Hidden carbon storage in the continental lithosphere revealed by Mg-Zn isotopes of adakite-like magmas

Carbon cycling into Earth’s interior regulates long-term climate and habitability, yet the fate of subducted sedimentary carbon remains widely debated. Competing models predict either efficient carbon release from the subducting slab and surface return or massive deep sequestration into the mantle. Here, we present coupled Mg and Zn isotope systematics of Oligocene adakite-like granites derived from partially melted metasediments previously relaminated into the lower continental crust. The granites display variable δ26Mg (−0.41‰ to +0.02‰) and δ66Zn (0.27‰ to 0.46‰) values that define a negative correlation, trending from silicate sediment signatures toward isotopically lighter Mg and heavier Zn characteristic of carbonates. These signatures indicate recycled contribution from carbonate-bearing sediments to the magma source. We propose that diapiric relamination of carbonate-bearing sediments during subduction represents an efficient mechanism for storing carbon in the deep crust, thereby helping to resolve the imbalance between subducted and released carbon at convergent margins. Partial melting of such relaminated carbonate during the postcollision stage may subsequently act as a potential driver of atmospheric CO2.