Carbon’s Role in Mantle Melting – Where it is Important and Where it is Not, and Implications for the Carbon Heterogeneity of the Mantle

Carbon is present in the Earth’s mantle as a trace element; yet, the mantle is the largest reservoir of carbon, which modulates the composition of the Earth’s atmosphere. Unlike other trace elements, however, carbon, in oxidized form, affects mantle melting phase equilibria. Therefore, the compositions of the mantle-derived partial melts can, in theory, be used to decipher the presence and even concentrations of carbon in the mantle source regions for various volcanic centers. However, such an approach requires both a careful estimation of the primary melt compositions from natural samples and reliable experimental constraints on the partial melt compositions of mantle-equilibrated melts in the presence of carbon.

Here, using experimental phase equilibria and major element compositions of intraplate ocean island basalts, I will discuss how the mantle source regions of intraplate volcanism are generally more carbon-rich compared to the ambient mantle (Sun and Dasgupta, 2023 – EPSL). Future studies will need to assess whether such carbon-enriched deep mantle domains reflect primordial reservoirs or reservoirs modified by subducted carbon. I will also present recently published experimental results on mantle melting with low and variable bulk molar XCO₂ [CO₂/(CO₂+H₂O)] (0.0-0.17) at 2-4 GPa and 1200-1350 °C, aimed at constraining the effects of variable CO₂ in slab-derived H₂O-rich fluid fluxing the mantle wedge (Lara and Dasgupta, 2022 – EPSL; 2023 – JPet). The experimental partial melts show systematic evolution toward silica undersaturation with increasing bulk XCO₂ of the system. A comparison between our experimental partial melt compositions and a global dataset of the most primitive arc magmas suggests that the upper limit of XCO₂ in fluids inducing melting in mantle wedges is ∼0.10 at 2–4 GPa. This suggests that the sub-arc mantle domains are carbon-poor despite slab modification. Application of these new constraints to an H₂O and CO₂ mass balance model for subduction zones reveals that ∼35–85% of CO₂ entering subduction zones bypasses the sub-arc melt generation zone and is subducted to the convecting mantle, either carried by the slab or by the down-dragged limb of the mantle wedge directly above the slab.