Quantitative thermal infrared emission spectroscopy applied to granitoid petrology

Thermal infrared emission spectroscopy has the potential to be a powerful tool in studying the petrology of granitoids. Previous descriptions of the emission method have presented techniques that vary in accuracy and reproducibility. Contributions of thermal energy from the instrument and environment are major calibration factors that limit accuracy in emissivity determination. Reproducibility is related to the stability of these quantities. Sample temperature determination also is a significant factor in arriving at accurate emissivity. All of the factors which impact the measurement of emissivity using an interferometric spectrometer with an uncooled detector are isolated and examined. An experimental apparatus is presented along with a description of a simplified measurement and calibration scheme used to arrive at quantitative emissivity of minerals.; The ability to distinguish between different feldspars is crucial to the petrologic study of granitoids. Results of the current work clearly indicate that thermal infrared emission spectroscopy can be used to detect the compositional variations in feldspars. Potassic, sodic, and calcic endmember species are easily distinguished. The variations in feldspars that arise from polymorphism, exsolution, and solid solution can be studied with this technique. Crystal axis orientation effects also can be examined readily.; Eighty granitoid samples have been measured using emission spectroscopy. Their spectra have been deconvolved into mineral modes using a least squares linear retrieval algorithm with a suite of endmember minerals tailored to this application. The modes have been converted to bulk oxides and compared to the actual oxides of the rocks in order to evaluate the success of the deconvolution algorithm as well as to investigate the information content in the emissivity spectra of granitoids. The five most important granitoid bulk oxides can be derived successfully using this technique, with normalized errors ranging from {dollar}sim{dollar}4% for SiO{dollar}sb2{dollar} to 27% for CaO relative to the actual data. The results demonstrate the potential of the technique for providing both mineral and chemical information on granitoids with one rapid and easily obtained measurement.; The determination of alkalinity in granitoid rocks has application to a range of geological investigations. Traditional methods of alkalinity determination rely on bulk oxide chemistry and Harker diagrams, but thermal infrared emissivity spectra of granitoids can provide similar results. Spectral details in granitoids along with ratios of modes produced from spectral deconvolution can distinguish alkali-calcic from calc-alkalic alkalinity types. When used in combination, the five spectral and modal alkalinity indicators developed for this application successfully classify 89% of the granitoids examined.