Sound wave velocities and elastic constants for magnesiowustite using gigahertz interferometry

Ultrasonic interferometry at near-gigahertz frequencies is used to investig ate longitudinal sound wave velocities parallel to [100] and [111] and the elastic constant c(11) for high quality single crystals of magnesiowustite (Mg,Fe)O. The measurements were made at room pressure and temperature on sa mples spanning the full compositional range from periclase (MgO) to wustite (Fe1-xO). A strong P-wave anisotropy of similar to 10% was measured for pe riclase, increasing about 1% for Mg0.94Fe0.06O then decreasing linearly to similar to 0% for wustite. The elastic modulus c(11) of magnesiowustite dec reases linearly with iron content by -0.72(3) GPa/mol% FeO with the excepti on of a: sharp increase for pure periclase. Thus, the addition of a small a mount of iron to periclase has a major effect on its elastic behavior. This observation indicates that the use of a simple interpolation of elastic co nstant data between periclase and wustite [Jackson, 1998] may yield erroneo us predictions for lower mantle compositions based on seismic data.