Re-determination of the post-spinel phase transition boundary in Mg2SiO4 by calorimetric measurements and thermodynamic calculation

Abstract

It is thought that the 660-km seismic discontinuity in the Earth's mantle is caused by the post-spinel phase transition. The Clapeyron slope of the post-spinel boundary is an important parameter which affects mantle convection style. In this study, the phase transition enthalpy for the post-spinel transition was determined by measuring drop-solution enthalpies of Mg2SiO4 ringwoodite, MgSiO3 perovskite and MgO. Using the newly re-determined enthalpy, the post-spinel phase boundary was re-examined by thermodynamic calculation. The Clapeyron slope was obtained to be -1.2±0.3 MPa/K at 1873 K.  It is suggested that the post-spinel phase transition is a less effective barrier to the mantle convection.