Carbon budget of Earth's deep mantle constrained by petrogenesis of silica-poor ocean island basalts

Abstract

The convecting mantle is the Earth's largest carbon reservoir that controls the carbon outflux from the interior to surface, but its carbon content remains largely uncertain due to limited constraints on carbon inventory of the deep mantle. This knowledge gap can be filled by a global assessment of CO2 in ocean island basalts (OIBs), which are associated with upwelling mantle plumes from the deep mantle. However, this approach is hindered because of limited samples of undegassed, primary melts from intraplate magmatic settings and the debate on the origin of volatile-rich alkaline OIBs from plume versus lithospheric mantle. Here we examine the origin of silica-poor, alkaline OIBs and constrain CO2 in the primary melts of global OIBs by applying the newly developed liquid-based thermobarometer and primary melt correction scheme for silica-poor mantle-derived magmas. The averaged primary melt compositions of alkaline OIBs from individual island groups are more CO2-rich (∼3–11 wt% CO2) than those of subalkaline OIBs (∼0–7 wt% CO2), but both series from the same island groups yield final equilibration with the mantle at ∼3–5 GPa and mantle potential temperatures of ∼1430–1530°C. Our findings indicate that alkaline and subalkaline OIBs are generated from carbon-rich and carbon-poor domains, respectively, in the mantle plume sources, rather than deriving from two different sources in the plume and lithosphere mantle or variable extents of melting of a source with identical carbon content. Despite the source heterogeneity between alkaline and subalkaline OIBs, CO2 in the averaged primary melts of individual island groups displays strong positive correlations with Nb and Ba, which define robust CO2/Nb (1850 ± 196) and CO2/Ba ratios (226 ± 22) for the less-degassed deep, OIB source mantle. Combining these ratios with primitive mantle Nb and Ba contents, we estimate that the deep mantle on average has ∼330–400 ppm carbon. With an averaged CO2 content of ∼4 wt% in their primary melts, global OIBs are likely produced from the deep mantle source by ∼3% melting. The mean CO2 content in the primary melts of global OIBs, together with the previous estimate of plume magma production rate, yields a flux of ∼5.5 × 1012 mol/yr CO2 or ∼66 Mt/yr carbon through ocean island volcanism. If the OIB source mantle carbon budget is mostly primordial, our estimation provides a useful reference point for undifferentiated bulk silicate Earth to further examine carbon distribution and/or loss through Earth's history.