5.2 WALTER Michael

The Fate of Carbonate in Oceanic Crust Subducted into Earth’s Deep Mantle

Michael J. Waltera, James W. E. Drewitta, Sorcha C. McMahona, Hongluo Zhanga,b, Oliver T. Lorda, Benedict Heinena, and Annette K. Kleppec

a School of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, BS8 1RJ, United Kingdom

b Faculty of Geochemistry and Environment Science, Research Building, ROM 723, School of Earth and Space Sciences, University of Science and Technology of China, 96 Jinzhai RD, Hefei, Anhui 230026, P. R. China

c Diamond Light Source Ltd, Diamond House, Harwell Science and Innovation Campus, Chilton, OX11 0DE, United Kingdom

The H/C ratio is higher in earth’s exosphere than it is in the primitive mantle, suggesting an enriched carbon reservoir in the mantle. A plausible explanation is that subduction of carbon may have enriched the mantle over time. Average basaltic crust contains ~ 2.5 wt.% CO2, and modeling of slab devolatilisation suggests that more than half of subducted carbonate may be transported into the deeper mantle. Carbonated oceanic crust may melt in the transition zone due to a deep trough in the carbonated basalt solidus, and mineral inclusions in superdeep diamonds testify to carbonate melt in their formation. Along cooler subduction geotherms carbonate may subduct into the lower mantle. Here we report on laser-heated diamond anvil cell experiments in the CaO-MgO-SiO2-CO2 and FeO-MgO-SiO2-CO2 systems at lower mantle pressures where we test for decarbonation and diamond forming reactions. We find that carbonate reacts with silica to form bridgmanite ± Ca-perovskite + CO2 at pressures in the range of ~40 to 70 GPa. These reactions form an impenetrable barrier to subduction of carbonate into the deeper lower mantle, however, slabs may carry solid CO2 (Phase V) into the deep lower mantle. We also identify reactions where carbonate or CO2 dissociate to form diamond plus oxygen. At the predicted oxygen fugacity of the lower mantle diamond is stable along a mantle adiabat. We suggest that the deep lower mantle may become enriched in diamond over time due to subduction of carbonate and solid CO2 and its eventual dissociation to form diamond plus oxygen.