大别山双河超高压变质大理岩及其包裹榴辉岩的Li同位素地球化学研究

Li isotopes were considered as an ideal tracer of recycling processes in the subduction zones and have become one of the focuses in recent years. It has been suggested that subducted sedimentary carbonates can deliver “heavy Li” signatures to the mantle, therefore may significantly affect the Li isotope compositions of the mantle and cause mantle heterogeneity. However, Li isotopes in the ultrahigh pressure (UHP) marble have not been previously investigated, although they may provide important information on the global “Li cycle” processes. In addition, the genesis of the extremely light Li compositions in eclogites is still under debate. The Dabie-Sulu orogenic belt in China is the largest known UHPM belt on Earth, in which UHP marble and eclogite are well exposed in the Shuanghe area. This project investigates Li isotope compositions of UHP marbles, eclogites and wall rock gneisses, particularly focus on the systematic Li elemental and isotopic variations from eclogite to marble and marble to gneiss. Together with in-situ Li zonation studies on minerals and Li isotope analysis on minerals separated, the study aims to reveal the mechanism and the scale of Li isotope transfer during deep subductions, and find the decisive factors that control the Li isotope compositions in UHP marbles and eclogites, such information may help to improve our understandings on the Li isotopic recycling in the subduction zones.

俯冲带的Li同位素示踪是近年来国际Li同位素研究的焦点，理论上俯冲下去的表层沉积碳酸盐可携带重的Li同位素组成再循环进入地幔，进而影响地幔Li同位素组成并造成地幔不均一性。然而，对于经历深俯冲超高压变质碳酸盐的Li同位素组成目前仍缺乏研究，此外，学界对于榴辉岩极端轻Li同位素组成的成因也存在争议。大别-苏鲁造山带是全球出露面积最大的超高压变质带，在双河地区发育有非常典型的超高压变质大理岩和榴辉岩。本项目拟对大别山双河经历超高压变质作用的大理岩、大理岩内的榴辉岩以及围岩片麻岩进行系统的Li同位素剖面研究，探讨深俯冲过程中榴辉岩-大理岩以及大理岩-片麻岩之间Li同位素交换的方式及尺度，并结合矿物的Li元素环带和单矿物的Li同位素分析确定控制榴辉岩和大理岩Li同位素组成的决定因素，为更深刻理解俯冲带Li同位素地球化学循环提供关键信息。

板块俯冲作用是地表物质，尤其是碳酸盐，发生壳幔物质循环的重要场所，因此对俯冲碳酸盐的研究对于我们理解深部碳循环过程至关重要。本项目主要研究了大别山地区出露的超高压变质大理岩以及大理岩中榴辉岩的Li含量、Li同位素和微量元素组成。我们首次发现了，双河大理岩中的小尺度榴辉岩具有系统的重Li同位素组成（为+8至+15‰），比地幔值显著偏高（δ7Li = +2.0至+5.7‰, 均值 +3.7‰），甚至高于大理岩的Li同位素组成（+7至+11‰）。而较大尺度的榴辉岩透镜体具有高度变化的Li同位素组成（从-13至+9‰）。通过Li同位素组成和微量元素的分析，我们得出结论，双河地区的榴辉岩经历显著的扩散作用，导致观测到的Li同位素和Li含量的负相关关系。大别苏鲁的超高压变质大理岩在Li同位素-Li含量图解中落在了两端元混合的趋势线上，分别为纯的海相沉积碳酸盐和上地壳硅酸盐，表明在超高压变质过程中Li同位素的分馏不显著。因此，深俯冲的变质碳酸盐可以携带重的Li同位素信息进入地幔，由此造成地幔的不均一性。考虑到大理岩和地幔储库的较大Li同位素差异，Li同位素示踪深部碳循环过程具有很大的应用潜力。