In situ X‐ray observations of phase assemblages in peridotite and basalt compositions at lower mantle conditions: Implications for density of subducted oceanic plate

Abstract

Phase relationships in KLB‐1 peridotite, normal‐type mid‐oceanic ridge basalt (NMORB) and K‐rich basalt compositions were investigated experimentally from 28 to 143 GPa and 2050 to 3000 K. A laser‐heated diamond anvil cell with the synchrotron X‐ray diffraction was developed to perform in situ observations of samples at simultaneous high pressures and temperatures. We confirmed that the peridotite composition crystallized into an assemblage of Mg perovskite + Ca perovskite + magnesiowüstite at pressures below 97 GPa and that NMORB and K‐rich basalt compositions both crystallized as Mg perovskite + Ca perovskite + stishovite + an aluminous phase with a CaFe2O4‐type structure at pressures below 68 GPa. The orthorhombic Mg perovskite is stable in natural rock compositions at pressures less than 97 GPa, corresponding to about 2200 km depth. Phase transition of silica phase from stishovite to CaCl2‐type phase was observed at 78 GPa in NMORB composition. Moreover, we observed that NMORB crystallized in to an assemblage of CaIrO3‐type (Mg,Fe)SiO3 + Ca perovskite + α‐PbO2‐type silica + an aluminous phase with a calcium CaTi2O4‐type structure at 143 GPa, corresponding to the core‐mantle boundary depth. We measured the unit cell volumes of minerals in the NMORB composition at high P‐T and thereby estimated the density of subducted MORB to a depth of 1800 km in the lower mantle. In this depth regime, MORB is denser than model mantle densities, e.g., PREM and AK135. The estimated density of MORB may contribute to an understanding of dynamics of the subducted slab in the deep lower mantle.