An Experimental Study Of The Reaction Between Carbon And Bridgmanite Under The Lower Mantle Pressure-Temperature Conditions

Carbon can be transported into Earth's deep interior by subduction slab.The abundance,distribution,and species of carbon could significantly affect the magma activities,redox conditions and the composition and structures of major minerals in the deep mantle.Previous studies show that the "redox freezing" of carbon could initiate at depths greater than 150 km in the upper mantle,which means the carbonates or carbonatite melts are unstable and can be reduced to immobile diamond.In this study,we investigated the interaction of carbon and(Mg,Fe)SiO3 bridgmanite under the conditions corresponding to Earth's lower mantle(35.5-88.4 GPa,1850-2800 K)by laser-heated diamond anvil cells.The run products were identified by synchrotron X-ray diffraction(XRD)measurements and ex situ transmission electron microscope X-ray energy-dispersive spectrometer(TEM-EDX).Results from XRD phase identification,unit cell analysis of bridgmanite and ex situ TEM-EDX show that(1)the Fe2+ in bridgmanite can be reduced to metallic iron(Fe0)by carbon in the SiO2 saturated system in the lower mantle pressure and temperature(P-T)conditions.(2)The iron content in bridgmanite at the carbon-bearing system decreased with increasing P-T(depth)conditions,and the oxygen fugacity of the run products dropped accordingly based on the thermodynamic calculation.(3)CO2 could be the stable phases in the subducted MORB composition in the lower mantle,the differences in redox properties between CO2 and carbonate could lead to the redox heterogeneity in the Earth's deep mantle.(4)Accumulation of carbon in the lower mantle caused by subduction slab can provide us a potential explanation for the locally highly reducing conditions in the lower mantle which have been revealed by some diamond inclusions.