Thermal equation of state of magnesiowustite (Mg0.6Fe0.4)O

Volume measurements for magnesiowustite(Mg(0.6)Fe(0.4))O, were carried out up to pressures of 10.1 GPa in the temperature range 300-1273 K, using energy-dispersive synchrotron X-ray diffraction. These data allow reliable determination of the temperature dependence of the bulk modulus and good constraint on the thermal expansitivity at ambient pressure which was previously not known for magnesiowustite. From these data, thermal and elastic parameters were derived from various approaches based on the Birch-Murnaghan equation of state (EOS) and on the relevant thermodynamic relations. The results from three different equations of state are remarkably consistent. With (partial derivativeK(T)/partial derivativeP)(T) fixed at 4, we obtained K(o) = 158(2) GPa, (partial derivativeK(T)/partial derivativeT)(P) = -0.029(3) GPa K(-1), (partial derivativeK(T)/partial derivativeT)V = -3.9(+/-2.3) x 10(-3) GPa K(-1), and alpha(T) = 3.45(18) x 10(-5) + 1.14(28) x 10(-8)T. The K(o), (partial derivativeK(T)/partial derivativeT)(p), and (partial derivativeK(T)/partial derivativeT)(V) values are in agreement with those of Fei et al. (1992) and are similar to previously determined values for MgO. The zero pressure thermal expansitivity Of (Mg(0.6)Fe(0.4))O is found to be similar to that for MgO (Suzuki, 1975). These results indicate that, for the compositional range x = 0-0.4 in (Mg(1-x)Fe(x))O, the thermal and elastic properties of magnesiowustite exhibit a dependence on the iron content that is negligibly small, within uncertainties of the experiments. They are consequently insensitive to the Fe-Mg partitioning between (Mg, Fe)SiO(3) perovskite and magnesiowustite when applied to compositional models of the lower mantle. With the assumption that (Mg(0.6)Fe(0.4))O is a Debye-like solid, a modified equation of heat capacity at constant pressure is proposed and thermodynamic properties of geophysically importance are calculated and tabulated at high temperatures. Published by Elsevier Science B.V.