Carbon-rich polyphasic inclusions in postcollisional mafic magmatic rocks from the Dabie Shan, China: Implications for the carbon cycle in continental subduction zones

Abstract

Subduction is a fundamental geodynamic process that transfers carbon from Earth’s surface into the mantle. However, current understanding of the migration mechanisms, final storage region, and species involved in carbon recycling from continental crust remains limited. Here, we investigated the compositions of polyphasic inclusions and Mg isotopes in postcollisional mafic magmatic rocks from the Dabie Shan region of China. The main rock-forming minerals contained two distinct types of polyphasic inclusions, which displayed systematic differences in daughter mineral/gaseous phase assemblages, including host-like silicates ± carbonates (magnesite, dolomite, and calcite) + CH4 and carbonates + talc ± SiO2 (aqueous) + CH4, respectively. These inclusions indicate that carbon-rich silicate melts and carbon-rich magmatic fluids were trapped by host minerals during magmatic processes. The abundant carbonates and CH4 in both types of inclusions suggest that the mantle source of these postcollisional mafic magmatic rocks was rich in carbon, most likely existing in the forms of CO2 and CH4. Moreover, the studied postcollisional mafic magmatic rocks have mantle-like Mg isotope compositions, with δ26Mg values ranging from −0.23‰ to −0.16‰. The combined observations of polyphase inclusions and Mg isotopes indicate that a substantial carbon-rich mantle domain arose from the metasomatism of silicate melts derived from subducted continental slabs that had dissolved a certain quantity of CO2 and CH4. We proposed that continental subduction is an efficient pathway for transporting crustal carbon into an orogenic subcontinental lithospheric mantle wedge, where the recycled carbon can be stored for >100 m.y. and eventually released to the surface during postcollisional magmatism.