Thermal elasticity of (Fex,Mg1-x)2SiO4 olivine and wadsleyite

We present first-principles results for elastic moduli (bulk, K, and shear, G) and acoustic velocities (compressional, V-P, shear, V-S, and bulk V-Phi) of olivine (alpha) and wadsleyite (beta) (Fe-x,Mg1-x)(2)SiO4, at high pressure (P) and temperature (T) with varying iron content (0 <= x <= 0.125). Pressure and temperature derivatives of these properties are analyzed. We show that adding 12.5% of Fe in forsterite softens V-P and V-S by similar to 3-6%, the same effect as raising temperature by similar to 1000 K in dry olivine at 13.5 GPa-the same is true in wadsleyite. This study suggests that Fe is effective in producing seismic velocity heterogeneity at upper mantle and transition zone conditions and should be another key ingredient, in addition to temperature and water content variations, in interpreting seismic heterogeneities in the transition zone. The effect of Fe on density, elastic, and velocity contrasts across the alpha ->beta transition is also addressed at relevant conditions. We show that simultaneous changes of composition, temperature, and pressure do not affect significantly the relative density contrasts. We also find that compressional and shear impedance contrasts result primarily from velocity discontinuities rather than density discontinuity. Citation: Nunez-Valdez, M., Z. Wu, Y. G. Yu, and R. M. Wentzcovitch (2013), Thermal elasticity of (Fe-x,Mg1-x)(2)SiO4 olivine and wadsleyite, Geophys. Res. Lett., 40, 290-294, doi:10.1002/grl.50131.