On the Occurrence of (Mg, Fe)SiO3 in the Lower Mantle

Abstract

Summary The purpose of this investigation involves the estimation of compression of the solid solution series (Mg, Fe)SiO, within a shock-wave induced high pressure phase, and the application of the results to an assessment of the gross chemical composition ofthe lower mantle. Raw data were provided in the form of Hugoniot measurements on several rocks of general ABO, bulk chemistry, which were available in the literature; corrections were applied according to Anderson’s ‘additivity of 0’ rule to compensate for the deviation in chemistry of each individual rock from that of an idealized (Mg, Fe)SiO, composition. The results are critically dependent upon the interpretation that the structural form of (Mg, Fe)Si03 in the high pressure modification appropriate for the lower mantle is most likely related closely to corundum or ilmenite. Reduction and analysis of the pertinent shock-wave data yield the following results for the isentropic bulk moduli of the end member compositions: KS(MgSiO,) = 2.55 Mb, and KS (FeSiO,) = 3.28 Mb; the estimated uncertainty in these values is 10 per cent. The results of the study were used to determine the composition of the lower mantle in terms of the relative proportions of pyroxene (Mg, Fe)SiO, and olivine (Mg, Fe)2Si04. Comparison of the seismic parameter @ and density for the high pressure phase of pyroxene considered in this investigation, with the elastic requirements of the lower mantle, indicate that the pyroxene component of the lower mantle, relative to olivine, must be minor. Therefore, in terms of this simple two-component model, a fundamental olivine (Mg, Fe),Si04 chemistry remains the most likely possibility for the bulk composition of the lower mantle.