蚀变洋壳和俯冲带变质流体的Fe-Mg同位素组成  

Fe-Mg Isotopic Compositions of Altered Oceanic Crust and Subduction-Zone Fluids

作　　者：黄建[1] 黄方[1] 肖益林[1] Huang Jian;Huang Fang;Xiao Yilin(CAS Key Laboratory of Crust-Mantle Materials and Environments,School of Earth and Space Sciences,University of Scietice and Technology of China,Hefei 230026,China)

机构地区：[1]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

出　　处：《地球科学》2019年第12期4050-4056,共7页Earth Science

基　　金：国家重点基础研究发展计划项目(No.2015CB856102);国家自然科学基金项目(No.41573018)

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δ^56Fe-δ^26Mg值,而且早期到晚期变质脉的δ^56Fe-δ^26Mg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δ^56Fe-δ^26Mg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.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 δ^56Fe and δ^26Mg compared to the host eclogites and that the δ^56Fe and δ^26Mg gradually decrease from the early omphacite-epidote through epidote-quartz to the late kyanite-epidotequartz 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 δ^56Fe and δ^26Mg 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.

关 键 词：铁一镁同位素 蚀变洋壳 榴辉岩 变质流体 流体演化 弧岩浆岩 地球化学. 

分 类 号：P581[天文地球—岩石学]