Micro-FTIR imaging: an advanced method for the determination of CO2 and H2O concentration gradients in silicate glasses

Glass samples of diffusion couple experiments with rhyolitic to hawaiitic melts were analyzed by FTIR microscopy with a common single element and a 64 x 64 element focal plane array detector to obtain total CO2 and H2O concentration distance profiles. In contrast to the single element detector, the array detector facilitates mid-infrared spectral imaging with a resolution on a micrometer scale. The imaged sample information was converted to multiple parallel concentration distance profiles, using the absorption band areas of the dissolved volatile species. The bulk CO2 and H2O diffusivities from measurements with different array resolutions range from log D = -11.03 to -11.33 m(2).s(-1) at 500 MPa and 1523-1623 K. All determined diffusion coefficients are within the error identical to single element detector analysis. Further test measurements were performed on a partially degassed, vesiculated phonolite in order to visualize the concentration of H2O around bubbles. Both applications confirm that FTIR imaging is a convenient tool to investigate the spatial distribution of CO2 and H2O in silicate glasses, especially in terms of diffusion-related studies.