The stability field and composition of immiscible sulfate melts in equilibrium with silicate magmas has been determined using experiments over a range of crustal pressures, allowing an assessment of their possible role in transporting sulfur into the sub-volcanic arc and porphyry copper deposit systems. Experimental starting materials were based on natural trachyandesite and trachydacite compositions, with 3.5–7 wt% H2O and 3.5–5.5 wt% sulfur added to produce large, analyzable amounts of sulfate phases. Conditions ranged over 800–1200 °C, 0.2-1GPa and ƒO2 > NNO + 2.5. Sulfate melts formed at temperatures above 1000 °C at 0.75 and 1 GPa and above 900 °C at 0.2 GPa, suggesting some pressure dependence on their stability. At temperatures below 1100 °C sulfate melts and anhydrite crystals commonly coexist. Sulfate melts quenched to an intergrowth that was difficult to prepare for analysis. However, the composition was approximated by EPMA and further constrained by mass balance calculations. Sulfate melts were dominated by CaO and SO3, but also contained, in order of decreasing abundance, Na2O, K2O, MgO, FeO, Cl and P2O5. Chlorine showed a particular preference for the sulfate melt relative to the coexisting silicate melt, and calculated partition coefficients for sulfate/silicate melts were 5–13 at 1200 °C, 0.75–1 GPa. Experimental data show that, in the absence of an exsolved, hydrous fluid phase, sulfate melts can form in natural arc magmas at near-liquidus temperatures ≥ 1000 °C, assuming that magmas are oxidized and contain sufficient sulfur (> 2000–3000 ppm S). These results suggest that sulfate melt could be an important component in transporting sulfur as well as chlorine to shallow levels in the crust for hydrous magmas under a specific range of conditions. Both the non-quenchable and water-soluble nature of sulfate melts (and anhydrite) make them difficult to identify, unless trapped as mineral inclusions similar to the “wormy anhydrite” trapped in high-temperature amphiboles from Yanacocha, Peru.

中文翻译：

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弧岩浆中硫酸盐熔体的稳定性和组成

与硅酸盐岩浆平衡的不混溶硫酸盐熔体的稳定性场和组成已通过在一系列地壳压力范围内的实验确定，允许评估它们在将硫输送到亚火山弧和斑岩铜矿床系统中的可能作用。实验起始材料基于天然粗面英岩和粗面英石组成，添加 3.5-7 wt% 的 H2O 和 3.5-5.5 wt% 的硫以产生大量可分析的硫酸盐相。条件范围超过 800–1200 °C、0.2-1GPa 和 ƒO2 > NNO + 2.5。硫酸盐熔体在 0.75 和 1 GPa 时在 1000 °C 以上和 0.2 GPa 时在 900 °C 以上形成，这表明它们的稳定性存在一定的压力依赖性。在低于 1100 °C 的温度下，硫酸盐熔体和硬石膏晶体通常共存。硫酸盐熔体骤冷至难以准备分析的共生体。然而，该组成由 EPMA 近似，并进一步受到质量平衡计算的限制。硫酸盐熔体以 CaO 和 SO3 为主，但也包含 Na2O、K2O、MgO、FeO、Cl 和 P2O5，按丰度递减的顺序。相对于共存的硅酸盐熔体，氯对硫酸盐熔体表现出特别的偏好，并且计算出的硫酸盐/硅酸盐熔体的分配系数在 1200 °C 时为 5-13，0.75-1 GPa。实验数据表明，在没有溶出的含水流体相的情况下，假设岩浆被氧化并含有足够的硫（> 2000-3000 ppm S ）。这些结果表明，在特定条件下，硫酸盐熔体可能是将硫和氯输送到地壳中含水岩浆浅层的重要组成部分。硫酸盐熔体（和硬石膏）的不可淬火和水溶性性质使它们难以识别，除非被困在类似于秘鲁亚纳科查高温角闪石中“蠕虫状硬石膏”的矿物包裹体中。