Limits on the Constitution of the Lower Mantle *

Summary The limits on the constitution of the lower mantle which can be inferred from currently available data are investigated. The density and elasticity of the mantle are compared to those of various model mineral assemblages at zero pressure and at high pressures, the former case using extrapolations of mantle properties. The models are based on equations of state of MgO (periclase), SiO 2 (stishovite) and FeO (wüstite), and of the Twin Sisters and Mooihoek dunite high‐pressure phases. The bounds on the elastic properties of mixtures are discussed. The properties of hypothetical phases denser than their isochemical simple oxide mixtures are estimated relative to those of the oxide mixtures. The uncertainties in all quantities are discussed and the interdependence of some inferred mantle characteristics is considered. The inferred silica content, temperature and phase assemblage are strongly interdependent. The inferred FeO content depends to a small extent on the phase assemblage and temperature corrections and more strongly on the presence of iron in the ‘low spin’ electronic state. Models which best fit the data used range between (1) an oxide mixture, or phase assemblage with similar properties, with pyroxene composition and 9 mole per cent FeO, and (2) a phase assemblage about 5 per cent denser than an isochemical simple oxide mixture with olivine composition and 6 mole per cent FeO. Combining all uncertainties and interdependences allows the FeO content to vary between about 3 and 14 mole per cent, and the temperature to vary beyond the independently estimated bounds of 2000 °K and 6000 °K. The possibilities of an isentropic temperature gradient, chemical homogeneity, or some iron enrichment in the lower mantle relative to the upper mantle are allowed, but not required, by the data.