大别超高压榴辉岩矿物间的Fe同位素分馏机制及对氧逸度的响应

Iron is a redox-sensitive major element, and the geochemistry of iron isotope has the potential to trace oxygen fugacity variation during geological process. Geochemical behavior of iron isotope in subduction zone and whether oxygen fugacity effects inter-mineral iron isotope fractionation have become two of the focuses in recent years. My previous study suggested that oxidized geofluid infiltration resulted in elevation of of δ56Fe values of omphacite during retrograde metamorphism. However, only a few researches investigated inter-mineral iron isotope fractionation during redox process in subduction zones. This project studies on the iron isotopic composition and Fe3+/ΣFe ratio of whole rock and mineral seperates of eclogites from Dabie orogen belt. Together with major and trace element contens of bulk eclogites and minerals, this study aims to reveal whether oxygen fugacity changes during exhumation and how redox process effects iron isotope fractionation between garnet-omphacite-amphibole-phengite-epidote. Such consequence may help to improve our understanding on the inter-mineral iron fractionation at high temperature and viaration of oxygen fugacity during exhumation in subduction zones.

铁（Fe）是地球的主要组成元素之一。作为常见的变价元素，Fe及其同位素地球化学具有示踪地质过程中氧逸度变化的潜力。俯冲带的铁同位素地球化学行为及氧逸度变化能否影响岩石本身及矿物间的铁同位素分馏是近年来国际Fe同位素研究领域刚刚开始的研究热点。申请人在博士期间的工作表明高氧逸度流体参与的退变质过程会使绿辉石的δ56Fe升高，进而影响绿辉石-石榴石之间的铁同位素分馏。但是，折返过程中氧逸度变化对矿物间Fe同位素分馏的影响和分馏机制仍缺乏系统研究。本项目拟采集大别造山带不同地区的峰期变质榴辉岩和经历过不同程度角闪岩相退变质作用的榴辉岩，对其全岩和单矿物进行Fe同位素和Fe3+/ΣFe值的测量，配合开展全岩和单矿物的主微量元素含量分析，探讨折返过程中氧逸度的变化及对石榴石-绿辉石-角闪石-多硅白云母-帘石等矿物间Fe同位素分馏的影响，为氧逸度变化影响矿物间Fe同位素分馏的作用机制提供重要信息。

作为常见的变价元素，Fe及其同位素地球化学具有示踪俯冲板片折返过程中氧逸度变化的潜力，但是目前对于退变质榴辉岩全岩及单矿物的Fe同位素组成所知甚少。本项目系统分析了大别造山带退变质榴辉岩全岩及单矿物的Fe同位素组成，取得主要以下观察成果：1）板块折返退变质过程中，榴辉岩的Fe3+/ΣFe值会上升，但全岩铁同位素组成不发生分馏（δ56Fe=0.07±0.09‰, 2SD）；2）退变质过程不会改变石榴石的Fe同位素组成，本项目观察到的绿辉石和石榴石Fe同位素分馏值变化较小，但不能代表两者之间达到的Fe同位素平衡；3）白云母和绿辉石之间在退变质阶段达到了Fe同位素平衡，可以得到白云母和绿辉石之间的Fe同位素温度计方程：Δ56FePhg-Omp= 0.10±0.06×10^6/T2。本项目工作系统地研究了退变质榴辉岩全岩及单矿物的Fe同位素组成，限定了板片折返阶段退变质过程中铁同位素的地球化学行为，对加深理解俯冲带变质岩Fe同位素组成变化成因具有重要意义。