THERMAL EXPANSIVITY IN THE LOWER MANTLE

The pressure dependence of the thermal expansion coefficient, alpha, previously reported as (partial derivative lnalpha/partial derivative lnV)T = 5.5 +/- 0.5 by Chopelas and Boehler [1989] is refined, using systematics in the volume dependence of (partial derivative T/partial derivative P)s measured for a large number of materials at high pressures and high temperatures. Since (partial derivative ln(partial derivative T/partial derivative P)s/partial derivative (V/V0))T is found to be constant and material independent over a very large compression range, (partial derivative lnalpha/partial derivative ln V)T is proportional to the compression, V/V0. We find a decreases by a factor of 5 for MgO throughout the mantle, reaching a value of 1.0 . 10(-5) K-1 at its bottom. Densities of perovskite (PV) and magnesiowustite (MW) are calculated for lower mantle conditions using our new alpha(P,T), a room temperature finite strain equation, and recent data on the Mg-Fe partitioning in.the PV-MW system. Both minerals have nearly identical densities to those of PREM throughout the entire lower mantle, which allows variable PV:MW ratios. A lower mantle made entirely of PV with a molar ratio of Mg:Fe of 88:12 would be about 0.11 g/cm3 or 2.5% denser than this mixture, but this density would just be within the uncertainty in PREM, A change in chemistry at 660 km depth to a PV mantle requires a thermal boundary which would improve the match in the densities between PV and PREM. These density agreements therefore preclude evaluation of a mineralogical model for the lower mantle using density comparisons. Recent measurements on melting of Fe, FeO, and FeS, however, suggest temperatures at the core-mantle boundary below 3500 K, which tends to favor a geotherm without a large thermal boundary at 660 km depth.