Elastic properties of iron-bearing wadsleyite to 17.7 GPa: Implications for mantle mineral models

The sound velocities and single-crystal elastic moduli of iron-bearing wadsleyite with [Fe]/[Fe + Mg] molar ratio of 0.075 have been measured by Brillouin scattering experiments at high pressures up to 17.7 GPa. Pressure derivatives for the adiabatic bulk modulus (KS0) and shear modulus (μ0) are 4.1(1) and 1.45(4), respectively. A comparison of our results with previous Brillouin scattering results on the Mg end-member wadsleyite shows that incorporating 7.5 mol% iron in wadsleyite at high-pressure conditions decreases the shear moduli by ∼4–5%, but does not have a discernable effect on the bulk modulus. The effects of iron on the elastic moduli of wadsleyite at ambient pressure persist to high-pressure conditions at a relatively constant level. Using our results on iron-bearing wadsleyite at high pressure, we conclude that less olivine than in the pyrolite model of mantle composition provides a satisfactory explanation for 410 km seismic discontinuity at the top of the transition zone.