MgSiO_3 phase boundaries measured in the laser-heated diamond cell

摘要

New data on phase equilibria in the MgSiO_3 system relevant to the stability of Mg–Si-perovskite (perovskite) are obtained in the laser-heated diamond cell using improved techniques for accurate pressure measurement and novel heating techniques eliminating temperature gradients. The data of the MgSiO_3-ilmenite (akimotoite)–perovskite phase boundary, measured in the range of 19–25.5 GPa and 1800–2350 K, are consistent with the pressure and temperature conditions generally assumed at 660 km depth and agree within the experimental uncertainties with earlier multi-anvil quench experiments. The triple point wadsleyite+stishovite–akimotoite–perovskite is at 21 GPa and 2300 K. The slope of the akimotoite–perovskite boundary is not well constrained by reversal experiments alone due to large hysteresis, but results using glass starting materials yield a value of -0.004±0.002 GPa/K. The slopes of the phase transitions to Mg–Si-perovskite in both olivine and pyroxene systems are less negative than those required to cause layering in mantle convection. At the pressure of the 660 km seismic discontinuity (23.8 GPa) the phase boundary between akimotoite and perovskite yields a slightly lower temperature than that of the ringwoodite to perovskite+periclase transition in the Mg2SiO_4 system (1900±100 K). Garnet (majorite) is not observed above 21 GPa.