地幔橄榄岩中碳酸盐熔体交代作用及其鉴定特征

碳酸盐熔体交代作用是指在地幔碳酸盐熔体与橄榄岩之间的相互作用，是改造地幔的重要方式之一.碳酸盐熔体交代会显著改变地幔橄榄岩的岩石学和地球化学特征.首先，碳酸盐熔体交代作用会改变地幔橄榄岩中的矿物组成和比例.尽管碳酸盐熔体与橄榄岩的反应结果受控于初始反应物成分和反应的温压条件，但多数反应会导致橄榄岩中辉石的比例增加，而且有时还会出现磷灰石、独居石等副矿物.另外，在有些受碳酸盐熔体交代显著的橄榄岩的矿物中不仅可发现大量CO₂流体包裹体和碳酸盐熔体包裹体，也会出现特殊的反应边结构和熔体囊.其次，碳酸盐熔体在改造地幔橄榄岩过程中，会在地幔矿物中留下明显的地球化学指纹.在主量元素特征上，受到碳酸盐熔体交代的橄榄岩中的单斜辉石往往具有偏高的Mg#和Ca/Al比值（>5）；而在微量元素组成特征上的变化更为显著，包括单斜辉石具有高的（La/Yb）_N、Eu/Ti、Zr/Hf、Y/Ho比值，并显著亏损HFSE等.另外，值得注意的是，碳酸盐熔体与地幔橄榄岩反应的程度不同也会导致这些地球化学特征存在差异，因此在判别碳酸盐熔体交代作用时要采用岩石学和地球化学特征相结合，多方面对比分析.对于引起地幔碳酸盐熔体交代作用的交代介质来源的识别主要用Mg-Zn-Ca-Sr等多种同位素体系进行示踪研究，尤其是近年来微区Sr同位素分析方法的建立为地幔碳酸盐熔体交代作用研究提供了重要手段. 

Carbonate Metasomatism and Its Identification Characteristics in Mantle Peridotite

Carbonate metasomatism, one of the important ways to modify the mantle, is the interaction between carbonate melt and peridotite in the mantle. It can significantly change the petrology and geochemistry of the mantle peridotite. Firstly, composition and proportion of minerals in peridotite can be modified by carbonate metasomatism. Although results of carbonate metasomatism depend on the initial reactant composition and temperature and pressure conditions, most reactions result in the pyroxene enrichment in peridotite, and occurrence of accessory minerals such as apatite and monazite. In addition, minerals of peridotite having experienced significant metasomatism by carbonate melts are generally featured with abundant CO₂-fluid and -melt inclusions, and distinctive spongy texture and melt pockets. Secondly, the carbonate metasomatism can be well identified by some geochemical fingerprints as well. As to major elements, the clinopyroxenes in peridotite having experienced carbonate metasomatism are characterized by high Mg# value and Ca/Al ratio (>5). In terms of trace elements, clinopyroxenes in peridotite having experienced carbonate metasomatism generally have higher (La/Yb)_N, Eu/Ti, Zr/Hf and Y/Ho ratios, and show depletions in HFSE. It is worth noting that the geochemical features may vary with the degree of carbonate metasomatism. In order to trace the source of carbonate melt caused metasomatism, Mg-Zn-Ca-Sr isotopic systems can be well used. Especially, in-situ Sr isotopic analysis method established in recent years provides us an important way to unravel the overlap of multiple carbonate metasomatism.