Effect of pressure, temperature, and composition on lattice parameters and density of (Fe,Mg)SiO₃‐perovskites to 30 GPa

Abstract

High‐pressure, high‐temperature properties of MgSiO3, (Fe0.1Mg0.9)SiO3, and (Fe0.2Mg0.8)SiO3 perovskites have been investigated using a newly developed X ray diffraction technique involving monochromatic synchrotron radiation. The first direct measurements of unit cell distortions and equation‐of‐state parameters of the orthorhombic perovskite as functions of composition and simultaneous high pressure and high temperature were obtained. The experiments were conducted under hydrostatic pressure up to 30 GPa, into the stability field of the perovskite. The results demonstrate that the perovskite is elastically anisotropic, with the lattice parameter b being 25% less compressible than a and c. Under increasing pressures the orthorhombic perovskite is distorted further away from the ideal cubic structure in agreement with theoretical predictions. The 298‐K isothermal equations of state of the three perovskites are indistinguishable within the uncertainty limits of the experiment. The zero‐pressure bulk modulus KT0 = 261 (±4) GPa with its pressure derivative KT0′ = 4 is close to that determined in previous static high pressure measurements. The thermal expansion obtained from the high P ‐ T experiments are consistent with previous measurements carried out at zero pressure but shows a strong volume dependence. The temperature derivative of the isothermal bulk modulus at constant pressure (∂KT/∂T)p is −6.3(±0.5)×10−2 GPa/K. Analyses of the high‐temperature data give a value for the Anderson‐Grüneisen parameter δT of 6.5–7.5, which is significantly higher than that used in recent lower mantle models.

Does Lower Mantle Tomography Indicate Temperature Variations That Affect the Core?

“…These values are consistent with the values obtained by ISAAK et al (1992) and ANDERSON and ISAAK (1993) for MgO. They are not compatible with the much higher value of ST obtained for perovskite up to 30 GPa by MAO et al (1991). It is suggested that the value of 6s by AGNON and BUKOwINSKI (1990) is slightly too low.…”

“…But we may suppose that the temperature variations extend further into the lower mantle and that evidence of them may be sought in seismic tomography. Central to this discussion are the temperature dependences of incompressibility, Ks, and rigidity, µ, under lower mantle conditions, which are the subject of several recent discussions (AGNON and BUKOWINSKI, 1990;MAO et al, 1991;ISAAK et al, 1992;ANDERSON and ISAAK, 1993). They are represented by dimensionless parameters 1 (OKs (1) b s a vKs 8T )P, 1 aµ e = avq 8T P…”

Electronic spin state of iron in lower mantle perovskite

“…Taking ͟ from Burns (4) and the thermoelastic parameters of perovskite from Mao et al (22), we found that the partial transition pressure of 80 GPa at room temperature would increase by about 30 GPa at 2,000 K (10 GPa increase due to the thermal expansion effect, and 20 GPa increase due to the entropy effect). The estimated thermal expansion effects rely on the assumption ⌬ ϳ R…”

Geodynamically consistent seismic velocity predictions at the base of the mantle

“…An analysis of various reports shows that the range of reported values seem to have a bimodal nature, clustering around low values of about 1.7 x 10-5 K-1 [Wang et al, 1994;Chopelas, 1996;Stacey, 1996) and high values about 4.0 x 10-5 K-1 Mao et al, 1991;Patel et al, 1996).…”

“…First, all parameters involved, except p and G, are probably not very sensitive to iron content [Mao et al, 1991;Wang et al, 1994]. So in most cases we used the properties of the Mg end members for both (Mg,Fe)O and (Mg,Fe)Si0 3 (Table 1).…”