Geobarometers are commonly used to determine the pressure (and hence depth) of magmatic bodies. For instance, at equilibrium, the concentration of dissolved volatiles in a vapor-saturated melt can be used as a barometer: this is the pressure of vapor-saturation (⁠Psatv⁠). Most determinations of Psatv assume that melt and vapor contain only oxidized C-O-H species. However, sulfur is the third most abundant volatile element in magmas, and oxygen fugacity (fO2) exerts a strong influence on the speciation of the melt and vapor. To explore how S and fO2 affect calculations of Psatv⁠, we model a Hawaiian tholeiite that contains both reduced and oxidized C-O-H-S species in the melt and vapor. We find that excluding reduced C-O-H species in the system can result in significant underestimations of Psatv under reducing conditions (ΔFMQ < 0). The effect of S on Psatv is small except in the vicinity of the “sulfur solubility minimum” (SSmin; 0 < ΔFMQ < +2), where excluding S-bearing species can result in underestimates of Psatv⁠.The implications of these results depend on the volatile concentration of the system being investigated, its fO2, and the melt composition and temperature. Our results suggest there will be little impact on Psatv calculated for mid-ocean ridge basalts because their fO2 is above where reduced C-O-H species become important in the melt and vapor and yet below the SSmin. However, the fO2 of ocean island and arc basalts are close enough to the SSmin and their S concentrations high enough to influence Psatv⁠. However, high-CO2 and high-H2O concentrations are predicted to reduce the effect of the SSmin. Hence, Psatv calculated for shallowly trapped melt inclusions and matrix glass are more affected by the SSmin than deeply trapped melt inclusions. Lunar and martian magmas are typically more reduced than terrestrial magmas, and therefore accurate Psatv calculations for them require the inclusion of reduced C-O-H species.

中文翻译：

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氧逸度和硫对岩浆水汽饱和压力的影响

地质气压计通常用于确定岩浆体的压力（以及深度）。例如，在平衡状态下，蒸气饱和熔体中溶解的挥发物的浓度可以用作气压计：这是蒸气饱和压力（⁠Psatv⁠）。大多数 Psatv 测定都假设熔体和蒸气仅含有氧化的 COH 物质。然而，硫是岩浆中第三丰富的挥发性元素，氧逸度（fO2）对熔体和蒸气的形态有很大影响。为了探索 S 和 fO2 如何影响 Psatv⁠ 的计算，我们对夏威夷拉斑玄武岩进行了建模，该拉斑玄武岩在熔体和蒸气中同时含有还原和氧化的 COHS 物质。我们发现，排除系统中还原的 COH 物种可能会导致还原条件下 (ΔFMQ < 0) 下 Psatv 的显着低估。除了在“硫溶解度最小值”（SSmin；0 < ΔFMQ < +2）附近之外，S 对 Psatv 的影响很小，其中排除含硫物质可能会导致低估 Psatv⁠。这些结果的影响取决于受研究系统的挥发物浓度、fO2 以及熔体成分和温度的影响。我们的结果表明，对洋中脊玄武岩计算的 Psatv 影响不大，因为它们的 fO2 高于减少的 COH 物种在熔体和蒸气中变得重要的位置，但低于 SSmin。然而，海岛和弧玄武岩的 fO2 足够接近 SSmin，并且它们的 S 浓度足够高，足以影响 Psatv⁠。然而，高 CO2 和高 H2O 浓度预计会降低 SSmin 的影响。因此，针对浅层捕获的熔体夹杂物和基质玻璃计算的 Psatv 比深层捕获的熔体夹杂物更受 SSmin 的影响。月球和火星岩浆通常比陆地岩浆还原程度更高，因此对它们进行准确的 Psatv 计算需要包含还原的 COH 物质。