The sound velocity of wüstite at high pressures: implications for low-velocity anomalies at the base of the lower mantle

Abstract

The longitudinal sound velocity (VP) and the density (ρ) of wüstite, FeO, were measured at pressures of up to 112.3 GPa and temperatures of up to 1700 K using both inelastic X-ray scattering and X-ray diffraction combined with a laser-heated diamond-anvil cell. The linear relationship between VP and ρ, Birch’s law, for wüstite can be expressed as VP = 1.55 (1) × ρ [g/cm3] − 2.03 (8) [km/s] at 300 K and VP = 1.61 (1) × ρ [kg/m3] − 2.82 (10) [km/s] at 1700 K. The sound velocity of wüstite is significantly lower than that of bridgmanite and ferropericlase under lower mantle conditions. In other words, the existence of wüstite in the lower mantle can efficiently decrease the seismic velocity. Considering its slow velocity and several mechanisms for the formation of FeO-rich regions at the core–mantle boundary, we confirm earlier suggestions indicating that wüstite enrichment at the bottom of the Earth’s mantle may contribute to the formation of denser ultra-low velocity zones. [Figure not available: see fulltext.].