Zinc isotopic evidence for an Archean initiation of deep carbon cycling.

Carbonatite-forming magmas provide a critical window into the deep carbon cycle, but their ultimate carbon sources are debated. Here, we present first-principles calculations based on density functional theory and demonstrate that incipient mantle melting in the presence of primordial carbon produces carbonated melts enriched in light zinc (Zn) isotopes relative to the mantle. In contrast, our Zn isotopic data for a suite of Phanerozoic-to-Precambrian carbonatites and associated silicate rocks reveal systematically heavy isotopic signatures. The discrepancy implies that carbonatite-forming magmas must have incorporated isotopically heavy Zn from their mantle sources, probably in the form of recycled sedimentary carbonates, which suggests a Mesoarchean onset of deep carbon cycling. The common presence of recycled crustal carbon components in Earth's mantle might also account for the heavy Zn isotopic compositions of many other mantle-derived rock types globally.