In situ ultrasonic velocity measurements across the olivine-spinel transformation in Fe₂SiO₄

Compressional (P) and shear (S) wave velocities across the olivine-spinel transformation in Fe₂SiO₄ were investigated in situ using combined synchrotron X-ray diffraction, X-ray imaging, and ultrasonic interferometry up to 5.5 GPa along the 1173 K isotherm. The onset of the spinel to olivine transformation at 4.5 GPa and olivine to spinel transition for Fe₂SiO₄ at 4.8 GPa was concurrently observed from X-ray diffraction, the amplitude of the ultrasonic signals, the calculated velocities, and the ratio of P and S wave velocities (v_P/v_S). No velocity softening was observed prior to the fayalite to spinel transition. The velocity contrasts across the Fe₂SiO₄ spinel to fayalite phase transition are derived directly from the measured velocities, which are 13 and 12% for P and S waves, respectively, together with a density contrast of 9.4%. A comparison with literature data indicates that the changes in compressional-wave velocity and density across the olivine-spinel transformation in Fe₂SiO₄ are comparable to those with different iron concentrations in the (Mg,Fe)₂SiO₄ solid solution, whereas the shear wave velocity contrast decreases slightly with increasing iron concentration.