Thermoelastic Properties of (Mg,Fe)SiO 3 Perovskite
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Thermoelastic Properties of (Mg,Fe)SiO3 Perovskite
Boris Kiefer and Lars Stixrude Department of Geological Sciences, University of Michigan, Ann Arbor, MI, 48104-1063, U.S.A ABSTRACT Magnesium rich (Mg1−x ,Fex )SiO3 perovskite is thought to be the most abundant mineral in the earth’s lower mantle between 660 km and 2900 km depth. We discuss (Mg,Fe) solid solutions and their elastic properties at lower mantle pressures. The differences of the elastic constants between the Mg-endmember and the iron bearing perovskite with x=0.25 are used to predict the compositional contribution to lateral variations of elastic wave-velocities at high pressures. These predictions are compared and discussed in the context of seismic observations.
INTRODUCTION One major goal of high-pressure mineralogy is to understand the chemistry of the earth’s interior. This chemistry, if known, would provide a major constraint on the thermal structure and the dynamical state inside our planet today and over geological time. The earth’s lower mantle (56% by volume) is the largest single region within the earth’s interior and it is thought that the mineralogy of this region is largely dominated by magnesium rich ustite and CaSiO3 perovskite (Mg1−x ,Fex )SiO3 perovskite with lesser amounts of magnesiow¨ [1]. Perovskite rich compositions are known to match reasonably well the seismic constraints on density and bulk-sound velocity [2,3]. However, recent experimental observations show that the solubility limit of iron in (Mg,Fe)SiO3 perovskite increases strongly with increasing pressure and increasing temperature [4]. It is therefore important to include iron in the formulation of compositional models for the earth’s deep interior and to understand the chemistry of iron in perovskite and its effect on geophysically important parameters such as the shear-modulus. Previous experimental studies have largely focused on the equation of state of (Mg,Fe)SiO3 perovskite at lower mantle conditions [3,5,6]. The elastic constants of the Mg-endmember have been measured at ambient conditions with Brillouin scattering experiments [7] and the shear-modulus has been determined up to 800o C from ultrasonic measurements [8]. The pressure dependence of the elastic constants for x=0 at static conditions [9,10] and at high temperatures [11,12] have been predicted. The effect of iron on the zero pressure volume has been determined experimentally [13-16] and up to 30 GPa and 900 K [17] only one study was done at higher pressure [18] and the elastic properties of iron-bearing perovskite remain unknown. We have determined the ground-state structure, and all nine elastic constants of (Mg1−x ,Fex )SiO3 perovskite for x=0 and x=0.25 for a pressure range that encompasses the the earth’s lower mantle. The results allow us to evaluate the dependence of equation of state parameters on iron-content. From the elastic constants we determine the iron-dependence
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