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

Abstract

25Volcanogenic halogens, in particular bromine, potentially play an important role in the ozone 26 depletion of the atmosphere. Understanding bromine behaviour in magmas is therefore crucial 27 to properly evaluate the contribution of volcanic eruptions to atmospheric chemistry and their 28 environmental impact. To date, bromine partitioning between silicate melts and the gas phase 29 is very poorly constrained, with the only relevant experimental studies limited to investigation 30 of synthetic melt with silicic compositions. In this study, fluid/melt partitioning experiments 31 were performed using natural silicate glasses with mafic, intermediate and silicic 32 compositions. For each composition, experiments were run with various Br contents in the 33 initial fluid (H 2 O-NaBr), at T -P conditions representative of shallow magmatic reservoirs in 34 volcanic arc contexts (100-200 MPa, 900-1200°C). The resulting fluid/melt partition 35 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 -36 200 MPa for the andesite and 20.2 ± 1.2 at 900°C -200 MPa for the rhyodacite. Our 37 experiments show that D Br f/m increases with increasing SiO 2 content of the melt (as for 38 chlorine) and suggest that it is also sensitive to melt temperature (increase of D Br f/m with 39 decreasing temperature). We develop a simple model to predict the S-Cl-Br degassing 40 behaviour in mafic systems, which accounts for the variability of S-Cl-Br compositions of 41 volcanic gases from Etna and other mafic systems, and shows that coexisting magmatic gas 42 and melt evolve from S-rich to Cl-Br enriched (relative to S) upon increasing degree of 43 degassing. We also report first Br contents for melt inclusions from Etna, Stromboli, Merapi 44 and Santorini eruptions and calculate the mass of bromine available in the magma reservoir 45 prior to the eruptions under consideration. The discrepancy that we highlight between the 46 mass of Br in the co-existing melt and fluid prior to the Merapi 2010 eruption (433 and 73 47 tons, respectively) and the lack of observed BrO (from space) hints at the need to investigate 48 further Br speciation in 'ash-rich' volcanic plumes. Overall, our results suggest that the Br 49 3yield into the atmosphere of cold and silicic magmas will be much larger than that from hotter 50 and more mafic magmas. 51 52 Keywords: bromine, fluid/melt partitioning, degassing, arc magmas, atmospheric chemistry 53 4 1. Introduction 54 Volcanic degassing is an important process in sustaining the composition of Earth's 55 atmosphere (e.g., Gaillard and Scaillet, 2014; Mather, 2015). Whilst much progress has been 56 made constraining global volcanic fluxes, uncertainties remain regarding the emissions of the 57 key halogen species, especially the trace Br-and I-bearing species (Pyle and Mather, 2009). 58 However, improvements in remote sensing techniques and analytical techniques, and their 59 application to an increasing number of active volcanoes, have provided new data on the 60 concen...

Advances in Bromine Speciation in Volcanic Plumes

“…The challenge to interpreting BrO observations is highlighted by Cadoux et al (2018). GOME-2 observations of Merapi's eruption in Indonesia 2010 resulted in extremely low BrO/SO 2 ratios (maximum 8 × 10 −6 ; Hörmann et al, 2013), much lower than expected based on petrological estimates of the halogen/sulfur emission.…”

“…GOME-2 observations of Merapi's eruption in Indonesia 2010 resulted in extremely low BrO/SO 2 ratios (maximum 8 × 10 −6 ; Hörmann et al, 2013), much lower than expected based on petrological estimates of the halogen/sulfur emission. Cadoux et al (2018) discussed that BrO formation could be limited by the restriction of photochemical driven reactions caused by the ash-rich paroxysmal phase and the absence of UV light during the eruption in the night (the BrO was detected by GOME-2 in the following day). According to Cadoux et al (2018) other possibilities include: (a) scavenging of bromine in the troposphere (e.g., adsorption of bromine on ash), (b) kinetic factors lead to favored S degassing from the magma, (c) brine saturation leading to Br uptake when magma rises, and (d) changes in Br partitioning caused by other volatile species in the magma.…”

“…Cadoux et al (2018) discussed that BrO formation could be limited by the restriction of photochemical driven reactions caused by the ash-rich paroxysmal phase and the absence of UV light during the eruption in the night (the BrO was detected by GOME-2 in the following day). According to Cadoux et al (2018) other possibilities include: (a) scavenging of bromine in the troposphere (e.g., adsorption of bromine on ash), (b) kinetic factors lead to favored S degassing from the magma, (c) brine saturation leading to Br uptake when magma rises, and (d) changes in Br partitioning caused by other volatile species in the magma. To this, we add the important role of plume chemistry in converting emitted HBr into BrO, as emphasized in this review.…”

“…Volcanic sulfate particles in the stratosphere also act as surfaces to activate halogens causing ozone layer depletion (Brasseur and Granier, 1992). More recently, the potentially chemistry-climate importance of volcanic halogen emissions has started to be of increased interest again (von Glasow et al, 2009;Kutterolf et al, 2013Kutterolf et al, , 2015Cadoux et al, 2015Cadoux et al, , 2018Vidal et al, 2016;Klobas et al, 2017). Some eruptions to the stratosphere have been observed to co-inject halogens alongside sulfur (Theys et al, 2009;Hörmann et al, 2013;Carn et al, 2016), leading to volcanic halogen impact on stratospheric ozone and NO X (Lurton et al, 2018).…”

The sulfur- and halogen-rich super eruption Los Chocoyos and its impacts on climate and environment

“…Halogen compounds, like chlorine and bromine, contributes to catalytic ozone depletion in the stratosphere (Brasseur and Solomon, 2005;Solomon, 1999). There is welldocumented petrological evidence that large to extremely large explosive volcanic eruptions emitted environmentally significant amounts of chlorine and bromine (Cadoux et al, 2015(Cadoux et al, , 2018Krüger et al, 2015;Kutterolf et al, 2013Kutterolf et al, , 2015Vidal et al, 2016). Furthermore, recent atmospheric observations revealed that even relatively small volcanic eruptions can inject significant amounts of halogen compounds in the stratosphere (for review and overview discussions see (von Glasow et al, 2009;Krüger et al, 2015;WMO, 2018).…”