Abstract
In this study, we have measured the compressional and shear wave velocities of (Mg1.77Fe0.22Ca0.01) Si2O6 natural orthopyroxene up to 13.5 GPa and 873 K using ultrasonic interferometry in conjunction with in situ synchrotron X-ray diffraction and imaging techniques. Previous acoustic experiments on orthoenstatite (OEn) MgSiO3 indicated that both compressional and shear velocities (VP and VS) of OEn undergo continuous velocity softening above 9 GPa at room temperature, which has been attributed to the phase transition from OEn to the metastable, high-pressure clinoenstatite HPCEn2. For the first time, our results suggest that pressure-induced velocity softening can occur in natural orthopyroxene at high-temperature conditions relevant to the Earth’s cold subduction zones. Estimates of the impedance and velocity contrasts between orthopyroxene (Opx) and high-pressure clinopyroxene (HPCpx) have been calculated, and the possibility of this phase transformation being a plausible candidate for seismic X-discontinuities at depth around 250–350 km is re-evaluated.
Acknowledgments
The authors thank Yanbin Wang and Tony Yu for help in conducting the ultrasonic experiment at the Advanced Photon Source and David Welch for valuable discussions. Ting Chen thanks the Geodynamic Research Center (Ehime University, Japan) for a summer internship during which the SEM analysis was done.
Funding: This project is supported by the National Science Foundation (EAR-1524078) and DOE-NNSA (DE-NA0002907). GeoSoilEnviroCARS is supported by the National Science Foundation—Earth Sciences (EAR-1634415) and Department of Energy-GeoSciences (DE-FG02-94ER14466). This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. MPI publication number 510.
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