Mg-Li-Fe-Cr Isotopic Fractionation during Subduction
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摘要: 金属稳定同位素体系是示踪板块俯冲对壳幔物质再循环影响的全新工具，因此其在俯冲带的地球化学行为备受关注.Mg同位素在俯冲过程中不发生显著分馏，但大陆玄武岩具有低于洋中脊玄武岩的Mg同位素，这可能是碳酸岩的俯冲再循环导致的.与角闪岩继承原岩的Li同位素组成不同，榴辉岩具有轻于原岩的Li同位素组成，可归因于俯冲折返过程中的动力学扩散、脱水反应或低Li同位素的流体交代.作为变价元素，Fe和Cr的同位素在榴辉岩的形成过程中均不发生显著分馏，但是蛇纹岩的Fe同位素和Cr同位素与氧逸度指标具有相关性，指示氧化还原条件变化时脱水过程或流体交代会导致同位素分馏.
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关键词:
- 俯冲带 /
- Mg-Li-Fe-Cr同位素 /
- 分馏 /
- 地球化学
Abstract: In order to investigate mantle heterogeneities induced by subducted crustal plate, many studies have been focused on geochemical characteristics of stable metal isotopic systematics in subduction zone in recent years. Limited Mg isotopic fractionation occurred during subduction and exhumation, thus lighter Mg isotopic composition of continental basalts might be caused by carbonatite metasomatism. Instead of inherited Li isotopes of amphibolite, eclogite has lighter Li isotopies, which are attributed to kinetic diffusion, dehydration, or rehydration. Isotopes of Fe and Cr have insignificant fractionation during formation of eclogite, however, Fe and Cr isotopic composition of serpentinite are related to oxidation index, suggesting dehydration or alteration of fluids during serpentinization could shift Fe and Cr isotopic composition of serpentinite.-
Key words:
- subduction zone /
- Mg-Li-Fe-Cr isotope /
- fractionation /
- geochemistry
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图 1 大别造山带不同变质级别变质岩的烧失量与MgO和δ26Mg
据李曙光(2015)；数据引自Wang et al.(2014)
Fig. 1. MgO vs. LOI and δ26Mg vs. LOI for greenschists, amphibolites and eclogites from the Dabie orogen
图 3 碧溪岭榴辉岩单矿物的δ56Fe值和Fe3+/ΣFe
Fig. 3. Plot of δ56Fe vs. Fe3+/ΣFe for mineral separated from the Bixiling eclogites
图 4 大别山榴辉岩的δ53Cr vs.烧失量和δ53Cr vs. Cr含量
Fig. 4. δ53Cr vs. LOI and δ53Cr vs. Cr content for eclogites from the Dabie orogen
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