Interaction of carbonates with peridotite containing iron metal: Implications for carbon speciation in the upper mantle

Due to the significantly reduced conditions in the mantle below ~250 km, carbonates transported by the subducting oceanic plates will inevitably interact with Fe-saturated mantle peridotites to form diamonds or other reduced carbon species. In this study, we present the reaction experiments at 6 and 10 GPa, and 1100–1400°С between carbonates and peridotite with iron to model the behaviors of subducted carbonates in the reduced mantle. Magnesiowüstite (Mws), ferropericlase, metastable graphite, or diamond crystallization showed the occurrence of the redox reactions. The reactions in Ca‑carbonate and dolomite-containing systems were accompanied by the appearance of carbonate melt already at 1100 and 1200 °C. Formation of the melt at such a low temperature in the studied systems shows that the redox gradient between iron-containing mantle peridotites and carbonate-containing subduction lithologies could trigger deep mantle melting at the hot subduction zone conditions, even in the anhydrous and alkali-free systems. Carbonate melts, and oxides, formed in the redox reactions, would further interact with olivine, pyroxenes, and garnet with the formation of merwinite, Ca-rich garnet, and Fe-rich olivine. Here we confirm that merwinite could be an indicator of the mantle metasomatism and may originate via interaction of Ca-rich carbonate melt with peridotite. The kinetic calculations reflect that, in natural systems where carbonates and iron are separated by silicate media, the redox reactions are limited due to slow chemical diffusion rates in silicates and will not have any significant effect on the preservation of carbonates in the anhydrous and melt free environment. On the contrary, the presence of carbonate melt during the reaction between carbonate and metallic Fe will lead to significant consumption of carbonates.