Pressure-induced Pbca-P2₁/c phase transition of natural orthoenstatite: The effect of high temperature and its geophysical implications

In-situ high-pressure (P) high-temperature (T) Raman spectroscopy has been used to investigate the effect of temperature on the high-pressure phase transition of Mg-rich orthoenstatite (OEN) to a newly-discovered P2₁/c phase (HPCEN2) up to 673K and 18.2(10)GPa. Two natural orthoenstatite samples were used in this study: near end-member Mg orthoenstatite (Zabargad Island, Egypt), and Al+Fe-bearing orthoenstatite (San Carlos, Arizona). For San Carlos OEN (SC-OEN), the experiment was performed at room temperature, 373, 573 and 673K; For Zabargad Island OEN (Zabg OEN), experiments were performed at 573 and 673K. The three phases OEN, HPCEN2, and another high-pressure phase with space group C2/c (denoted by HPCEN) are readily distinguished by a characteristic doublet, triplet, or singlet, respectively, in the 660-680cm^-1 range. Similarly, splitting of a peak near 1100cm^-1 is indicative of an OEN→HPCEN2 transition. For both samples, no phase other than OEN and HPCEN2 was observed within the investigated P-T range. The recovered products after slow cooling for over 24h from 673K and 16.6(9)GPa were OEN. The Clapeyron slope (dP/dT) of this transition is bracketed between +0.020 to -0.0026GPa/K for Zabg-OEN, and +0.0023 to -0.0049GPa/K for SC-OEN. Our results suggest a possible stability field for HPCEN2 at the bottom of the upper mantle.