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首页> 外文期刊>Contributions to Mineralogy and Petrology >Metasomatism and graphite formation at a lithological interface in Malaspina (Alpine Corsica, France)
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Metasomatism and graphite formation at a lithological interface in Malaspina (Alpine Corsica, France)

机译：马拉皮西纳（法国科西嘉岛）岩性界面上的交代作用和石墨形成

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摘要

Multiple pieces of geologic evidence suggest that interfaces between contrasted lithologies exert a strong control on the fate of volatiles in subduction zones. Here we present results from a contact between serpentinites and sediments, located in Corsica and metamorphosed in the blueschist facies during the alpine orogeny. It was shown previously that carbonates in the sediments have been reduced to graphitic carbonaceous material within a 5–10-cm-thick reaction zone at the contact with serpentinites. In an effort to investigate the mechanisms governing this unusual process, bulk rock geochemical analyses incorporating a statistical analysis of compositional data are presented. Observations show that the fate of C was decoupled from that of other elements such as O, H, and large-ion lithophile elements—e.g. K, Sr, Ba...,As—that were extensively leached from the reaction zone. Notably, Na is strongly enriched in the reaction zone and structurally linked to pectolite. Reducing conditions, manifested by the depletion of O in the reaction zone compared to the bulk metasediment, were likely maintained by the presence of Fe(II) in the serpentinite. Moreover, thermodynamic calculations show that the low solubility of carbon in COH fluids at high-pressure and low-temperature conditions was the main driver for graphite precipitation synchronously with carbonate destabilization. This may have been kinetically favored by the presence of already existing graphitized carbonaceous material and phengite in the metasediment. Limited lateral flow might have contributed as well to the geochemical and petrological patterns observed in these rocks.
机译：多个地质证据表明，对比岩性之间的界面对俯冲带中的挥发物的命运具有强烈的控制作用。在这里，我们介绍了在高山造山过程中位于科西嘉岛并在蓝片岩相中变质的蛇纹岩和沉积物之间的接触结果。先前已表明，在与蛇纹岩接触的5-10厘米厚的反应区内，沉积物中的碳酸盐已被还原为石墨碳质物质。为了研究控制这一异常过程的机制，提出了结合成分数据的统计分析的块状岩石地球化学分析。观察结果表明，C的命运与其他元素（例如O，H和大离子亲石性元素）的命运脱钩。 K，Sr，Ba ...，As –从反应区大量浸出。值得注意的是，Na在反应区中高度富集，并在结构上与方沸石连接。蛇纹石中Fe（II）的存在可以维持还原条件，该条件的减少是由于反应区中O的减少（与体金属沉淀相比）。此外，热力学计算表明，在高压和低温条件下，碳在COH流体中的低溶解度是与碳酸盐失稳同时发生的石墨沉淀的主要驱动力。在变质沉积物中，已经存在的石墨化碳质物质和亚辉石的存在可能在动力学上是有利的。有限的侧向流动也可能对这些岩石中观察到的地球化学和岩石学特征有所贡献。

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