Differentiation between carbonate and silicate metasomatism based on lithium isotopic compositions of alkali basalts

Abstract

Abstract Carbonate and silicate metasomatism occurring in subduction zones is an important mechanism underlying mantle heterogeneity and compositional diversity of mantle-derived rocks. However, distinguishing between the two kinds of metasomatism is often difficult. Lithium (Li) and its isotopes have great potential in this regard because of the different Li isotopic compositions of recycled marine carbonate and silicate components. We report Li isotopic data from Cenozoic and Mesozoic alkali basalts of the West Qinling orogen in central China. Relative to those for normal basalts, very high δ7Li values (up to +11.2‰) were observed for the Cenozoic alkali basalts, but significantly and systematically lower values (as low as −3.3‰) were estimated for the Mesozoic alkali basalts. Their abnormal Li isotopic compositions, combined with major- and trace-element contents and Sr-Mg isotope ratios, indicate that the Cenozoic and Mesozoic alkali basalts originated from carbonated and silicated mantle sources, respectively, reflecting metasomatism of the mantle by slab-derived carbonate and silicate melts during Paleotethyan oceanic subduction. Interactions of such melts with the mantle peridotite in subduction channels can account for the elemental and isotopic differences of the studied alkali basalts. The present study demonstrates an effective way to distinguish between carbonate and silicate metasomatism in subduction zones by studying Li isotopic compositions of alkali basalts and highlights the prospect of Li isotopes in tracing the deep carbon cycle.