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 $D_{\mathrm{Br}}^{\mathrm{f}/\mathrm{m}}$ 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, [BrNa_x(H₂O)_y], in hydrous silicate melts and supercritical fluids released at greater depth (> 200 km). In denser fluids (ρ > 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.

中文翻译：

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平板状水性流体和硅酸盐熔体中的溴形态和分配以及对俯冲带中卤素转移的影响

理解俯冲带中卤素（Cl，Br和I）的行为对于限制这些挥发物和相关的痕量金属的地球化学循环，以及量化通过火山脱气向大气中的卤素通量至关重要。在这里，在840年前就地研究了共存的水性流体和含水花岗岩熔体之间溴的分配及其在板坯衍生流体中的形态。^{一种？？？？}C和2.2？GPa通过同步加速器X射线荧光（SXRF）和X射线吸收光谱（XAS）在钻石砧细胞中。分配系数$d_{\mathrm{溴}}^{\mathrm{F}/\mathrm{米}}$范围从A？？¼2到 â??¼15，具有的平均值6.7A±3.6（1 Ï??）在整个pressureâ????温度（P â???? Ť）范围，表示中等与先前的工作一致，在水溶液中富集Br。扩展X射线吸收精细结构（EXAFS）分析进一步证据从水合溴离子[BR（BR形态的逐步演变ħ ₂ ö）₆ ] ^â???? 在平板脱水液中形成包含Na离子和水分子的更复杂结构，[ BrNa _x（H ₂ O）_y]，在含水硅酸盐熔体和超临界流体中释放的深度更大（> 200千米）。更致密的流体（Ï?? â????>â???? 1.5A ????Gâ????厘米^{â???? 3}）含有60A ???? WTA ????％的溶解碱金属硅酸盐和含水的Na ₂ Si ₂ O ₅熔体（10wt％ H ₂ O），发现Br处于“盐状”状态。该结构涉及六个最接近的Na离子和几个近邻的O邻居，这些邻居来自水分子和/或硅酸盐网络。因此，与碱配合的溴（以及可能的氯和碘）是通过含水硅酸盐熔体和富含二氧化硅的超临界流体来移动和运输卤素的有效机制。我们的研究结果表明，浅层脱水液和深层含硅酸盐的液都可以有效地从亚弧区域的板坯中除去卤素，从而有利于卤素在整个俯冲区的有效转移。