Carbonates from subducted oceanic lithosphere may be remixed into Earth’s deep mantle via both redox “freezing” and melting that is driven by carbon and iron redox coupling, caused by redox capacity changes in the mantle. A recent study  demonstrated experimentally that carbonatite melts are unstable when introduced to the ambient mantle—becoming reduced to immobile diamond below ~250 km, where the mantle is metal saturated. These melts cannot escape through the surrounding metal-bearing average mantle but will “freeze” until redox control shifts from metal saturation back to bulk Fe2+/Fe3+ controlled at ~ 250km in depth.This redox “freezing” process leads to diamond-enriched mantle domains where Fe3+ is preserved and Fe0 is consumed, resulting from Fe2+ disproportionation in perovskites and garnet. When these carbon-enriched mantle heterogeneities become part of the upwelling mantle, the diamond reacts with Fe3+ leading to true carbonatite redox melting to form deep mantle melts at ~660 km and ~250 km—levels where the capacity of the mantle phases to incorporate Fe3+ changes drastically.
Figure: Carbonatitic redox freezing and redox melting caused by redox capacity changes in Earth’s mantle.