Abstract

In situ carbon isotopes are a powerful but under-utilised tracer of mantle processes, in part due to analytical challenges and lack of reference materials. In this talk, I present a new SIMS method for concurrently measuring carbon isotopes and carbon concentration in basaltic glass, enabling improved precision at low carbon concentrations and high spatial resolution. Applying this method to olivine-hosted melt inclusions, I first describe recent work constraining the convecting upper mantle, which yields a more positive and more tightly defined δ13C value than previously assumed, along with a lower organic burial fraction of carbon. I then present results from the Icelandic primordial mantle reservoir sampled at Miðfell, revealing anomalously heavy carbon isotopic compositions coupled with depleted incompatible element concentrations, potentially linking early core formation to modern mantle heterogeneity. Together, these results refine the mantle carbon baseline and deep carbon cycle and demonstrate how melt inclusions retain isotopic memory of Earth’s interior processes.