深部碳循环的Ca-Mg同位素示踪

Recycling of marine carbonates into the mantle through subduction zone is an important part of the global C recycles. This process significantly modifies the volueme of surface CO2 sinks, which has important significance for the CO2 budget of Earth's atmosphere. This process can also potentially modify Ca and Mg isotopic variations within the mantle, because marine carbonates typically have lighter Ca and Mg isotopes compared to typical mantle values. Therefore, it is possible to trace deep carbon recycles by using Ca-Mg isotopes on the base of understanding of the Ca-Mg isotopic fractionation during dehydration process in subduction zone. In order to test this hypothesis, this project includes three research contents: (1) Investigation of Ca-Mg isotopic compositions of ultrahigh pressure metanorphic rocks formed by continental subduction and oceanic suduction, respectively, to reveal the possible Ca-Mg.isotopic fractionation during dehydration process in subduction zone; (2) Investigation of Ca-Mg isotopic compositions of the Mesozoic and Cenozoic baslts developed in the eastern China and Ermeishan basalts to reveal that whether the Ca-Mg.isotopic compositions of their mantle sources have been modified by the recycling carbonetes; (3) Investigation of Ca-Mg isotopic compositions of the Mesozoic and Cenozoic mantle derived carbonite rocks and coexisting alkaline rocks developed in western Sichuan province and Shandong province to reveal the possible Ca-Mg isotopic fractionation during magma immiscibility process between carbonete melt and silicate melt and that whether the Ca-Mg isotopic compositions of their mantle sources have been modified by the recycling carbonetes.

俯冲洋壳携带沉积碳酸盐进入地幔，是全球碳循环的一个重要组成部分.这一过程显著改变了地表碳汇的体积，它对估计大气CO2收支有重要意义。这一过程还可改变地幔的Ca-Mg同位素组成，因为碳酸盐的Ca-Mg同位素显著较地幔轻。因此，在了解板块俯冲脱水过程中Ca-Mg同位素的分馏行为基础上，应用Ca-Mg同位素示踪深部碳循环是可行的。为检验这一推论，本项目包含3个方面研究内容:（1）调查陆壳和洋壳俯冲成因的超高压变质岩的Ca-Mg位素组成以揭示板块俯冲脱水过程可能导致的Ca-Mg同位素分馏；（2）调查中国东部中新生代玄武岩和峨眉山玄武岩的Ca-Mg同位素组成以揭示它们地幔源区Ca-Mg同位素组成是否受到再循环碳酸盐的影响；（3）调查川西和山东碳酸盐岩及其共生碱性岩的Ca-Mg同位素组成，观察碳酸盐和硅酸盐岩浆不混溶过程是否存在Ca-Mg同位素分馏，和其源区Ca-Mg同位素是否受到再循环碳酸盐的影响。

该项目在Mg同位素示踪深部碳循环取得了突破性进展，引领了该领域的研究。已获得的7项主要进展如下。.进展1、系统研究了洋壳俯冲过程Mg同位素地球化学，发现（1）玄武质地壳在俯冲变质过程中Mg含量和Mg同位素不发生变化；（2）再循环进入地幔的榴辉岩部分具轻Mg同位素组成；（3）在高压变质条件下榴辉岩与俯冲碳酸盐可发生Ca-Mg同位素交换反应，它导致碳酸盐化榴辉岩具有轻的Mg同位素组成，而俯冲钙质碳酸盐因此变重；（4）泥质沉积物具有比地幔重的Mg同位素组成，洋壳俯冲通道流体也具有偏重的Mg同位素组成。这些成果使得Mg同位素成为地球化学家示踪全球深部碳循环的有力工具。.进展2、通过对环太平洋岛弧玄武岩和中国东部中-新生代玄武岩的Mg同位素分析。厘定了中国东部地幔大尺度Mg同位素异常。它可划分为两个异常区：（1）大陆异常区，它与中国东部地幔过渡带的西太平洋俯冲滞留板片的分布区完全重合，证明该异常与西太平洋板块俯冲有关。玄武岩微量元素特征表明其成因与再循环碳酸盐有关. （2）海南异常区： 它与南海地幔柱吻合，玄武岩微量元素特征说明其成因与再循环碳酸盐化榴辉岩有关。这些结果证明俯冲碳酸盐可大量进入对流上地幔，很少进入下地幔。.进展3，发现Mg同位素不能示踪岛弧深部碳循环，这一问题是俯冲板片脱水流体选择溶解钙质碳酸盐，残留镁质碳酸盐造成的。.进展4：通过对华北南部岩石圈地幔包体和青藏冈底斯超钾火山岩和帕米尔碱性岩的Mg同位素研究，证明古特提斯和特提斯洋壳俯冲引发岩石圈地幔碳酸盐化交代。.进展5：分析了中国东部中-新生代玄武质岩的Zn同位素，发现锌同位素是可用来示踪深部碳循环的新的示踪剂。.进展6：建成了高精度Ca同位素分析方法，其长期外精度好于0.11‰ (2s)。 .进展7，对峨眉山和塔里木晚古生代大火成岩省的玄武岩和碳酸岩开展了Mg同位素地球化学研究，对下地幔是否存在再循环碳酸盐进行了初步探索。