Elastic properties of iron-bearing wadsleyite to 17.7GPa: 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 17.7GPa. Pressure derivatives for the adiabatic bulk modulus (K_S0) 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 7.5mol% 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 410km seismic discontinuity at the top of the transition zone.