碳酸岩浆作用过程中镁同位素的分馏行为

Knowledge on the behavior of Mg isotopes during carbonatite magmatism may provide important constraints on the Mg isotopic compositions of carbonatite melts involved in mantle metasomatism. Our recent studies on the Mg isotopic compositions of carbonatites and alkaline silicate rocks from the Oldoinyo Lengai volcano, Tanzania, demonstrate that olivine melilitites have mantle-like Mg isotopic compositions. By contrast, nephelinites have Mg isotopic compositions slightly higher than the mantle value, whereas carbonatites have variable Mg isotopic compositions that are remarkably higher than the mantle value. These preliminary data suggest that significant Mg isotope fractionation may occur during both “silicate-carbonatite liquid immisibility” and differentiation of carbonatite magma. This project intends to verify whether such a suggestion is right or not, through both studies of natural samples and high temperature and high pressure experimental studies. Studies of natural samples will provide high-precision Mg isotopic compositions of carbonatites and associated igneous silicate rocks from different areas in the world. Experimental studies will provide the equilibrium Mg isotope fractionation factors between the immiscible silicate and carbonatite melts under different temperatures, pressures and compositions of melts. The findings from these two aspects of this project will provide new constraints on the origin of local mantle Mg isotope heterogeneity and on the application of Mg isotopes for tracing mantle metasomatism.

对碳酸岩浆作用过程中镁同位素分馏行为的认识可以为参与地幔交代作用的碳酸盐熔体的镁同位素组成提供重要制约。申请人近期对坦桑尼亚Oldoinyo Lengai火山火成碳酸岩和碱性硅酸岩样品镁同位素组成的研究显示：橄榄石黄长岩的镁同位素组成与“地幔值”一致；霞石岩具有略高于“地幔值”的镁同位素组成；而火成碳酸岩具有变化较大且显著高于“地幔值”的镁同位素组成。这些初步的数据表明“硅酸盐—碳酸盐熔体不混溶”过程和碳酸岩浆分异过程中镁同位素都有可能发生显著的分馏。本申请拟通过天然样品和高温高压实验两方面的研究来判明这一结论是否正确。天然样品研究将给出全球多个地区共生的火成碳酸岩和硅质岩浆岩的高精度镁同位素数据；实验研究将给出不同温压条件和熔体成分下“不混溶”的硅酸盐和碳酸盐熔体之间的镁同位素平衡分馏系数。两方面的研究结果将为局部地幔镁同位素组成不均一性的成因及应用镁同位素示踪地幔交代作用提供新的制约。

查明不同地质过程中同位素的分馏行为是开展同位素地球化学示踪的前提。火成碳酸岩的Mg同位素和Ba同位素研究能够丰富我们对高温岩浆过程中同位素分馏行为的认识。本项目研究了加拿大、格陵兰、东非（包括目前世界上唯一喷发碳酸质岩浆的坦桑尼亚Oldoinyo Lengai火山）和德国等地不同时代（2680 ~ 0Ma）的火成碳酸岩和共生的幔源硅质岩浆岩样品的Mg和Ba同位素组成。主要研究结果和结论是：1）与“硅酸盐—碳酸盐熔体不混溶”过程无关的硅质岩浆岩（δ26Mg = -0.37 ~ -0.10‰）和“不混溶”过程所形成的硅质岩浆岩（δ26Mg = -0.06 ~ +0.09‰）两类样品的Mg同位素组成存在显著差异；火成碳酸岩样品的Mg同位素组成变化很大（δ26Mg = -1.11 ~ +0.37‰）。这些特征表明“硅酸盐—碳酸盐熔体不混溶”过程和碳酸岩浆分异过程中Mg同位素都会发生显著的分馏，因此Mg同位素是示踪碳酸岩浆作用过程的新工具。2）与“不混溶”过程无关的硅质岩浆岩和“不混溶”过程所形成的硅质岩浆岩两类样品具有一致的Ba同位素组成（δ137/134Ba = +0.01 ~ +0.03‰）；绝大多数火成碳酸岩样品的Ba同位素组成相对均一（δ137/134Ba = -0.03 ~ +0.09‰）。这些特征表明“不混溶”过程和碳酸岩浆分异过程中Ba同位素的分馏都很小。因此火成碳酸岩的Ba同位素组成直接反映其地幔源区的组成；根据本项目数据估算获得地幔这一重要储库的平均Ba同位素组成为δ137/134Ba = +0.04 ± 0.06‰（2SD）。