| 蚀变洋壳和俯冲带变质流体的Fe-Mg同位素组成 |
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| 引用本文： | 黄建,黄方,肖益林.蚀变洋壳和俯冲带变质流体的Fe-Mg同位素组成[J].地球科学,2019,44(12):4050-4056. |
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| 作者姓名： | 黄建 黄方 肖益林 |
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| 作者单位： | 中国科学院壳幔物质与环境重点实验室, 中国科学技术大学地球和空间科学学院, 安徽合肥 230026 |
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| 基金项目： | 国家重点基础研究发展计划项目2015CB856102国家自然科学基金项目41573018 |
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| 摘 要： | 贫碳酸盐的蚀变洋壳具有与新鲜洋中脊玄武岩一致的Mg同位素组成，说明低温和高温洋壳蚀变不会导致Mg同位素分馏.大别山港河和花凉亭的早期变质脉比榴辉岩具有偏高的δ56Fe-δ26Mg值，而且早期到晚期变质脉的δ56Fe-δ26Mg值逐渐降低.这些结果说明，在流体-岩石反应和流体演化过程中，Fe-Mg同位素发生了显著的分馏，且矿物溶解-再沉淀是同位素分馏的控制因素.相比洋中脊玄武岩，蚀变洋壳和变质脉具有相似或偏高的δ56Fe-δ26Mg值，说明蚀变洋壳脱水产生的流体富集重Fe-Mg同位素，不能解释弧岩浆岩的轻Fe/重Mg同位素组成.因此，弧岩浆岩异常的Fe-Mg同位素组成是熔体提取和富集54Fe-26Mg的蛇纹岩流体交代地幔楔两个过程共同作用的结果.
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| 关 键 词： | 铁-镁同位素 蚀变洋壳 榴辉岩 变质流体 流体演化 弧岩浆岩 地球化学 |
| 收稿时间： | 2019-08-16 |
Fe-Mg Isotopic Compositions of Altered Oceanic Crust and Subduction-Zone Fluids |
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| Abstract: | The origin of the light Fe and heavy Mg isotope enrichments in arc lavas remains unclear because of the lack of constraints on the Fe-Mg isotope compositions of altered oceanic crust (AOC) and metamorphic fluids in subduction zones. Carbonate-barren AOC has Mg isotope compositions similar to those of fresh mid-ocean ridge basalts, suggesting that low-to-high temperature alteration of oceanic crust by seawater and hydrothermal fluids results in limited Mg isotope fractionation. Fe-Mg isotope measurements show that the early omphacite-epidote veins have higher δ56Fe and δ26Mg compared to the host eclogites and that the δ56Fe and δ26Mg gradually decrease from the early omphacite-epidote through epidote-quartz to the late kyanite-epidote-quartz veins. These results indicate significant Fe-Mg isotope fractionation during fluid-rock interaction and fluid evolution due to the dissolution-precipitation processes of minerals in subduction zones. Compared to mid-ocean ridge basalts, the similar or higher δ56Fe and δ26Mg of AOC and metamorphic veins suggest that AOC-derived fluids are probably enriched in heavy Fe-Mg isotopes. Thus, contribution from AOC-derived fluids is unlikely to explain the light Fe and heavy Mg isotope compositions of arc lavas. We propose that the Fe-Mg isotope anomaly of arc lavas may result from a combination of prior melt depletion and addition of serpentinite-derived 54Fe-26Mg-rich fluids into the overlying mantle wedge. |
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