Thermodynamic, Petrologic, Geochemical and Isotopic Constraints on Metal (Re, Pt, Pd, Ir, Ru, Os and Other Metals) Mobility During Hydrothermal Serpentinization of Ultramafics

摘要超镁铁质热液循环系统中金属（Re, Pt, Pd, Ir, Ru， Os等金属）在热液蚀变过程中的流动性的热力学、岩石学、地球化学和同位素限制。了解超镁铁质岩石热液蚀变过程中的金属流动性对于定量确定微量金属进出海洋的热液通量等地球科学的突出问题具有重要意义。量化金属在俯冲和地幔交代过程中的溶解度和运移过程，最终了解贱金属矿床的成因。这里测试的假设是，蛇纹石化热液可以导致超镁铁质岩性中金属的显著浸出。提出了一项为期三年的研究，将理论和观测资料相结合，研究超镁铁质岩性蛇纹石化过程中微量金属的地球化学行为（400℃）。特别强调的是获得铂族元素（PGEs）和其他亲铜元素的高质量数据。利用加利福尼亚北部一个连续辉长岩-蛇纹岩过渡露头，研究了超镁铁质岩石热液蚀变过程中微量金属的迁移和输运路径。详细的野外测绘、岩石学、同位素（Os、Sr和O）和地球化学数据相结合，以估计蛇纹石化过程中的金属动员程度、变质温度和水岩比。相对金属迁移率是通过直接比较蛇纹岩化哈尔茨伯基岩和新鲜哈尔茨伯基岩之间的金属含量来确定的，金属溶解度的数量级绝对估计是通过水岩比的估计来计算的，受岩石体的Os、Sr或O同位素系统的限制。将非均相水平衡热力学模型与基于场的平衡固相约束相结合，推断出蛇辉岩化热液中的fO2、pH、主、小元素组成和物种形成。理论溶解度和形态的微量金属，特别是Re和PGEs，也计算使用适当的结合常数的各种配合物，如果知道。这种观察和理论的结合使我们更好地了解热液流体的哪些特性（如盐度、fO2、pH和温度）促进了金属的运输。

AbstractThermodynamic, petrologic, geochemical, and isotopic constraints on metal (Re, Pt, Pd, Ir, Ru, Os, and other metals) mobility during serpentinization in ultramafic-hosted hydrothermal circulation systemsUnderstanding metal mobility during hydrothermal alteration of ultramafic rocks bears importance to a number of outstanding problems in the earth sciences, such as quantifying hydrothermal fluxes of trace-metals into or out of the ocean, quantifying the solubilities and transport processes of metals during subduction and mantle metasomatism, and finally, understanding the genesis of base metal ore deposits. The hypothesis that is tested here is that serpentinizing hydrothermal fluids can result in significant leaching of metals from ultramafic lithologies. A three-year study, in which theoretical and observational data are combined to investigate the geochemical behavior of trace metals during the serpentinization of ultramafic lithologies (at temperatures 400 o), is proposed. Particular emphasis is placed on obtaining high quality data for the platinum group elements (PGEs) and other chalcophile elements. The mobility and transport pathways of trace metals during hydrothermal alteration of ultramafic rocks are investigated using a continuous harzburgite to serpentinite transition outcrop in northern California. Detailed field mapping, petrography, and isotopic (Os, Sr, and O) and geochemical data are combined to estimate the degree of metal mobilization, metamorphic temperatures, and water-rock ratios during serpentinization. Relative metal mobilities are determined by direct comparison of the metal contents between serpentinized harzburgites and fresh harzburgites and order-of-magnitude absolute estimates of metal solubilities are calculated using estimates of the water-rock ratio, as constrained by bulk-rock Os, Sr, or O isotopic systematics. The fO2, pH, and major- and minor-element composition and speciation in the serpentinizing hydrothermal fluids are inferred by combining thermodynamic modeling of heterogeneous aqueous equilibria with field-based constraints on equilibrium solid phases. The theoretical solubilities and speciations of trace metals, particularly of Re and the PGEs, are also calculated using appropriate association constants for various complexes, if known. This integration of observation and theory provides a better understanding of what properties of hydrothermal fluids (e.g., salinity, fO2, pH, and temperature) facilitate metal transport.