P-V-T EQUATION OF STATE OF (MG,FE)SIO3 PEROVSKITE - CONSTRAINTS ON COMPOSITION OF THE LOWER MANTLE

Unit-cell volumes (V) of Mg1-xFexSiO3 perovskite (x = 0.0 and 0.1) have been measured along several isobaric paths up to P = 11 GPa and T = 1300 K using a DIA-type, cubic anvil high-pressure apparatus (SAM-85). With a combination of X-ray diffraction during heating cycles and Raman spectroscopy on recovered samples, pressure and temperature conditions were determined under which the P-V-T behavior of the perovskite remains reversible. At 1 bar, perovskites of both compositions begin to transform to amorphous phases at T almost-equal-to 400 K, accompanied by an irreversible cell volume contraction. Electron microprobe and analytical electron microscopy studies revealed that the iron-rich perovskite decomposed into at least two phases, which were Fe and Si enriched, respectively. At pressures above 4 GPa, the P-V-T behavior of MgSiO3 perovskite remained reversible up to about 1200 K, Whereas the Mg0.9Fe0.1SiO3 exhibited an irreversible behavior on heating. Such irreversible behavior makes equation-of-state data on Fe-rich samples dubious. Thermal expansivities (alpha(V)) of MgSiO3 perovskite were measured directly as a function of pressure. Overall, our results indicate a weak pressure dependence in av for MgSiO3. Analyses on the P-V-T data using various thermal equations of state yielded consistent results on thermoelastic properties. The temperature derivative of the bulk modulus, (partial derivative K(T)/partial derivative T)P, is -0.023(+/- 0.011) GPa K-1 for MgSiO3 perovskite. Using these new results, we examine the constraints imposed by alpha(V) and (partial derivative K/partial derivative T)P on the Fe/(Mg + Fe) and (Mg + Fe)/Si ratios for the lower mantle. For a temperature of 1800 K at the foot of an adiabat (zero depth), these results indicate an overall iron content of Fe/(Mg + Fe) = 0.12(1) for a lower mantle composed of perovskite and magnesiowustite. Although the (Mg + Fe)/Si ratio is very sensitive to the thermoelastic parameters of the perovskite and it is tentatively constrained between 1.4 and 2.0, these results indicate that it is unlikely for the lower mantle to have a perovskite stoichiometry.