Bromine speciation in hydrous silicate melts at high pressure

Abstract

International audienceBr speciation in hydrous silicate melts at high pressure has been investigated up to 7.6 GPa using X-ray Absorption Spectroscopy (XAS) at the Br K-edge in a Paris-Edinburgh press. Br in silicate melts is surrounded by an average of 6 Na cations, a number slightly increasing with pressure (5.8 to 6.6), with a Br-Na distance increasing from 3.49 to 3.72 Å. Two oxygens, either from a water, an –OH molecule or from the tetrahedral silicate network, with an average Br-O distance of 1.80 Å, form the closest coordination shell around Br ions. The persistence of an alkali shell around Br, in a structure similar to crystalline NaBr, throughout the pressure range investigated shows that Br can be retained in the melt structure at relatively high pressure and supports the idea of its deep recycling. Finally, our results confirm that Br could be efficiently degassed with water at low pressures and that Br may also have been efficiently degassed along with water during the early stages of an oxidized magma ocean

Bromine speciation and partitioning in slab-derived fluids and melts: Implications for halogens recycling in subduction zones

“…The EXAFS-derived parameters suggest that Br in NS2 and haplogranite glasses is coordinated to an average of 6 Na cations in the first shell at an average distance of ~2.95 Å and 6 O second neighbors located at ~3.4 Å ( Table 3). The fitted Br-Na bond length is consistent, within errors, with theoretical Br-Na distances in crystalline NaBr (2.987 Å, Deshpande, 1961) and is close to that fitted for aluminosilicate glasses in a previous study (Cochain et al, 2015), suggesting Br is incorporated in the silicate glasses in a "salt-like" structure, similar to NaBr. The similarities https://doi.org/10.5194/se-2019-205 Preprint.…”

“…Similarly, current knowledge of halogens solubility and speciation in fluids and melts is mostly limited to pressures below 0.3 GPa (equivalent to ~10 km depth), which are relevant to volcanic degassing and ore deposit formation in the shallow crust, but not to slab dehydration or melting beneath arcs at far greater depth (Webster, 1990;Métrich and Rutherford, 1992;Webster, 1992;Bureau et al, 2000;Signorelli and Carroll, 2002;Bureau and Métrich, 2003;Carroll, 2005;Evans et al, 2009;Cadoux et al, 2018). Only recently Bureau et al (2010Bureau et al ( , 2016 reported fluid-melt partition coefficients for Br and I in the haplogranite-H 2 O system up to 1.7 GPa while Cochain et al (2015) investigated the speciation of Br in aqueous fluids or haplogranitic melts up to 7.6 GPa.…”

Variation of the BrO/SO2 Molar Ratio in the Plume of Tungurahua Volcano Between 2007 and 2017 and Its Relationship to Volcanic Activity

“…For bromine in contrast, the degassing processes are still not fully understood and lack comparison of measurements with model data. Several studies investigated melt-fluid partitioning of bromine with respect to sulphur using artificial melts (Bureau et al, 2000(Bureau et al, , 2010Cochain et al, 2015) and concluded that the partition coefficient highly depends on the melt composition. A recent study using natural melts suggested efficient degassing of bromine at shallow depths beneath the volcanic vent similar to chlorine behaviour (Cadoux et al, 2018).…”

Incorporation et rôle des halogènes dans les silicates vitreux et fondus

“…Très récemment, uneétude de la spéciation du brome dans des liquides silicatés hydratés a montré que le brome est entouré des ions sodium et que sa coordinence augmente avec la pression (passage de 3à 6 ions sodium autour du brome) [37]. L'iode s'associe sous forme iodure avec les alcalins et principalement les ions sodium au sein du réseau vitreux [18,29].…”