核墁边界碳同位素平衡分馏的理论计算研究

The well-recognized stable carbon isotope composition (δ13C) of the bulk silicate earth (BSE) is about 5 ‰, which is different from the isotope signals (about 20 ‰) of Mars, Vesta or chondrite meteorites in the solar system. The huge difference (i.e., 15 ‰) of the carbon isotope signal most likely comes from the process of core formation in the Earth. When the core was separated from the mantle, possible carbon isotope fractionations took place between silicate melts and the liquid iron outer core. To the best of our knowledge, there is no experimental or theoretical research so far focused on carbon isotope fractionations between the mantle and the core. This proposal is trying to theoretically investigate possible carbon isotope fractionations under extremely high temperature and pressure conditions just like the core-mantle boundary. Another goal of this project is to improve or modify the calculation method of equilibrium isotope fractionation under very high temperature and pressure.

目前公认的整体硅酸盐地球(BSE)的碳稳定同位素信号δ13C约为-5‰，与来自太阳系其他星体Mars和Vesta的陨石以及球粒陨石中所携带δ13C同位素信号(约为-20‰) 存在15‰巨大差异。这15‰的差异的一种可能成因是：在地球的地核形成过程中，碳同位素在硅酸盐熔体和液态铁之间发生了大的分馏。目前国际上尚未出现对硅酸盐熔体和液态铁之间碳同位素分馏的实验和理论研究，本申请拟率先使用理论计算方法对极高温度和压力下碳同位素在硅酸盐熔体和液态铁之间平衡分馏展开定量研究，并改进高温高压下同位素平衡分馏的计算方法。

本研究利用第一性方法计算了硅酸盐熔体，铁熔体，碳酸镁，金刚石，莫桑石和各种碳化铁晶体的碳同位素分馏系数。我们发现压力对熔体和晶体的碳同位素分馏系数的影响是明显不同的。我们认为，使用晶体作为熔体的替代物来研究碳同位素分馏可能得到有瑕疵的结果。. 使用本研究的计算结果显示：在原始地球的岩浆洋底部的温度和压力条件下，无论使用平衡分类模型还是瑞利分馏模型，硅酸盐熔体和铁熔体间的碳同位素平衡分馏不能够解释地幔碳同位素组成。地幔的碳同位素组成比球粒陨石偏重的现象需要用星胚的核幔分异来解释。我们建议地球的碳来源是一个已经分异的星胚，而且这个星胚可能与火星类似。