Elasticity and Composition of the Upper Mantle

Summary This paper is concerned with the elasticity and composition of the upper mantle as inferred from recent laboratory ultra‐sonic and phase‐stability data. The seismically‐determined compressional and shear velocity structure of the upper mantle, as well as the density distribution, is initially discussed in terms of homogeneous olivine models of variable fayalite content. The elastic properties of the olivine solid solution series are defined by a least squares linear fit to existing acoustic and static compression data from several single‐crystal and polycrystal specimens of forsterite and fayalite. Compressional and shear velocity profiles are calculated for olivine models of 0, 10 and 20 mol per cent fayalite using geothermal models defined by requiring a satisfactory fit between the model V p profile and a standard seismic compressional wave profile. The significance of the calculated velocity curves is discussed in terms of the estimated uncertainties in the ultra‐sonic input data. On a basis of the elastic and density requirements alone, the 10 mol per cent fayalite model appears to be most satisfactory. In addition to consideration of the elastic requirements, the stability relations of the olivine‐spinel phase transformation provide an additional constraint on the proportion of fayalite fraction. At the 370‐km seismic discontinuity, assumed to be the change in velocity associated with the olivine‐spinel transition, the combined elastic, density, and geochemical requirements are satisfied with an olivine model of composition Fo 88 Fa 12 . Assuming that the olivine approximation is reasonably valid, the Mg/Mg+Fe ratio at the top of the 370‐km discontinuity is 0.88 ± 0.03, and the temperature is 1450±120°C. Lastly, the effects of other mineral phases such as pyroxene and garnet are assessed. It is concluded that the addition of relatively minor fractions of other phases, for example, an assemblage of garnet pyrolite, would not significantly alter the present results in terms of presently available data. That is, the elasticity, density, and structure of the upper mantle below the low‐velocity zone, are satisfied self‐consistently by a peridotite model.