Determination of the phase boundary between ilmenite and perovskite in MgSiO3 by in situ X-ray diffraction and quench experiments

Determination of the phase boundary between ilmenite and perovskite structu res in MgSiO3 has been made at pressures between 18 and 24 GPa and temperat ures up to 2000 degrees C by in situ X-ray diffraction measurements using s ynchrotron radiation and quench experiments. It was difficult to precisely define the phase boundary by the present in situ X-ray observations, becaus e the grain growth of ilmenite hindered the estimation of relative abundanc es of these phases. Moreover, the slow reaction kinetics between these two phases made it difficult to determine the phase boundary by changing pressu re and temperature conditions during in situ X-ray diffraction measurements , Nevertheless, the phase boundary was well constrained by quench method wi th a pressure calibration based on the spinel-post-spinel boundary of Mg2Si O4 determined by in situ X-ray experiments. This yielded the ilmenite-perov skite phase boundary of P (GPa) = 25.0 (+/-0.2) 0.003 T (degrees C) for a t emperature range of 1200 1800 degrees C, which is generally consistent with the results of the present in situ X-ray diffraction measurements within t he uncertainty of similar to +/-0.5 GPa, The phase boundary thus determined between ilmenite and perovskite phases in MgSiO3 is slightly (similar to 0 .5 GPa) lower than that of the spinel-post-spinel transformation in Mg2SiO4 .