Bromine speciation and partitioning in slab-derived aqueous fluids and silicate melts and implications for halogen transfer in subduction zones

Louvel, Marion; Sanchez-Valle, Carmen; Malfait, Wim J.; Pokrovski, Gleb S.; Borca, Camelia N.; Grolimund, Daniel

Understanding the behavior of halogens (Cl, Br, and I) in subduction zones is critical to constrain the geochemical cycle of these volatiles and associated trace metals, as well as to quantify the halogen fluxes to the atmosphere via volcanic degassing. Here, the partitioning of bromine between coexisting aqueous fluids and hydrous granitic melts and its speciation in slab-derived fluids have been investigated in situ up to 840 inline-formulaC and 2.2 GPa by synchrotron x-ray fluorescence (SXRF) and x-ray absorption spectroscopy (XAS) in diamond anvil cells. The partition coefficients inline-formula M2inlinescrollmathml D chem normal Br normal f / normal m 24pt20ptsvg-formulamathimg1a6dde59dbd94d322c69d3092af548b0 se-11-1145-2020-ie00001.svg24pt20ptse-11-1145-2020-ie00001.png range from inline-formula∼2 to inline-formula∼15, with an average value of inline-formula6.7±3.6 (inline-formula1σ) over the whole pressure–temperature (inline-formulaPinline-formulaT) range, indicating a moderate Br enrichment in aqueous fluids, in agreement with previous work. Extended x-ray-absorption fine-structure (EXAFS) analysis further evidences a gradual evolution of Br speciation from hydrated Br ions [Br(inline-formulaH2O)inline-formula6]inline-formula in slab dehydration fluids to more complex structures involving both Na ions and water molecules, [inline-formulaBrNax(H2O)y], in hydrous silicate melts and supercritical fluids released at greater depth (> 200 km). In denser fluids (inline-formulaρ > 1.5 g cminline-formula−3) containing 60 wt % dissolved alkali–silicates and in hydrous inline-formulaNa2Si2O5 melts (10 wt % inline-formulaH2O), Br is found to be in a “salt-like” structure involving the six nearest Na ions and several next-nearest O neighbors that are either from water molecules and/or the silicate network. Bromine (and likely chlorine and iodine) complexing with alkalis is thus an efficient mechanism for the mobilization and transport of halogens by hydrous silicate melts and silica-rich supercritical fluids. Our results suggest that both shallow dehydration fluids and deeper silicate-bearing fluids efficiently remove halogens from the slab in the sub-arc region, thus favoring an efficient transfer of halogens across subduction zones.

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Louvel, Marion / Sanchez-Valle, Carmen / Malfait, Wim J. / et al: Bromine speciation and partitioning in slab-derived aqueous fluids and silicate melts and implications for halogen transfer in subduction zones. 2020. Copernicus Publications.

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