Merapi and Santorini eruptions and estimate the mass of bromine in the pre-eruptive magmas, this allows us to address the atmospheric contribution of open-vent mafic volcanoes versus that of intermediate-silicic volcanoes.