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

In this study, we describe the first FTIR measurements of CO 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 CO-rich sample. Single-crystal slabs were heat-treated up to 1000 °C, and the intensity of the ν antisymmetric stretching mode of CO at 2348 cm 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: −log D = 4.2 ± 0.5 m/s and E = 208 ± 11 kJ/mol. Comparison with diffusion data from the literature (Lepezin and Osorgin in Rossijskaa Akademia Nauk 339:658-661, ) shows that at 800 °C HO diffuses almost two orders of magnitude faster than CO along the structural channels of cordierite, with the difference in diffusion coefficients increasing at lower temperatures, and this implies that at lower temperatures HO mobility is more favored compared to CO. Therefore, the volatile contents measured in exhumed rocks may not reflect the HO/CO ratio upon cordierite crystallization, and this could significantly affect the thermodynamic calculations for fluid activities or peak metamorphic conditions. [ABSTRACT FROM AUTHOR]