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 673 K 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 673 K; For Zabargad Island OEN (Zabg OEN), experiments were performed at 573 and 673 K. 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 660-680 cm−1 range. Similarly, splitting of a peak near 1100 cm−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 24 h from 673 K and 16.6(9) GPa were OEN. The Clapeyron slope (dP/dT) of this transition is bracketed between +0.020 to −0.0026 GPa/K for Zabg-OEN, and +0.0023 to −0.0049 GPa/K for SC-OEN. Our results suggest a possible stability field for HPCEN2 at the bottom of the upper mantle.