高温高压下橄榄岩蛇纹石化作用以及蛇纹石脱水—对俯冲带氧逸度的制约

The oxygen fugacity in subduction zones has attracted wide attention but remains poorly constrained. The production of molecular hydrogen (H2) during serpentinization and the mobility of iron during serpentinization and serpentine dehydration can be used to constrain the oxygen fugacity in subduction zones. Previous investigations have shown that H2 production and the mobility of iron are influenced by many factors such as temperature, pressure and fluid compositions. However, the P, T, and composition ranges investigated so far are very limited, e.g., previous experiments on serpentinization processes were mostly conducted at temperatures of ≤300 °C and pressures of ≤500 bars. In this project, we will experimentally study serpentinization and serpentine dehydration processes. Our investigations include two parts: (1) we will conduct experiments at 200-500 °C and 1.0-40 kbar on peridotite serpentinization to study the effects of temperature, pressure, and compositions of starting fluids on the production of molecular hydrogen, hydrocarbon and iron oxide, and iron isotope fractionation; (2) We will perform experiments at 600-1100 °C and 1.0-40 kbar on serpentine dehydration to investigate the effects of temperature and pressure on iron contents of olivine formed after dehydration, and iron isotope fractionation. The results of this investigation will be important in understanding the oxygen fugacity in subduction zones, and they may be also important in understanding the mechanisms of serpentinization and serpentine dehydration processes.

近年来地质学研究的热点之一是俯冲带的氧逸度。研究蛇纹石化作用过程中氢气和烷烃形成以及蛇纹石化和蛇纹石脱水过程中铁的活动性可用于制约俯冲带的氧逸度。实验表明，温度、压力和初始流体组成等因素影响氢气和烷烃产量以及铁的活动性。但是，已有研究的温度、压力和初始流体组成变化非常有限：比如，大多蛇纹石化实验温度低于300℃，压力低于0.50kbar。本项目拟通过高温-高压实验研究蛇纹石化和蛇纹石脱水反应，研究内容包括两方面:(1)200-500℃、1.0-40kbar条件下橄榄岩的蛇纹石化反应，研究温度、压力和初始流体组成对氢气和烷烃产量、铁氧化物形成以及铁同位素分馏的影响；(2)600-1100℃、1.0-40kbar条件下蛇纹石脱水反应，探讨温度和压力对脱水形成橄榄石的铁含量以及铁同位素分馏的影响。本项目的执行会制约俯冲带的氧逸度，同时对理解蛇纹石化和蛇纹石脱水反应机制产生重要影响。

蛇纹石化对大洋岩石圈的物理和化学性质、俯冲带氧逸度、元素再循环以及岛弧岩浆的形成起到重要的作用，蛇纹石化速率是俯冲带蛇纹石比例和水含量计算所需的重要参数之一。目前国际上地球物理领域使用的蛇纹石化速率，主要采用Martin and Fyfe (1970)的实验数据。Martin and Fyfe (1970)实验初始物是人工合成的镁橄榄石和纯水。实际地质环境中主要是橄榄岩，纯橄岩非常少，地质流体常包含其他组分(如NaCl、硅、铝和铬)。本项目开展了流体化学组成对蛇纹石化速率和氢气形成影响的研究，发现酸性流体、碱性流体和盐水溶液加快了橄榄岩的蛇纹石化速率。硅的影响较为复杂：产物矿物组成为蛇纹石+滑石，流体中的硅降低了橄榄石的水热蚀变速率；产物的矿物为滑石时，流体中的硅促进了橄榄石的水热蚀变速率。酸性和碱性流体促进氢气的形成，含硅流体和高盐度流体抑制氢气的形成。橄榄岩的蛇纹石化速率随着压力提高而增加，这主要是因为压力以及辉石释放的二氧化硅导致橄榄石蛇纹石化反应的吉布斯自由能降低。蛇纹石化橄榄岩的铁同位素随着温度和压力的升高而增加。本项目研究了尖晶石、铝和铬对蛇纹石化过程中烷烃形成的影响。发现尖晶石、铝和铬促进了蛇纹石化过程中烷烃的形成。同时，本项目研究了不同温度和压力下蛇纹岩脱水过程中铁的地球化学行为，发现磁铁矿参与了蛇纹岩脱水反应，低压下(3.0 kbar)橄榄石的铁含量随着温度增加而降低，高压下(8.0 kbar)橄榄石的铁含量随着温度增加而增加。橄榄石的FeO与MgO含量成反相关，表明蛇纹岩脱水释放了富MgO流体。本项目获得的蛇纹石化速率更加符合地质事实，可用于俯冲带中计算蛇纹石的比例，氢气可用于制约俯冲带氧逸度。本项目表明蛇纹石脱水形成的富镁流体交代地幔楔、岛弧岩浆的形成可能起到了重要的作用。