Thermal equation of state of magnesite to 32 GPa and 2073 K

Abstract

Pressure–volume–temperature relations have been measured to 32 GPa and 2073 K for natural magnesite (Mg 0.975Fe 0.015Mn 0.006Ca 0.004CO 3) using synchrotron X-ray diffraction with a multianvil apparatus at the SPring-8 facility. A least-squares fit of the room-temperature compression data to a third-order Birch–Murnaghan equation of state (EOS) yielded K 0 = 97.1 ± 0.5 GPa and K′ = 5.44 ± 0.07, with fixed V 0 = 279.55 ± 0.02 Å 3. Further analysis of the high-temperature compression data yielded the temperature derivative of the bulk modulus (∂ K T /∂ T) P = −0.013 ± 0.001 GPa/K and zero-pressure thermal expansion α = a 0 + a 1 T with a 0 = 4.03 (7) × 10 −5 K −1 and a 1 = 0.49 (10) × 10 −8 K −2. The Anderson–Grüneisen parameter is estimated to be δ T = 3.3. The analysis of axial compressibility and thermal expansivity indicates that the c-axis is over three times more compressible ( K Tc = 47 ± 1 GPa) than the a-axis ( K Tc = 157 ± 1 GPa), whereas the thermal expansion of the c-axis ( a 0 = 6.8 (2) × 10 −5 K −1 and a 1 = 2.2 (4) × 10 −8 K −2) is greater than that of the a-axis ( a 0 = 2.7 (4) × 10 −5 K −1 and a 1 = −0.2 (2) × 10 −8 K −2). The present thermal EOS enables us to accurately calculate the density of magnesite to the deep mantle conditions. Decarbonation of a subducting oceanic crust containing 2 wt.% magnesite would result in a 0.6% density reduction at 30 GPa and 1273 K. Using the new EOS parameters we performed thermodynamic calculations for magnesite decarbonation reactions at pressures to 20 GPa. We also estimated stability of magnesite-bearing assemblages in the lower mantle.