Thermal expansivity and Raman spectra of natural cordierite: A potential cordierite-garnet elastic thermometer

Abstract

Cordierite is widely found in metamorphic rocks, and has important application for geothermometry and geobarometry, as well as a monitor of volatile concentrations in fluids or melts. In this study, we measured high-temperature powder X-ray diffraction (XRD) patterns on a natural sample (Na0.06[(Mg1.87Fe0.19)Al3.94Si5.00O18.00](CO2)0.09(H2O)0.22) up to 1500 K. The thermal expansion coefficient is fitted with an average value of 7.39(19) × 10−6 K−1. In situ high-temperature and high-pressure Raman spectra were also obtained on this sample, on the basis of which the isobaric and isothermal Grüneisen parameters are calculated. The anharmonic correction has positive contribution to the heat capacity, since the intrinsic anharmonic parameters show systematically negative values. Next, the isothermal bulk modulus (KT) at high temperatures is modeled by density functional theory (DFT), which is essential for constructing the P-V-T equation of state (EOS) as well as thermodynamic Grüneisen parameter (γV) for cordierite. The axial thermoelastic parameters and strain evolution along isochors at high temperatures are also evaluated. Cordierite is substantially softer with weaker anisotropy for axial compressibilities as compared with zircon and rutile, and consequently, significantly smaller strain effect is expected on the vibrational frequencies. Due to its smaller thermal expansivity and larger compressibility than garnets, the assemblage of cordierite trapped in garnet minerals can potentially be an elastic thermometer in the pressure range below 1 GPa. Entrapment isomekes are calculated with negative slopes of (∂P/∂T)V(inc-host), and positive residual inclusion pressures are expected at the ambient condition.