Mineral physics constraints on the chemical composition of the Earth's lower mantle
Mineralogical modeling of the lower mantle critically depends on thermoelastic parameters, such as the expansion coefficient and the Grüneisen parameters. Significant experimental discrepancies exist for these properties of silicate perovskite. We calculate density and seismic velocities of mineral assemblages of (Mg, Fe)SiO_3 perovskite and (Mg, Fe)O magnesiowüstite at lower-mantle pressures and temperatures. The results are compared with seismological results (PREM; Preliminary Reference Earth Model) and a statistical test of goodness of fit is applied. We plot the chemical composition of the lower mantle as a function of the thermoelastic parameters of perovskite. The inferred chemical composition depends on the adopted properties. The present uncertainty in the data accommodates most proposed compositional models. A better understanding of lower-mantle mineralogy requires better thermoelastic data, particularly of silicate perovskite. A preferred compositional model is constrained by combining mineral physics with seismological observations of the Earth's deep interior. It appears that the upper mantle and lower mantle are chemically distinct. A chondritic lower mantle cannot be ruled out with present data.
© 1994 Elsevier Science B.V. Received 28 October 1993; revision accepted 8 March 1994. We thank Dr. Ian Jackson and two anonymous referees for their helpful reviews. Useful discussions with Drs. X. Feng, L.-S. Zhao, J. Ita and E.J. Gamero are greatly appreciated. Y. Zhao, working in the Helen and Roland Lindhurst Laboratory of Experimental Geophysics of Caltech, has benefited from a Caltech post-doctoral research fellowship. This research was partially supported by Eleanor and John R. MacMillian and NSF Grants EAR-9002947 and EAR-9218390. This paper is Contribution 5338 of the Division of Geological and Planetary Sciences, California Institute of Technology.