P-V-T equation of state of CaCO3 aragonite to 29 GPa and 1673 K: In situ X-ray diffraction study

Abstract

Pressure–volume–temperature relations have been measured to 29GPa and 1673K for CaCO3 aragonite 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 the Vinet-Rydberg equation of state (EOS) yielded KT0=65.7±0.8GPa and KT'=5.1±0.1, with fixed V0=227.11Å3. Further analysis of the high-temperature compression data led to the temperature derivative of the bulk modulus (∂KT/∂T)P=−0.016±0.001GPa/K and zero-pressure thermal expansion α=a0+a1T with a0=4.98 (22)×10−5K−1 and a1=2.81(38)×10−8K−2. The Mie-Gruneisen-Debye approach revealed the Gruneisen parameter γ0=1.39 at a fixed Debye temperature θ0=516K and the parameter q=1. Analysis of axial compressibility and thermal expansion indicates that the c-axis is two times more compressible than the b-axis and four times more compressible than the a-axis, whereas zero-pressure thermal expansion of the a-axis (a0a=2.6×10−5K−1 and a1a=2.3×10−8K−2) is weaker than that of the b-axis axis (a0b=6.3×10−5K−1 and a1b=0.1×10−8K−2) and c-axis axis (a0c=5.2×10−5K−1 and a1c=9.5×10−8K−2). A full set of thermodynamic parameters (including heat capacity, enthalpy and free energy) for aragonite and updated equations of state for magnesite and siderite was obtained using the Kunc-Einstein approach. The new EOS parameters were used for thermodynamic calculations for aragonite decarbonation reactions. The present thermal EOS provides accurate calculations of aragonite density to deep mantle. Decarbonation of subducting oceanic crust containing 2wt% aragonite would result in a 0.5% density reduction at 30GPa and 1273K. Aragonite becomes denser than magnesite at pressures about 16GPa along the 1500K isotherm and at 9GPa along the 298K isotherm.