A NEW HIGH-PRESSURE PHASE TRANSITION IN IRON-BEARING ORTHOENSTATITE: AN ANISOTROPY DISCONTINUITY IN THE UPPER MANTLE?

Sound velocities and elastic moduli have been measured on iron-bearing orthoenstatite single crystals up to 12.00 GPa by Brillouin spectroscopy. A fourth order finite strain fit to the data yields pressure derivatives of Ks = 9.2(2) and G = 3.0(1) for the adiabatic bulk modulus (Ks) and shear modulus G respectively. These values are significantly higher than those for other major mantle minerals. A phase transition from orthoenstatite to a new monoclinic high pressure phase with space group P2₁/c was demonstrated by in-situ X-ray diffraction. The transformation was reversed and occurs between 9.96 and 11.54 GPa. This transformation is accompanied by a pronounced increase of velocity anisotropy, and a density increase of ~1.6(3)%. The seismic signature of this phase transformation to a high-pressure pyroxene phase in the upper mantle may be a jump in anisotropy.