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The redox filter beneath magmatic orogens and the formation of continental crust

机译：岩浆造山带下的氧化还原过滤器与大陆壳形成

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The two most important magmatic differentiation series on Earth are the Fe-enriching tholeiitic series, which dominates the oceanic crust and island arcs, and the Fe-depleting calc-alkaline series, which dominates the continental crust and continental arcs. It is well known that calc-alkaline magmas are more oxidized when they erupt and are preferentially found in regions of thick crust, but why these quantities should be related remains unexplained. We use the redox-sensitive behavior of europium (Eu) in deep-seated, plagioclase-free arc cumulates to directly constrain the redox evolution of arc magmas at depth. Primitive arc cumulates have negative Eu anomalies, which, in the absence of plagioclase, can only be explained by Eu being partly reduced. We show that primitive arc magmas begin with low oxygen fugacities, similar to that of mid-ocean ridge basalts, but increase in oxygen fugacity by over two orders of magnitude during magmatic differentiation. This intracrustal oxidation is attended by Fe depletion coupled with fractionation of Fe-rich garnet. We conclude that garnet fractionation, owing to its preference for ferrous over ferric iron, results in simultaneous oxidation and Fe depletion of the magma. Favored at high pressure and water content, garnet fractionation explains the correlation between crustal thickness, oxygen fugacity, and the calc-alkaline character of arc magmas.
机译：地球上最重要的两个岩浆分异系列是，富集铁质的高品位系列（以洋壳和岛弧为主）和贫铁钙碱性系列（以大陆壳和大陆弧为主）。众所周知，钙碱性岩浆喷发时会被更多地氧化，并且优先在厚壳区域发现，但是为什么这些量应该相关仍是无法解释的。我们使用deep（Eu）对深层，无斜长石的电弧累积物中的氧化还原敏感行为来直接约束深度深处的岩浆的氧化还原演化。原始弧形累积量具有负Eu异常，在没有斜长石的情况下，只能通过部分还原Eu来解释。我们显示原始弧岩浆以低氧逸度开始，类似于中海脊玄武岩，但在岩浆分化过程中氧逸度增加了两个数量级。这种壳内氧化伴随着铁耗竭以及富铁石榴石的分馏。我们得出的结论是，由于石榴石的分馏，因为它偏爱亚铁而不是三价铁，因此会导致岩浆同时氧化和铁耗竭。石榴石分馏在高压和高含水量的帮助下，解释了地壳厚度，氧逸度和弧岩浆的钙碱性特征之间的关系。

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