深部碳循环的Mg同位素示踪  

Tracing deep carbon recycling by Mg isotopes

作　　者：李曙光[1,2] 

机构地区：[1]中国地质大学地质过程与矿产资源国家重点实验室,北京100083 [2]中国科学技术大学地球与空间科学学院中国科学院壳幔物质与环境重点实验室,安徽合肥230026

出　　处：《地学前缘》2015年第5期143-159,共17页Earth Science Frontiers

基　　金：国家自然科学基金项目(41230209)

摘　　要：大洋板块俯冲导致的深部碳循环可影响地球历史的大气CO2的收支情况及气候变化。沉积碳酸盐岩是地球中轻镁同位素的主要储库,它通过板块俯冲再循环进入地幔有可能引起地幔局部的Mg同位素组成不均一性。因此,在这样一个基本假设基础上,即俯冲岩石的镁同位素在变质脱水和岩浆过程中不发生显著变化,镁同位素有可能成为深部碳循环的示踪剂。前人研究已经证明岩浆过程不会发生显著镁同位素分馏。然而,至今对俯冲、变质过程镁同位素的分馏程度以及低δ26 Mg玄武岩成因还属未知。为此,本研究聚焦在高温高压条件下碳酸盐的稳定性和相转换、板块俯冲过程中的镁同位素行为、循环碳酸盐对地幔镁同位素组成可能产生的影响。Deep carbon recycling through oceanic subduction influences the atmospheric CO2 budget and climate changes through geologic time. Sedimentary carbonate is the main light-δ26 Mg sink of Earth＇s reservoirs, and therefore its recycling into the mantle by plate subduction has potential to cause local mantle Mg isotopic heterogeneity. Therefore, Mg isotopes are potential traces for deep carbon recycling, based on a primary assumption that Mg isotopic systematics of the subdueting rocks underwent little change during metamorphic dehydration and magmatic process. Previous studies demonstrated that magmatic processes do not significantly fractionate Mg isotopes. However, to date, the extent to which Mg isotopes fractionate during metamorphic process and origin of low δ26 Mg basalts remain largely unknown. Here we focus on these topics by introducing the stability and transformation of carbonates in high p-T condition, the behavior of Mg isotopes during plate subduction, and the possible influences on Mg isotopic compositions o{ the mantle by deep recycled carbonates.

关 键 词：深部碳循环 碳酸盐 镁同位素 板块俯冲 地幔地球化学 

分 类 号：P597.2[天文地球—地球化学]