Phase transition and density of subducted MORB crust in the lower mantle

Phase relations, mineral chemistry, and density of a natural mid-oceanic ridge basalt (MORB) composition were investigated up to 134 GPa and 2300 K by a combination of in-situ X-ray diffraction measurements and chemical analyses using transmission electron microscope (TEM). Results demonstrate that the MORB composition consists of MgSiO3-rich perovskite, stishovite, CaSiO3 perovskite, and CaFe2O4-type Al-phase in the upper part of the lower mantle. The most abundant mineral of MgSiO3-rich perovskite undergoes phase transition to a CaIrO3-type post-perovskite phase above 110 GPa and 2500 K. Post-perovskite phase is similar in composition to perovskite except considerably high Na2O content. Stishovite transforms to CaCl2-type SiO2 phase above 62 GPa and 2000 K and further to α-PbO2-type phase above 110 GPa. α-PbO2-type SiO2 phase includes large amount of Al2O3, which significantly expands its stability relative to CaCl2-type phase. Phase transition of CaSiO3 perovskite from tetragonal to cubic was also observed with increasing temperature. CaFe2O4-type Al-phase is stable to the bottom of the mantle. The density of MORB crust was calculated using volume data, combining with measured chemical compositions and calculated mineral proportions. The former MORB crust is denser than the average lower mantle at all depths greater than ∼ 720 km, contrary to earlier predictions. The subducted basaltic crust may have accumulated at the base of the mantle.