New High‐Pressure Phase of CaCO3 at the Topmost Lower Mantle: Implication for the Deep‐Mantle Carbon Transportation

Abstract

AbstractIn this study, we have investigated the stability of CaCO3 at high pressures and temperatures using synchrotron X‐ray diffraction in laser‐heated diamond anvil cells. Our experimental results have shown that CaCO3 in the aragonite structure transforms into CaCO3‐VII (P21/c) at 27 GPa and 1,500 K with a negative Clapeyron slope of −4.3(9) MPa/K. CaCO3‐VII is stable between 23 and 38 GPa at 2,300 K and transforms into post‐aragonite at 42 GPa and 1,400 K. Furthermore, it reacts with stishovite, an abundant form of SiO2 in subducted oceanic crust, forming CaSiO3‐perovskite. The occurrence of CaSiO3‐perovskite via the reaction of CaCO3‐VII and stishovite provides an explanation for the observation of the high concentrations of CaSiO3‐perovskite and some amount of CaCO3 in deep‐mantle inclusions. CaCO3‐VII is thus an important carbon‐bearing phase at the topmost lower mantle and may provide necessary carbon to produce deep‐mantle diamonds.