Mantle eclogite evidence of subducted carbonate sediments in Earth’s deep carbon cycle

Abstract

Mantle eclogite represents a rare and unique archive for understanding the dynamics and implications of subduction, but its origin is still debated especially for high-MgO eclogites. We report new chemical and Mg–O isotopic data on high-MgO mantle eclogites from the Obnazhennaya kimberlite, Siberia craton. Obnazhennaya high-MgO eclogites have lower bulk δ²⁶Mg isotopes (-1.47– -0.36‰) and mantle-like garnet δ¹⁸O isotopes (+5.18–+6.39‰). A bulk mixing calculation among oceanic crust, carbonate sediment and kimberlite, and its effects on Mg isotopes and MgO content indicate that metasomatism can only increase the MgO content but not δ²⁶Mg, and more than 50% dolostone by mass is needed, under conditions with no kimberlitic melt metasomatism, to account for both high-MgO contents, which is unrealistically high for natural settings. The ternary mixing model on both O–Mg isotopic compositions and MgO–Mg isotopic compositions suggest that subducted oceanic crusts modified by mantle metasomatism (e.g., kimberlite-like melt) as well as carbonate sediments involvement collectively provides the best explanation for such a high magnesian origin. This model is also compatible with global mantle eclogite observations, where there is also a clear chemical composition distinction between high- and low-MgO eclogites globally. In light of our new results and global compilation, we propose that mantle eclogites represent a spectrum of metasomatized subducted oceanic crust with variable amounts of the involvement of carbonate sediments, the latter of which represents a novel ingredient in the recipe for making previously enigmatic high-MgO eclogite xenoliths in kimberlites.