Subducted magnesite in serpentinite carries fluid-mobile elements and carbon into the lower mantle

Cycling incompatible elements and fluids into the mantle plays a crucial role in shaping its compositional heterogeneity through time and crustal evolution. Ocean island basalts (OIBs) and silicate inclusions in diamonds are enriched in incompatible fluid-mobile elements (FME) relative to normal mid-ocean ridge basalt (N-MORB) and primitive mantle, which is commonly interpreted to reflect the presence of recycled oceanic crust (the HIMU endmember) and/or sediment (EM endmembers) in their lower mantle sources. However, the specific mineral phases that transport these FME into the lower mantle are poorly understood. Carbonatized serpentinites have attracted relatively little attention. These rocks represent a major source of FME that may be recycled into the deep mantle. In addition, magnesite is the main carbonate phase in subducted carbonatized serpentinites. It has been found to be an inclusion in deep diamonds and, with microdiamonds, in carbonatized peridotite and can be stable at depths of at least 700 km. Here, we present a comprehensive mineralogical and geochemical investigation of magnesite (MgCO₃) within subducted Neoproterozoic carbonatized serpentinites from the Arabian–Nubian Shield, which is enriched in FME (e.g., B, Sb, As, Pb, and Mo) relative to primitive mantle. Atom probe tomography shows that these elements are more-or-less homogeneously distributed within magnesite and, thereby, structurally bound. Given that the experimentally determined stability of magnesite extends to lower mantle pressures, our findings indicate that magnesite is a major carrier of fluid-mobile elements (including carbon) into Earth’s deep interior, where it contributes to the lower mantle source of some ocean island basalts (OIBs) and superdeep diamonds.