Elasticity of olivine and constitution of the Earth's mantle

The object of this paper is to show that Birch's recent suggestions on the occurrence of phase changes in the upper mantle are based on a pressure-density relation that is not likely to be applicable to silicate minerals, and are inconsistent with the assumption that the depth-velocity curve is continuous. It is suggested that the pressure coefficient of incompressibility (dK/dP) of a homogeneous phase may increase at low pressures and go through a maximum. Bridgman's experimental data on quartz and olivine suggest that this is so; yet the accuracy of his measurements is not sufficient to warrant any definite conclusion. There is still no way of extrapolating to zero pressure the elastic properties of the lower mantle.