Using micro-FTIR spectroscopy to measure volatile contents in small and unexposed inclusions hosted in olivine crystals

A new technique is described that makes it possible to use micro-Fourier Transform Infra-Red (micro-FTIR) spectroscopy to determine H2O and CO2 concentrations in unexposed melt inclusions hosted in olivine crystals. Previously, inclusions have had to be doubly polished, and even then it has been problematic to obtain pure glass spectra through inclusions smaller than 100 μm. The new technique involves calculating the proportion of inclusion in transmittance spectra taken through the inclusion and the olivine host, which, when combined with accurate overall thickness determinations from interference fringe patterns on reflectance spectra, allows the inclusion thickness to be calculated. This thickness can then be used with the absorbance of volatile species in the transmittance spectra to calculate accurate and precise analyte concentrations. Application of this technique will greatly ease sample preparation and allow the H2O and CO2 concentrations to be measured in many more olivine hosted inclusions than was previously possible by FTIR. The minimum size of an inclusion that can be completely analysed will be limited by the analytical sensitivity of FTIR, and the resolution of other in situ analytical techniques used to fully characterise the geochemistry of inclusions.