Sound velocities and elastic properties of Fe‐bearing wadsleyite and ringwoodite

Abstract

The sound velocities and single‐crystal elastic moduli of β phase (wadsleyite) and γ phase (ringwoodite) of (Mg,Fe) 2 SiO 4 with Fe/(Fe+Mg) ratios of ∼0.075 and ∼0.09, respectively, have been determined at ambient conditions by Brillouin spectroscopy. Both compressional and shear wave aggregate velocities decrease with increasing Fe content in both phases, but the magnitude of this decrease is different for the two phases. The adiabatic bulk modulus, K s , of Fe;‐bearing β‐Mg 2 SiO 4 ( K s = 170±2 GPa) is indistinguishable from that of the Mg end‐member within experimental uncertainty, whereas K s of γ‐(Mg,Fe) 2 SiO 4 increases rapidly with increasing iron content. The shear moduli of both phases decrease with increasing Fe content. Our measurements indicate that the velocity and impedance contrasts between olivine and β‐(Mg,Fe) 2 SiO 4 are independent of Fe content for Mg‐rich compositions, but the contrast for the β → γ‐(Mg,Fe) 2 SiO 4 transition increases significantly with increasing Fe content. The new data support a previous estimate of 40±10% for the olivine content of the upper mantle and suggest that less than 50% (Mg,Fe) 2 SiO 4 is sufficient to account for the observed impedance contrasts at depths of both 410 km and 520 km. Unless the effect of Fe on elastic properties is accounted for, it is difficult to account for both the 410 and 520 km discontinuities with a single olivine content.