Survived carbonates in orogenic dunites from recycling of subduction-related sediments

Abstract

The carbon recycling between the Earth's surface and its interior is accomplished through plate subduction and magmatic processes. The orogenic mantle peridotites document multi-stage metasomatism in the subduction zone and play a significant role in revealing the material transportation and migration between the Earth's different spheres. However, how the migration process of carbon-bearing species influenced the subcontinental lithosphere mantle remains a mystery. To address these issues, we conducted detailed petrography, whole-rock and mineral geochemical compositions, and Sr–C–O isotope studies on two types of peridotites (silicate melt pocket-bearing and carbonate melt pocket-bearing dunites) of the Maowu ultramafic massif in the Dabie orogen. The silicate-melt pockets were captured earlier than the carbonate-melt pockets, and were derived from a large proportion of silicate melts during the formation of garnet pyroxenites. The subsequent carbonate metasomatism was mainly divided into two stages: the stage 1 reflects the reaction between dolomite melts and orthopyroxenes to generate clinopyroxenes with high CaO content and Mg# value (> 92); the stage 2 is the injection of Ca-rich fluids into dunites, forming dolomite veins and volatile-rich assemblages of tremolite, barite and monazite. The clinopyroxenes display high 87Sr/86Sr (0.7079–0.7097) and (La/Yb)N ratios, low Ti/Eu ratios, and different enrichment in Th, U, Sr and light rare earth elements (LREEs), indicating a carbonate metasomatic origin. The equilibrium melts of them are similar to sedimentary limestones with positive Sr anomaly and depleted high field strength elements (HFSEs). The Sr-O isotope mixing simulation shows a mixing of 80–90% carbonate sediments with the upper mantle peridotite. The C-O isotopes of the carbonate minerals have similar variation ranges (δ13CV-PDB: 19.7–15.6 ‰, δ18OV-SMOW: 18.1–28.1 ‰) within the range of sedimentary organic carbon. The 87Sr/86Sr ratios of limestone reservoir (0.708–0.709) suggest the addition of sedimentary carbonates into the mantle wedge at ∼370–500 Ma. Therefore, we propose that the sedimentary carbonates and organic carbon migrated into the deep mantle-wedge through the subducted Tethyan-Ocean slab in the Early Paleozoic, which experienced complex metasomatism and formed as carbonate minerals in the Maowu dunites to achieve the carbon recycling between the sedimentary carbon reservoir and the mantle wedge beneath the North China Craton.