Tracing carbonatite metasomatism in the source of arc magmatism using Li-Mg-Zn isotopes: Implications for deep carbon cycling

Abstract The subduction of carbonate-bearing components into Earth’s interior is a key part of the deep carbon cycle. However, the mechanisms of carbon transfer from the subducted slab to the sub-arc mantle remain unclear. Here we present a novel study using the Li-Mg-Zn isotopic systematics of Early Cretaceous olivine websterites from the northeastern North China Craton, China, to investigate cryptic carbonatite metasomatism. Clinopyroxene in the websterites has high Ca/Al and (La/Yb)N ratios, indicating their derivation from fluxed melting of a carbonated mantle peridotite. The samples exhibit lighter Li (δ7Li = −4.42‰ to 3.62‰) and Mg (δ26Mg = −0.40‰ to −0.36‰) isotopic compositions compared with the mantle, but similar Zn (δ66Zn = 0.25‰–0.29‰) isotopic compositions, suggesting the source was modified by ~14% dehydrated altered oceanic crust and ~6% carbonate-bearing silicate sediments. We propose that arc magmatism lacking high-δ7Li and -δ66Zn signatures does not necessarily indicate the absence of carbonates in the source. The combination of clinopyroxene and Li-Mg-Zn isotopic analyses can provide an improved understanding of deep carbon cycling.