Compressional and shear wave velocities of Fe₂SiO₄ spinel at high pressure and high temperature

Ultrasonic interferometric measurements on polycrystalline Fe₂SiO₄ spinel were conducted simultaneously with synchrotron X-ray diffraction and X-ray imaging up to 6.5 GPa, 1073 K. The compressional and shear wave velocity data and the volume data were fitted to the third-order finite strain equations to derive the bulk and shear modulus and their pressure and temperature derivatives. The fitting results are as follows: K _s0=204.5(7) GPa,=73.6(3) GPa, K′ _s =4.3(3), G′=1.2(1), (∂ K _s /∂ T) _p =−0.027(2) GPa/K, and (∂ G/∂ T) _p =−0.017(1) GPa/K. Comparison of our current results with previous data on (Mg,Fe)₂SiO₄ spinel with different compositions suggests that the bulk modulus (K _s ) increases slightly with increasing iron content, while the shear modulus (G), in contrast, shows a dramatic decrease. However, the pressure and temperature derivatives of K _s and G remain nearly constant from Mg₂SiO₄ to Fe₂SiO₄ spinel with average values of 4.2–4.4, 1.2–1.3,−0.024 GPa/K, and−0.016 GPa/K for K′ _s , G′, (∂ K _s /∂ T) _p , and (∂ G/∂ T) _p , respectively. The proposed version of equations to describe the effects of iron on the elastic moduli of ringwoodite are: K _s =184.7+18.0 X _Fe, and G=118.7−41.5 X _Fe.

Keywords

• ultrasonic interferometry,
• Fe2SiO4 spinel,
• elastic properties,
• iron content