SOUND VELOCITIES AND ELASTIC PROPERTIES OF FE-BEARING WADSLEYITE AND RINGWOODITE

The sound velocities and single-crystal elastic moduli of beta phase ( wadsleyite) and gamma phase (ringwoodite) of (Mg,Fe)(2)SiO4 with Fe/(F e+Mg) ratios of similar to 0.075 and similar to 0.09, respectively, ha ve been determined at ambient conditions by Brillouin spectroscopy. Bo th compressional and shear wave aggregate velocities decrease with inc reasing Fe content in both phases, but the magnitude of this decrease is different for the two phases. The adiabatic bulk modulus, K-S, of F e-bearing beta-Mg2SiO4 (K-S = 170+/-2 GPa) is indistinguishable from t hat of the Mg end-member within experimental uncertainty, whereas K-S of gamma-(Mg,Fe)(2)SiO4 increases rapidly with increasing lion content . The shear moduli of both phases decrease with increasing Fe content. Our measurements indicate that the velocity and impedance contrasts b etween olivine and beta-(Mg,Fe)(2)SiO4 are independent of Fe content f or Mg-rich compositions, but the contrast for the beta --> gamma-(Mg,F e)(2)SiO4 transition increases significantly with increasing Fe conten t. The new data support a previous estimate of 40+/-10% for the olivin e content of the upper mantle and suggest that less than 50% (Mg,Fe)(2 )SiO4 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 4 10 and 520 km discontinuities with a single olivine content.