The diffusion kinetics of CO2 in cordierite: an HT-FTIR microspectroscopy study

In this study, we describe the first FTIR measurements of CO2 desorption from cordierite, a microporous mineral stable up to high T conditions. To this purpose, we collected isothermal heating data on several (001) oriented sections of a well-characterized CO2-rich sample. Single-crystal slabs were heat-treated up to 1000 °C, and the intensity of the ν 3 antisymmetric stretching mode of CO2 at 2348 cm−1 was monitored as a function of time. The experimental data were modeled by using the mono-dimensional plane-sheet diffusion formalism; diffusion coefficients were plotted in the Arrhenius space and activation energies were calculated. Final data are as follows: −logD 0 = 4.2 ± 0.5 m2/s and E a = 208 ± 11 kJ/mol. Comparison with diffusion data from the literature (Lepezin and Osorgin in Rossijskaa Akademia Nauk 339:658–661, 1994) shows that at 800 °C H2O diffuses almost two orders of magnitude faster than CO2 along the structural channels of cordierite, with the difference in diffusion coefficients increasing at lower temperatures, and this implies that at lower temperatures H2O mobility is more favored compared to CO2. Therefore, the volatile contents measured in exhumed rocks may not reflect the H2O/CO2 ratio upon cordierite crystallization, and this could significantly affect the thermodynamic calculations for fluid activities or peak metamorphic conditions.