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Equation of state of (Mg_0.8,Fe_0.2)₂SiO₄ ringwoodite from synchrotron X-ray diffraction up to 20 GPa and 1700 K

¹ School of Science, University of Hyogo, Kouto, Kamigori, Hyogo 678–1297, Japan
² Institute for Study of the Earth's Interior, Okayama University, Misasa, Tottori, 682–0193, Japan
³ Department of Earth and Planetary Sciences, Faculty of Science, Kobe University, Kobe, Hyogo 657–8501, Japan
⁴ Japan Synchrotron Radiation Research Institute, Kouto, Hyogo 679–5198, Japan

^* E-mail: m.matsui{at}sci.u-hyogo.ac.jp

We present a temperature-pressure-volume (T-P-V) equation-of-state (EOS) of (Mg_0.8,Fe_0.2)₂SiO₄ ringwoodite based on in situ high-T and high-P synchrotron X-ray diffraction experiments up to 1700 K and 20 GPa with a multi-anvil apparatus at SPring-8. The third-order Birch-Murnaghan equation was applied to the data between 300 and 900 K, while a constant (P/T)_V fitting at temperatures higher than 900 K. By fixing previously measured volume thermal expansivities at 0 GPa and the isothermal bulk modulus at 300 K and 0 GPa of K_0,300k = 189.7 GPa, we derived the T-P-V EOS parameters of (Mg_0.8,Fe_0.2)₂SiO₄ ringwoodite using least squares to be (K₀/P)_T = 4.57(7) and (K_T/T)_P = –0.0283(13) GPa/K between 300 and 900 K, and (P/T)_V = 0.00535(11) GPa/K at temperatures above 900 K. These values compare very well with previously measured EOS data for Mg₂SiO₄ ringwoodite of (K₀/P)_T = 4.6(2), (K_T/T)_P = –0.029(1) GPa/K, and (P/T)_V = 0.0052 – 0.0055 GPa/K at high temperatures. At P = 20 GPa and T = 1800 K, as representative conditions in the lower part of the mantle transition zone, the relative V and K_T values of (Mg_0.8,Fe_0.2)₂SiO₄ ringwoodite with respect to the values at 300 K and 0 GPa are found to be V/V₀ = 0.9424, K_T/K_0,300K = 1.263, based on the present EOS. These results for (Mg_0.8,Fe_0.2)₂SiO₄ ringwoodite, combined with the corresponding data for Mg₂SiO₄ ringwoodite, describe that the effects of Fe substitution for Mg on the T-P-V EOS of ringwoodite with (Mg_0.9,Fe_0.1)₂SiO₄, thought to be the composition in the mantle transition zone, are virtually negligible for V/V₀, and less than 1% for K_T/K_0,300k.

Key-words: ringwoodite, equation of state, high temperature, high pressure, synchrotron X-ray diffraction.