New Constraints on Mantle Carbon from Mid-Atlantic Ridge Popping Rocks

Despite the influence of mantle carbon on melt formation and migration, global volatile budgets, and volcanic eruption styles, the carbon concentration in Earth's upper mantle remains highly debated, with estimates varying by more than an order of magnitude. The relationship between carbon and incompatible trace element (e.g., Nb, Ba) concentrations in rare, undegassed mid-ocean ridge basalts and melt inclusions provide primary constraints on upper mantle carbon content. Here we investigate whether the most volatile rich mid-ocean ridge basalts, termed ‘popping rocks’ represent undegassed magmas from the upper mantle and provide insight into upper mantle carbon inventory. We show that fourteen new popping rocks, collected in situ from the Mid-Atlantic Ridge rift valley near 14°N, contain highly variable CO₂/Nb and CO₂/Ba ratios despite similar mantle sources and extents of melting. We revise the original model for popping rock formation using seafloor observations, high-resolution bathymetry, vesicle size distributions, major and trace element geochemistry, and noble gas geochemistry. Highly variable volatile concentrations despite relatively homogeneous trace element ratios and low ⁴He/⁴⁰Ar^⁎ suggest that bubble accumulation affected these popping rocks. These results provide evidence for heterogeneity in the CO₂/Ba ratio of the depleted mantle and indicate that mantle carbon concentrations are lower and less heterogeneous than previously estimated, which influences models for mantle melting and CO₂ flux at mid-ocean ridges.