Carbon conservation in continental subduction zones revealed by zinc isotopes in ultrahigh-pressure marbles and carbonated eclogites

Carbon recycling in continental subduction zones is a fundamental component of the global deep carbon cycle, yet the efficiency of carbon transfer in these systems remains poorly constrained compared to oceanic subduction zones. Here we report the first zinc isotope data for ultrahigh-pressure (UHP) marbles and carbonated eclogites from the Dabie–Sulu orogenic belt to evaluate the behavior of carbonate minerals under conditions of continental subduction zones. Zinc isotope systematics of UHP marbles reveal contrasting behaviors of different carbonate species. Coupled declines of zinc concentrations and isotope ratios in calcite marbles compared with their sedimentary protoliths provide direct evidence for dissolution, but the dissolution fraction of calcite carbonates, as a whole, is almost negligible. For dolomite marbles, the elevated zinc concentrations and decreased zinc isotope ratios indicate that dissolved carbonates underwent strong refixation within the slab. These observations imply that carbonate dissolution occurs during continental subduction but is spatially restricted, despite some similarities to carbonate behavior in oceanic subduction zones. Carbonated eclogites exhibit zinc isotope compositions comparable to or heavier than those of MORB, reflecting variable extents of carbonate–silicate interaction within the subducted continental slab. These carbonate–silicate interactions further facilitate the stabilization and retention of carbonate minerals during deep continental subduction. As a whole, carbonates in subducted continental slabs remain relatively conservative. This can be attributed to (i) the low density of subducted continental crust, (ii) the limited availability of fluids, and (iii) the thick layers of carbonates in the continent. Consequently, most continental carbonates are retained within the slab and may be exhumed back to the crust, undergo diapiric ascent, or be underplated beneath thick continental lithosphere, rather than being extensively dissolved at sub-arc depths.