The elastic properties of β-Mg2SiO4 from 295 to 660 K and implications on the composition of Earth's upper mantle

New, high quality data are presented on the elastic properties of β-Mg2SiO4 (wadsleyite) from 295 to 660 K at ambient pressure. Elasticity measurements were carried out on a sintered polycrystal using resonant ultrasound spectroscopy (RUS). Room temperature values for the adiabatic bulk (KS) and shear (G) moduli are 170.2(1.9) and 113.9(0.7) GPa, respectively. The KS data exhibit linear dependence on temperature (T) with (∂KS/∂T)P = −1.71(5) × 10−2 GPa K−1. Our result for (∂KS/∂T)P is consistent with a relatively high magnitude for this derivative which contrasts with −1.20 × 10−2 to −1.30 × 10−2 GPa K−1 reported in some earlier studies. The average (∂G/∂T)P = −1.57(3) × 10−2 GPa K−1 over the temperature range studied. This result is consistent with most earlier measurements of (∂G/∂T)P for wadsleyite. The (∂KS/∂T)P and average (∂G/∂T)P for wadsleyite over 295-660 K are not measurably affected by the presence of iron as seen from comparing our results with those from a RUS study on β-(Mg0.91Fe0.09)2SiO4. Further, our results for (∂KS/∂T)P and average (∂G/∂T)P are consistent with olivine content of 44-54% at 410-km depth in Earth, given other assumptions made in recent studies about properties of the α- and β-olivine phases for P, T conditions at that depth. Our G(T) data appear to exhibit small, but persistent, nonlinear behavior; the magnitude of (∂G/∂T)P increases with T. We discuss the important implications on upper mantle mineralogy if this nonlinear effect is confirmed and is demonstrated to apply when extrapolating beyond the temperature range of 295-660 K.