FEATURES AND CONDITIONS FOR THE INTRACRYSTALLINE TRANSFORMATION FROM OLIVINE TO HIGH-PRESSURE POLYMORPHS IN SHOCK-METAMORPHOSED METEORITES

An investigation into two shock-metamorphosed chondritic meteorites indicates that the intracrystalline transformations from olivine to high pressure polymorphs could take place at relatively low temperature(1000 ℃) in a large pressure regime(14.5 to 23 GPa).The intracrystalline transformation of olivine produced lamellar ringwoodite.No direct intracrystalline transformation from olivine to wadsleyite was observed.The lamellar ringwoodite consisting of crystallite aggregates replaced parent olivine along staking faults and planar fractures.An association consisting of wadsleyite and ringwoodite platelets,respectively,has also been observed in olivine of some meteorites.The wadsleyite was produced by replacing ringwoodite during pressure release as a result of retrograde-metamorphism.This study provides information for understanding geological settings and pressure and temperature conditions for the polymorphic transformation of olivine in the Earth's lithospheric subducting slabs.