Bayerisches Geoinstitut, University of Bayreuth, Bayreuth D-95440, Germany.
Nature. 2013 Jan 3;493(7430):84-8. doi: 10.1038/nature11679.
Determining the oxygen fugacity of Earth's silicate mantle is of prime importance because it affects the speciation and mobility of volatile elements in the interior and has controlled the character of degassing species from the Earth since the planet's formation. Oxygen fugacities recorded by garnet-bearing peridotite xenoliths from Archaean lithosphere are of particular interest, because they provide constraints on the nature of volatile-bearing metasomatic fluids and melts active in the oldest mantle samples, including those in which diamonds are found. Here we report the results of experiments to test garnet oxythermobarometry equilibria under high-pressure conditions relevant to the deepest mantle xenoliths. We present a formulation for the most successful equilibrium and use it to determine an accurate picture of the oxygen fugacity through cratonic lithosphere. The oxygen fugacity of the deepest rocks is found to be at least one order of magnitude more oxidized than previously estimated. At depths where diamonds can form, the oxygen fugacity is not compatible with the stability of either carbonate- or methane-rich liquid but is instead compatible with a metasomatic liquid poor in carbonate and dominated by either water or silicate melt. The equilibrium also indicates that the relative oxygen fugacity of garnet-bearing rocks will increase with decreasing depth during adiabatic decompression. This implies that carbon in the asthenospheric mantle will be hosted as graphite or diamond but will be oxidized to produce carbonate melt through the reduction of Fe(3+) in silicate minerals during upwelling. The depth of carbonate melt formation will depend on the ratio of Fe(3+) to total iron in the bulk rock. This 'redox melting' relationship has important implications for the onset of geophysically detectable incipient melting and for the extraction of carbon dioxide from the mantle through decompressive melting.
确定地球硅酸盐地幔的氧逸度非常重要，因为它影响了内部挥发性元素的形态和迁移，并且自地球形成以来控制了从地球逸出的脱气物质的特征。太古岩石圈中石榴石橄榄岩包体记录的氧逸度特别有趣，因为它们限制了含挥发分的交代流体和熔体的性质，这些交代流体和熔体在最古老的地幔样品中是活跃的，包括那些发现钻石的样品。在这里，我们报告了在与最深地幔包体相关的高压条件下测试石榴石氧压计平衡的实验结果。我们提出了一个最成功的平衡的公式，并利用它来确定克拉通岩石圈中氧逸度的准确图像。最深岩石的氧逸度至少比以前估计的要氧化一个数量级。在可以形成钻石的深度，氧逸度与富含碳酸盐或甲烷的液体的稳定性不兼容，而是与贫碳酸盐、主要由水或硅酸盐熔体组成的交代液体兼容。平衡还表明，在绝热减压过程中，含石榴石岩石的相对氧逸度将随着深度的降低而增加。这意味着软流圈地幔中的碳将以石墨或钻石的形式存在，但通过硅酸盐矿物中 Fe(3+)的还原，将被氧化生成碳酸盐熔体，从而产生碳酸盐熔体。碳酸盐熔体形成的深度将取决于岩石中 Fe(3+)与总铁的比值。这种“氧化还原熔融”关系对地幔中物理可检测初始熔融的开始和通过减压熔融从地幔中提取二氧化碳具有重要意义。
