矿床地球化学国家重点实验室知识库

Whole rock lithium (Li) and Li isotopic compositions of coesite-bearing whiteschists and their country rock metagranites from the Dora-Maira Massif in theWestern Alps were investigated to understand their behavior during fluid-rock interaction in mantle depths of continental subduction zones. The whiteschists reveal relatively low d7 Li values (
6.4‰ to þ2.4‰, with an average value of
2.1‰) in comparison to the country rock metagranites (d7 Li ¼
1.2‰ to þ3.6‰, with an average value of þ1.4‰). The d7 Li values of whiteschists do not show any correlations withmajor elements (e.g. Mg, Fe), Li, or isotopes (i.e., d26Mg and d18O values) and cannot be explained by kinetic fractionation. Hence, the low d7 Li values of whiteschists result from interactions with metasomatic fluids containing isotopically light Li that occurred in deep portions of a continental subduction zone. Fluid-rock interaction modeling suggests that the metasomatic fluids contain low d7 Li values (down to
4‰) and moderate Li concentrations (50 to 150 ppm), which were probably derived from the dehydration of serpentinites in the subduction channel. The presence of such low d7 Li fluids in deep subduction zones could cause Li isotopic heterogeneity in the earth's interior.