Comparative Compressibilities of Silicate Spinels: Anomalous Behavior of (Mg,Fe)2SiO4

Compressibilities of five silicate spinels, including gamma-Mg(2)SiO(4), gamma-Fe(2)SiO(4), Ni(2)SiO(4) and two ferromagnesian compositions, were determined on crystals positioned in the same high-pressure mount. Subjection of all crystals simultaneously to the same pressure revealed differences in compressibility that resulted from compositional differences. Ferromagnesian silicate spinels showed an anomalous 13 percent increase in bulk modulus with increasing iron content, from Mg(2)SiO(4) (184 gigapascals) to Fe(2)SiO(4) (207 gigapascals). This result suggests that ferrous iron and magnesium, which behave similarly under crustal conditions, are chemically more distinct at high pressures characteristic of the transition zone and lower mantle.

Thermoelasticity of Silicate Perovskite and Magnesiowüstite and Stratification of the Earth's Mantle

Analyses of x-ray-diffraction measurements on (Mg,Fe)SiO(3) perovskite and (Mg,Fe)O magnesiowüstite at simultaneous high temperature and pressure are used to determine pressure-volume-temperature equations of state and thermoelastic properties of these lower mantle minerals. Detailed comparison with the seismically observed density and bulk sound velocity profiles of the lower mantle does not support models of this region that assume compositions identical to that of the upper mantle. The data are consistent with lower mantle compositions consisting of nearly pure perovskite (>85 percent), which would indicate that the Earth's mantle is compositionally, and by implication, dynamically stratified.

Phase Transition and Thermal Expansion of MgSiO 3 Perovskite

Results from in situ x-ray diffraction experiments with a DIA-type cubic anvil apparatus (SAM 85) reveal that MgSiO(3) perovskite transforms from the orthorhombic Pbnm symmetry to another perovskite-type structure above 600 kelvin (K) at pressures of 7.3 gigapascals; the apparent volume increase across the transition is 0.7%. Unit-cell volume increased linearly with temperature, both below (1.44 x 10(-5) K(-1)) and above (1.55 x 10(-5) K(-1)) the transition. These results indicate that the physical properties measured on the Pbnm phase should be used with great caution because they may not be applicable to the earth's lower mantle. A density analysis based on the new data yields an iron content of 10.4 weight percent for a pyrolite composition under conditions corresponding to the lower mantle. All current equation-of-state data are compatible with constant chemical composition in the upper and lower mantle; thus, these data imply that a chemically layered mantle is unnecessary, and whole-mantle convection is possible.