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 (vP/vS). 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.

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