Shallow mantle cycle of subducted sedimentary limestone indicated by carbonate xenoliths

Abstract

In the subduction zone, carbon (C) release from the subducted plate into the mantle wedge takes place by various mechanisms, including metamorphic degassing, carbonate dissolution, and hydrous melting. Recent studies have highlighted the significant role of buoyant limestone diapirs in facilitating C recycling during subduction processes. Here, we report that the Hannuoba carbonate xenoliths of eastern China were introduced into the shallow mantle in the form of sedimentary limestone diapirs. We conducted a comprehensive study of the Hannuoba carbonate xenoliths using petrological analysis, major- and trace-element analysis, in situ zircon dating, and C-O isotopic analysis techniques. The whole-rock major and trace elements, along with high δ18OSMOW values (22.8‰−23.2‰), indicate that the protolith of Hannuoba carbonate xenoliths consists of sedimentary limestones. The spectral age peaks of detrital zircons (400−100 Ma, 500−450 Ma, 2.0−1.6 Ga, and 2.8−2.2 Ga) suggest that the primary source of the protolith is the North China Craton. When sedimentary limestone diapirs enter into the mantle wedge, they undergo partial melting under high-temperature conditions. This process results in the melting of low-melting point pelitic components, while leaving behind resistant minerals such as pure calcite. Due to its low density and viscosity, this pure limestone component will continue to migrate upward and assimilate with the mantle peridotite. In this process, the pure calcites are preserved in the shallow mantle in a solid-state form rather than as carbonatitic melts. The discovery of graphite and spinel in the Hannuoba carbonate xenoliths indicates that the depth of formation of the sedimentary limestone diapirs does not exceed 2.5 GPa (<70 km). The calcite in Hannuoba carbonate xenoliths exhibits extremely low δ13CVPDB (−11.8‰ to −11.0‰) and high δ18OSMOW (22.8‰−23.2‰) contents. This characteristic C-O isotopic signature can be explained by the Rayleigh decarbonation of sedimentary limestones under upper-mantle conditions (<3 GPa). Based on calculations, the Hannuoba carbonate xenoliths are a product of subducted sedimentary limestone experiencing 99% volume decarbonation. Through this metamorphic decarbonation process, limestone diapirs release a significant amount of CO2 into the arc magma system. During subduction processes, sedimentary carbonate rocks can be recycled to the mantle wedge of island arcs as solid limestone diapirs. The C flux released by this mechanism cannot be ignored.