HIGH TEMPERATURE-PRESSURE PHASE BOUNDARIES IN SILICATE SYSTEMS USING IN SITU X-RAY DIFFRACTION

Abstract

Phase boundaries between olivine and spinel in Fe2SiO4, between coesite and stishovite in SiO2, and between pyroxene and ilmenite in ZnSiO3 were determined by in situ X-ray diffraction measurements using cubic-anvil-type high pressure apparatus. Pressures at high temperatures were determined by X-ray diffraction from NaCl, the internal pressure standard. The usual Debye-Scherrer configuration was used in the investigation of SiO2 and ZnSiO3, while an energy dispersive X-ray diffraction technique was used for preliminary investigation of the olivine-spinel phase boundary of Fe2SiO4. The following equations for the phase boundaries were determined: olivine-spinel transition in Fe2SiO4, P(kbar) = 34.6 + 0.025T(°C); coesite-stishovite transition in SiO2, P(kbar) = 80 + 0.011T(°C); pyroxene-ilmenite transition in ZnSiO3, P(kbar) = 91 + 0.02T(°C). Present results form a set of pressure fixed-points at high temperatures in the 50 to 130 kbar range.