Static compression and olivine flotation in ultrabasic silicate liquid

Piston-cylinder techniques have been used to observe static compressions of ultrabasic silicate liquids to 6 GPa and 2000°C. Compressions are measured by bracketing sinking and floating of olivine crystals of known density at run conditions. Komatiite, peridotite, and their liquid mixtures with Fe₂SiO₄ all show constant density changes with pressure of ≅ 0.07 g/cm³/GPa along their liquidi. These results confirm shock wave studies and ultrasonic studies which show weak compositional effects on compressibility of basic to ultrabasic silicate liquid. The techniques converge in supporting the conjecture of Stolper et al. (1981) that ultrabasic silicate liquids may be denser than their equilibrium coexisting olivines and pyroxenes at high compressions. The pressure of this density inversion is 7-8 GPa for the compositions studied. Olivine flotation provides an important constraint upon Earth evolution models that include extensive melting during early differentiation. It also provides a mechanism for concentrating high Mg/Si olivine in the upper mantle without unduly perturbing other geochemical parameters. A full evaluation of the physical and geochemical consequences is required at an early date.