The release of fluids from magmas at crustal depths is an essential process for the formation of magmatic-hydrothermal ore deposits. However, assessing the extent of volatile loss by magma degassing or volatile gain by fluid fluxing from deeper magmas remains challenging. To develop a new tool to quantitatively track these processes, we experimentally determined the partition coefficients of Cl, Br, and I between aqueous fluids and haplogranitic melts (Dif/m) as a function of fluid salinity and the aluminum saturation index [ASI = Al2O3/(Na2O + K2O)] of the silicate melt. The experiments were conducted in externally heated rapid-quench René 41 cold-seal pressure vessel apparatus at 200 MPa and 790 ± 10 °C. The Br and I concentrations in the run product glasses were determined by laser ablation inductively coupled plasma mass spectrometry and secondary ion mass spectrometry, whereas the concentration of Cl was determined by electron probe microanalysis. The results show that the partition coefficients of the three halogens in a system with chloride-dominated fluids increase with fluid salinity increasing from 1.49 to 60.6 wt% total NaCl equivalent. Specifically, DClf/m increases from 23 ± 5 to 168 ± 7 (1σ), DBrf/m increases from 57 ± 13 to 271 ± 14, and DIf/m increases from 198 ± 61 to 736 ± 159. As for the influence of melt composition, DClf/m and DBrf/m attain maximum values at ASI = 1, whereas DIf/m appears to be independent of ASI within error. Bromine and iodine partition into the fluid more strongly in the presence of significant Cl, indicating that these halogens compete for the same structural sites in the silicate melt. Empirical equations were derived to predict the Df/m of Cl, Br, and I in felsic magmatic systems as a function of fluid salinity and silicate melt ASI. These equations were in turn implemented in numerical models simulating the degassing of granitic magma reservoirs emplaced in the Earth’s upper crust. The results of these calculations show that the Br/Cl and I/Cl ratios in the silicate melt and the released fluid rapidly decrease during progressive magma degassing at depth due to the significantly increasing fluid/melt partition coefficients with increasing halide ion radius. On the other hand, a sudden increase of Br/Cl and I/Cl in the silicate melt indicates fluid fluxing of the magma. Therefore, halogen ratios may become particularly useful tools to track crystallization-driven degassing if the record of the variation of the silicate melt composition can be recovered from silicate melt inclusions in minerals. In addition, the evolving Br/Cl and I/Cl ratios of magmatic fluids feeding magmatic-hydrothermal ore-forming systems, as recorded by fluid inclusions in minerals, may inform about the evolution of the underlying magma reservoir.

中文翻译：

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长英质岩浆中氯、溴和碘的流体/熔体分配，以及卤素比在跟踪岩浆储层脱挥和流体熔融方面的应用


从地壳深处的岩浆中释放流体是形成岩浆-热液矿床的重要过程。然而，评估岩浆脱气引起的挥发性损失或来自更深的岩浆的流体通量引起的挥发性增加的程度仍然具有挑战性。为了开发一种定量跟踪这些过程的新工具，我们实验确定了水流体和单倍体熔体之间 Cl、Br 和 I 的分配系数 （Dif/m） 与流体盐度和硅酸盐熔体的铝饱和指数 [ASI = Al2O3/（Na2O + K2O）] 的函数。实验是在外部加热的快速淬火 René 41 冷封压力容器装置中进行的，压力为 200 MPa，温度为 790 ± 10 °C。 通过激光烧蚀电感耦合等离子体质谱和二次离子质谱法测定运行产物玻璃中的 Br 和 I 浓度，而 Cl 的浓度通过电子探针微量分析测定。结果表明，在以氯离子为主的流体体系中，三种卤素的分配系数随着流体盐度从 1.49 wt% 增加到 60.6 wt% 总 NaCl 当量而增加。具体来说，DClf/m 从 23 ± 5 增加到 168 ± 7 （1σ），DBrf/m 从 57 ± 13 增加到 271 ± 14，DIf/m 从 198 ± 61 增加到 736 ± 159。至于熔体成分的影响，DClf/m 和 DBrf/m 在 ASI = 1 时达到最大值，而 DIf/m 似乎与误差内的 ASI 无关。在存在大量 Cl 的情况下，溴和碘更强烈地分配到流体中，表明这些卤素在硅酸盐熔体中竞争相同的结构位点。 推导出经验方程来预测长英质岩浆系统中 Cl、Br 和 I 的 Df/m 与流体盐度和硅酸盐熔体 ASI 的函数关系。这些方程又在模拟位于地壳上部的花岗岩岩浆储层脱气的数值模型中实现。这些计算的结果表明，由于流体/熔体分配系数随着卤化物离子半径的增加而显著增加，因此在深层岩浆脱气过程中，硅酸盐熔体和释放流体中的 Br/Cl 和 I/Cl 比率迅速降低。另一方面，硅酸盐熔体中 Br/Cl 和 I/Cl 的突然增加表明岩浆的流体熔融。因此，如果可以从矿物中的硅酸盐熔体夹杂物中恢复硅酸盐熔体成分变化的记录，那么卤素比可能会成为跟踪结晶驱动的脱气的特别有用的工具。此外，矿物中的流体包裹体记录的为岩浆-热液成矿系统提供原料的岩浆流体的 Br/Cl 和 I/Cl 比率不断变化，这可能为底层岩浆储层的演变提供信息。