A NEW APPROACH TO LASER-HEATING IN HIGH-PRESSURE MINERAL PHYSICS

Single crystals of olivine, forsterite, and enstatite were heated to over 2000 K between 100 and 250 kbar with a 120 W CO2-laser in a diamond anvil cell using a hydrostatic, inert gas pressure medium. The conversion to some of their high pressure polymorphs, beta-phase, gamma-phase, perovskite, majorite, ilmenite, and clinoenstatite yielded single crystals large enough to obtain high quality Raman spectra, thus providing a quick, economic, and precise method to measure phase diagrams without pressure quenching. The phase boundaries between olivine or forsterite and beta-phase and between gamma-phase and peroviskite were accurately bracketed. Our preliminary results on the latter transition are within the P-T uncertainties of previous measurements using multi-anvil devices [Ito and Takahashi, 1989]. constraining the temperature at 670 km depth to 1900 +/- 100 K, which is about 300 K higher than using their best estimate of the phase boundary. The unquenchable phase of MgSiO3-clinopyroxene was synthesized at approximately 170 kbar; Raman spectra show that this phase is of C2/c symmetry. Its stability field is between 50 and approximately 165 kbar at ambient temperature. MgSiO3 perovskite was synthesized above 230 kbar and approximately 2000 K. Raman spectra were measured from 75 to 470 kbar. Above 370 kbar, two modes disappear and two others change slope with pressure, suggesting a change in symmetry from Pbnm at room temperature.