The effect of iron on the elastic properties of ringwoodite at high pressure

Elastic wave velocities of ringwoodite with compositions of Mg₂SiO₄, (Mg_0.8Fe_0.2)₂SiO₄ and (Mg_0.5Fe_0.5)₂SiO₄ have been measured to address the effect of iron on the elastic properties of silicate spinel under high pressure. Ultrasonic measurements on specimens produced by hot-pressing at about 19 GPa and at 1200 °C were conducted at pressures up to 14 GPa at room temperature in a multianvil apparatus. Pressure was estimated from a relationship between the travel time in an Al₂O₃ buffer rod and the pressure estimated from in situ X-ray diffraction measurements. Thus, measured bulk modulus (K) of ringwoodite slightly increases with increasing iron content, while the pressure derivative of the bulk modulus remains virtually the same (K′ = 4.4 for X_Fe = Fe/(Fe + Mg) = 0-0.5). In contrast, the shear modulus (G) decreases significantly with increasing iron content, while the pressure derivative of the shear modulus slightly decreases or remains almost unchanged (G′ = 1.4-1.0 for X_Fe = 0-0.5). The effects of iron content on the elastic moduli are somewhat different from those of an earlier study using Brillion scattering method, but are consistent with the elastic moduli of the Fe₂SiO₄ end-member measured in a piston-cylinder apparatus using ultrasonic interferometry. The effects of iron on the elastic moduli of ringwoodite are described as K = 184(1) + 16(1)X_Fe (GPa) and G = 124(2) - 45(3)X_Fe (GPa), by combining the present and earlier results based on the ultrasonic interferometry at high pressure. The present result suggests that the temperature anomalies, rather than the variations of iron content in ringwoodite, are more likely causes for the observed variations in seismic velocities in the mantle transition region.