Carbonate Metasomatism and Its Identification Characteristics in Mantle Peridotite

Abstract

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.