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

Abstract

<jats:p>Abstract. 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 ∘C and 2.2 GPa by synchrotron x-ray fluorescence (SXRF) and x-ray absorption spectroscopy (XAS) in diamond anvil cells. The partition coefficients DBrf/m range from ∼2 to ∼15, with an average value of 6.7±3.6 (1σ) over the whole pressure–temperature (P–T) 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(H2O)6]− in slab dehydration fluids to more complex structures involving both Na ions and water molecules, [BrNax(H2O)y], in hydrous silicate melts and supercritical fluids released at greater depth (&gt; 200 km). In denser fluids (ρ &gt; 1.5 g cm−3) containing 60 wt % dissolved alkali–silicates and in hydrous Na2Si2O5 melts (10 wt % H2O), 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. </jats:p>

Interference-free electron probe micro-analysis of bromine in halogen-bearing minerals and glasses: high-resolution measurements and quantitative elemental mapping

“…In addition to the fact that there is no matrix-matched standardization required, the advantages of EPMA Br quantication over other micro-analytical approaches is its high spatial resolution (#10 mm), its non-destructiveness, and that EPMA instruments are readily available and relatively cost-effective compared to other methods. The approach described in this study can be a useful tool for experimental work such as studies conducted to determine Br solubility in magmas and/or partitioning between aqueous uids and melts; 21,26,37 experiments that help to better constrain the global Br cycle and especially Br degassing in volcanic systems, which directly impacts the composition of the atmosphere. Furthermore, high resolution spot analysis and quantitative Cl/Br mapping can be used to identify and trace uids in the crust, specically in mineralized systems where, for example, scapolite-group minerals are commonly present.…”

Atomic spectrometry update – a review of advances in X-ray fluorescence spectrometry and its special applications

“…A very interesting in situ SR-XRF/XAS study was conducted by Louvel et al 40 with the aim of understanding better the behaviour of halogens (Br, Cl, I) and associated trace-elements in subduction zones. By identifying the boundaries of the geochemical cycle of these volatiles, it will be possible to quantify the halogen fluxes to the atmosphere via volcanic degassing.…”