西南天山超高压俯冲带中不同含碳相同位素分馏效应

Carbon isotopes of mantle rocks always show obvious heterogeneity although Earth deep interior are under high temperature and high pressure conditions. Until now, there are still multiple interpretations for carbon isotope heterogeneity of Earth' mantle: such as subduction carbon-adding process; volcanism carbon-losing process and core fractionation process. In contrast to other two genesis explanation, the subduction carbon-adding process has the lowest geothermal, and more obvious carbon fractionation effect, so it has the biggest effect on the carbon isotope of Earth’s mantle. This project will choose the carbonate-bearing meta-ophiolites from southwestern Tianshan ultra-high pressure subduction zone as petrological studying target, we will analysis the carbon fractionation effect of different carbon-bearing phases (carbonate, graphite and hydrocarbon) during subduction process. Only relying on petrological study of natural samples to discuss carbon isotope fractionation effect under subduction zone condition will be affected by later geological process, while the high temperature and high pressure experiments technique can offset the shortage of petrological study. In this project, we will integrate the carbon isotope analysis of different carbon-bearing phases in meta-ophiolites from southwestern Tianshan subduction zone and high temperature and high pressure experimental simulation to study the carbon isotope fractionation effect between different carbon-bearing phases during subduction process, in order to interpret the effect of subduction carbon-adding process on carbon isotope heterogeneity of Earth’s mantle.

尽管地球深部处于高温高压环境下，但是地幔岩石的碳同位素却往往表现出明显的不均一性。目前为止，关于地幔碳同位素不均一性的成因依然有多种解释：如俯冲加碳作用，岩浆脱碳作用以及地核分馏作用等。相对于其它两种成因解释，俯冲加碳过程具有最低的地温梯度，碳同位素分馏作用也最明显，故对地幔碳同位素的影响也最大。本项目将选择西南天山超高压俯冲带含碳酸盐变质蛇绿岩作为岩石学研究对象，分析俯冲作用过程中不同含碳相（碳酸盐，石墨，碳氢化合物）的碳同位素分馏效应。单一依靠天然样品的岩石学来探讨俯冲带条件下含碳相碳同位素分馏往往会受到后期地质过程的影响，而高温高压实验模拟技术可以很好的弥补岩石学研究的不足之处。本研究拟结合对西南天山俯冲带变质蛇绿岩中不同含碳相碳同位素分析以及高温高压实验模拟来综合研究俯冲作用过程中不同含碳相之间的碳同位素分馏效应，用于解释俯冲作用对地幔碳同位素不均一性的影响。

为了研究俯冲加碳作用对地幔碳同位素的不均一性的影响，本项目选择了西南天山高压俯冲带含碳酸盐变质蛇绿岩作为研究对象。本项目对西南天山俯冲变质带中的大理岩，碳酸盐化榴辉岩，碳酸盐化泥质岩进行了系统的的岩石学分析，并通过原位和非原位方法分析了这些变质蛇绿岩中不同含碳相（方解石，白云石，菱镁矿，石墨）的碳同位素特征。分析结果显示西南天山俯冲带中不同含碳相具有明显不同的碳同位素特征。在进变质过程中，从方解石 (~ -0‰)，白云石 (~ -4‰)，菱镁矿 (~ -8‰) 到石墨 (~ -13‰)，碳同位素逐步降低。我们通过岩石学和热力学研究限定了西南天山俯冲带氧逸度的变化，发现西天山俯冲带具有比其它俯冲带低的多的氧化还原环境 (< FMQ-2.5)。同时我们设计并完成了含铁白云石和水在高温高压下产生无机石墨，碳氢化合物的模拟实验。岩石学和高压实验模拟都显示西南天山俯冲带石墨和碳氢化合物可能是来自俯冲碳酸盐在低氧逸度条件下的无机氧化还原作用。结合含碳相同位素分析，可以看出伴随俯冲带进变质脱碳过程，俯冲带含碳相同位素会逐步降低，最终转变为具有低碳同位素的石墨/金刚石 (~ -13‰)。此成果可以很好的解释地幔榴辉岩质变质金刚石显示的较低的碳同位素特征。俯冲脱碳作用也可明显的降低俯冲带榴辉岩质无机石墨/金刚石的碳同位素。