STABILITY OF CLINOPYROXENE AT PRESSURE-TEMPERATURE CONDITIONS OF THE TRANSITION REGION

The stability of diopside at conditions of 16-19 GPa and 1250-1900 degrees C was determined using a 6-8 type multi-anvil apparatus. At pressures of 17-18 GPa, diopside undergoes a decomposition reaction to an assemblage of CaSiO3-perovskite + (Ca,Mg)SiO3-garnet or -ilmenite, depending on temperature. The Clapeyron slope for the breakdown reaction: [GRAPHICS] is -2 MPa K-1. When coupled with a re-examination of clinopyroxene stability in other more complex peridotite systems, the experimental data are consistent with a recent proposal that the 520 km discontinuity in the transition region is caused by the exsolution of CaSiO3-rich perovskite from garnet, rich in majorite component.