Carbon in the asthenosphere is unevenly distributed, reflecting interactions between the depleted mantle and materials added by plumes or subducting slabs. Nevertheless, mid-ocean ridge basalts (MORB) from the Arctic Gakkel Ridge, far from any existent hotspots and subduction zones, are unusually carbon-rich. Here we show that the MORB along the Gakkel Ridge display systematically elevated Zn isotopes (δ66Zn = 0.27‰–0.41‰) relative to typical global MORB (0.26 ± 0.03‰). Their heavy δ66Zn ratios correlate with indicators for mantle source carbon enrichment. We attribute this signature to result from recycled surficial carbonates stored in deep upper mantle under the Arctic Ocean. These carbonate components were likely delivered by ancient oceanic subduction and subsequently entrained into the mantle upwelling feeding the Gakkel Ridge. Our results demonstrate that vestiges of past subduction can dominate carbon budgets beneath plume‑unaffected ridges, and act as a principal driver of large‑scale carbon heterogeneity in the upper mantle. Zinc isotopes in basalts along a 1,100-km transect of the Gakkel Ridge record widespread mantle recycling of surface carbonates via ancient oceanic subduction, explaining the unusually carbon-rich nature of the Arctic asthenosphere.
Zhang et al. (Mon,) studied this question.