Thermoelastic properties of (Mg0.64Fe0.36)O ferropericlase based on in situ X-ray diffraction to 26.7Â GPa and 2173Â K

We present 53 in situ measurements of the unit-cell volume (V) of polycrystalline (Mg0.64Fe0.36)O ferropericlase (FP) at simultaneous high pressures (P) and temperatures (T) up to P = 26.7 GPa and T = 2173 K, including pressure-temperature conditions at the top of Earth's lower mantle. FP volumes were determined through in situ energy-dispersive synchrotron X-ray diffraction in a multi-anvil press, using gold and MgO as pressure markers. Our data, combined with 112 previously published P-V-T measurements for the same FP sample, were fitted to high-temperature Birch-Murnaghan and Mie-Grüneisen equations of state (EOS). Experimental data are reproduced accurately, with a standard deviation lower than 0.31 GPa for both EOS. EOS calculations show that the thermal expansion and P-T derivatives of the bulk modulus of this iron-rich FP are virtually identical to those of pure MgO to pressures >55 GPa and temperatures of 3000 K. This result is confirmed by measurements of the normalised unit-cell volumes for FP and MgO at identical simultaneous high P-T conditions, which are identical to within 0.6 per cent relative to 26.7 GPa and 2173 K. The pressure and temperature derivatives of the bulk modulus, and thermal expansion are concluded to be independent of iron content across the range of plausible FP compositions in Earth's lower mantle.