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) hav e 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 Ram an spectroscopy on recovered samples, pressure and temperature conditi ons were determined under which the P-V-T behavior of the perovskite r emains 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 a nalytical electron microscopy studies revealed that the iron-rich pero vskite decomposed into at least two phases, which were Fe and Si enric hed, respectively. At pressures above 4 GPa, the P-V-T behavior of MgS iO3 perovskite remained reversible up to about 1200 K, Whereas the Mg0 .9Fe0.1SiO3 exhibited an irreversible behavior on heating. Such irreve rsible behavior makes equation-of-state data on Fe-rich samples dubiou s. Thermal expansivities (alpha(V)) of MgSiO3 perovskite were measured directly as a function of pressure. Overall, our results indicate a w eak pressure dependence in av for MgSiO3. Analyses on the P-V-T data u sing various thermal equations of state yielded consistent results on thermoelastic properties. The temperature derivative of the bulk modul us, (partial derivative K(T)/partial derivative T)P, is -0.023(+/- 0.0 11) 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 lo wer mantle. For a temperature of 1800 K at the foot of an adiabat (zer o depth), these results indicate an overall iron content of Fe/(Mg + F e) = 0.12(1) for a lower mantle composed of perovskite and magnesiowus tite. Although the (Mg + Fe)/Si ratio is very sensitive to the thermoe lastic parameters of the perovskite and it is tentatively constrained between 1.4 and 2.0, these results indicate that it is unlikely for th e lower mantle to have a perovskite stoichiometry.