TEMPERATURE AND CHEMISTRY OF THE CORE-MANTLE BOUNDARY

Abstract

Seismic anomaly structures at the bottom of the lower mantle may be interpreted in terms of large temperature variations based on new laboratory data at very high pressure: (1) sound velocity measurements at mantle pressures show an increase in (#lnρ#lnυ)p with depth (this and the strong decrease in the thermal expansion coefficient yield thermal anomalies δT of > 1000 K using the seismically measured lateral velocity variations); (2) high melting gradients in Fe and Fe-O compounds measured at core pressures yield a temperature increase across the core-mantle boundary in excess of 1300 K; (3) melting temperatures of (Mg,Fe)SiO3 perovskite measured to deep mantle conditions extrapolate to > 7500 K at the bottom of the mantle and result inTTm -values between 0.3 and 0.4 (this may strongly decrease the temperature dependence of viscosity if the average temperature profile of the lower mantle is nearly adiabatic, and if the viscosity systematics found at low pressure are applicable at lower mantle conditions); and (4) for dry conditions, MgSiO3, in contrast to MgO, does not react with molten iron at pressures of the core-mantle boundary (this suggests, that chemical interactions between the mantle and the core play a minor role).