Origin and Migration of Sulfur in Active Geothermal Systems: A Sulfur Isotopic Study Using the SHRIMP Ion Micro-probe

稳定同位素分析的微豆技术提供了对岩石成因过程的极大洞察，因为矿物包含了比传统同位素采样和分析技术容易获得的更多关于其来源和随后的修改的信息。S用SHRIMP离子探针进行的同位素研究表明，在索尔顿海地热系统中矿物内部和矿物之间存在着巨大的34S变化，识别出硫化物。研究了沉积硫化物矿物和硫酸盐矿物在进退变质过程中的同位素平衡，以及贫硫热液对沉积S的再循环程度。通过对S组分~(34)S的测定，评价了含盐热液中H_2S与SO_4之间的同位素平衡程度，以及H_2S对岩浆作用和主岩蒸发岩的热液还原作用所产生的热液的相对贡献。建议利用Valles火山口和COSO地热系统的新钻芯和流体样品，将S同位素系统的研究扩展到活动的硅质火山大陆地热系统。这项拟议的研究补充了我们正在运输贵金属和贱金属的研究。我们将继续使用SHRIMP离子微探测器无与伦比的30微米空间分辨率来研究矿物内部和矿物之间的微观~(34)S变化。将使用常规技术测定同时代地热流体中H_2S和SO_4的~(34)S，并表征矿物的整体S同位素组成。研究脉状矿物中的显微~(34)S分带模式将有助于约束成矿机制。阐明主岩中硫源的S同位素地球化学分布，以及热液温度下矿物与水相之间的同位素平衡程度，将有助于更准确地模拟火山热液系统中的金属迁移和沉积。

Microbean techniques of stable isotope analysis offer great insight into petrogenetic processes because minerals contain more information concerning their origins and subsequent modifications than can easily be obtained by conventional isotopic sampling and analytical techniques. S isotope studies with SHRIMP ion probe have revealed dramatic 34S variations within and among minerals in the Salton Sea geothermal system, identifying sulfides. The isotopic equilibration of sedimentary sulfide and sulfate minerals during prograde metamorphism, and the extent of recycling of sedimentary S by sulfur poor hydrothermal fluids will be examined. From determinations of 34S for aqueous S species the degree of isotopic equilibrium attained between H2S and SO4 in saline hydrothermal fluids and the relative contributions of H2S to hydrothermal fluids from igneous processes and hydrothermal reduction of host rocks evaporites will be evaluated. It is proposed to extend our studies of S isotope systematics to active silicic volcanic-hosted continental geothermal systems, making use of new drill cores and fluid samples from the Valles Caldera and Coso geothermal systems. The proposed study complements our research that are transporting precious as well as base metals. We will continue to use the unsurpassed 30 um spatial resolution of the SHRIMP ion microprobe for studying microscopic 34S variations within and among minerals. Conventional techniques to determine 34S of H2S and SO4 in coeval geothermal fluids and to characterize the bulk S isotope composition of minerals will be used. Study of microscopic 34S zoning patterns in vein minerals will help constrain mechanisms of ore precipitation. Elucidation of the distribution of S isotope geochemistry of sulfur sources in host- rocks and the degrees of isotopic equilibrium reached between minerals and aqueous species at hydrothermal temperatures will allow more accurate modeling of metal transport and deposition in volcanic-hosted hydrothermal systems.