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

超镁铁质热液循环系统中蛇纹岩作用过程中金属(Re、Pt、Pd、Ir、Ru、Os等)活动性的热力学、岩石学、地球化学和同位素约束。了解超镁铁质岩石热液蚀变过程中金属的活动性对地球科学中的一些突出问题具有重要意义，如定量痕量金属进出海洋的热液通量，定量确定俯冲和地幔交代过程中金属的溶解和迁移过程，最后了解贱金属矿床的成因。这里测试的假设是，蛇纹岩热液可以导致超镁铁质岩性中金属的大量浸出。提出了一项为期三年的研究，将理论数据和观测数据相结合，以研究超镁铁质岩性蛇纹岩(温度为400℃)蛇纹岩过程中痕量金属的地球化学行为。特别强调获得铂族元素和其他亲硫元素的高质量数据。利用加州北部方辉橄榄岩到蛇纹岩的连续露头，研究了超镁铁质岩石热液蚀变过程中痕量金属的迁移和迁移途径。详细的野外填图、岩石学、同位素(Os、Sr和O)和地球化学数据结合在一起，以估计蛇纹石化过程中金属的活化度、变质温度和水岩比。金属的相对迁移率是通过直接比较蛇纹岩化的方辉橄榄岩和新鲜的方辉橄榄岩之间的金属含量来确定的，而金属溶解度的数量级绝对估计值是使用水-岩比的估计值来计算的，受块状岩石的Os、Sr或O同位素体系的约束。通过结合非均相水相平衡的热力学模拟和基于场的平衡固相约束，推断了蛇纹岩热液流体中的fO2、pH和主、次元素的组成和形态。还利用各种络合物的适当缔合常数(如果已知)计算了痕量金属，特别是Re和Pge的理论溶解度和形态。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性质(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.