Magnesium isotope constraints on subduction contribution to Mesozoic and Cenozoic East Asian continental basalts

Abstract

To better understand the influence of subduction on geochemical heterogeneity of the mantle, Mg isotope ratios were measured in a suite of continental basalts from East Asia. In contrast to global oceanic basalts (δ26Mg=−0.25±0.07‰), these continental basalts display considerable Mg isotope variation that can be roughly related to the time and place of eruption. The Cenozoic basalts have slightly lower δ26Mg values (n=32; −0.46±0.05 to −0.25±0.06‰; average −0.38‰) than the Mesozoic basalts (n=12; −0.41±0.08 to −0.23±0.05‰; average −0.30‰) and display a decreasing δ26Mg trend with increasing distance from the present subduction trench, particularly for the basalts from South China Craton. Such variations may be related to westward subduction of Pacific oceanic plate beneath East Asia. Some outliers of Mg isotopic data in the North China Craton probably involve additional contributions from earlier subduction and collision from the south (Tethyan ocean and Yangtze Craton) and north (Paleo-Asian ocean). The correlations between δ26Mg and element indices (e.g., TiO2, Li/Y) of contributions of slab melt indicate input of a component from the slab that is progressively dehydrating in the source region. Highly variable Sr, Nd and Pb isotope ratios for a given δ26Mg value suggest continuous interaction rather than a simple binary mixing of the mantle with subduction components. Intra-continental magmatism in East Asian is thus derived from a mantle wedge affected by the ongoing subduction of the Pacific Plate. Mixing with the uniform Mg isotopic compositions of MORB and OIB sources, global mantle convection likely erases the enriched components, whereas mantle wedges might be able to retain some Mg isotope heterogeneity.