THERMAL INFRARED (TIR) EMISSION SPECTROSCOPY OF SILICIC MELTS: APPLICATION TO REMOTE SENSING OF ACTIVE VOLCANOES

TIR remote sensing is a useful tool for the detection and analysis of volcanic surfaces. The data have been used to determine heat flux, eruption rates, surface petrology, geochemistry, and textures, for example. The majority of past studies using the TIR to determine composition have focused primarily on crystalline minerals, which have unique and identifiable TIR spectral features. Silicate glasses and melts, although ubiquitous in active volcanic terranes, have received far less attention. Due to their amorphous and structurally-disordered nature, these materials display similar spectral features regardless of composition. Glasses are therefore difficult to distinguish in the TIR, especially with remote sensing instruments. However, these spectral features do change slightly with the changing physical state of the glass. For example, as a silicate glass transitions from a solid to a molten state, the glass structure becomes less polymerized, bond angles within the O-Si-O structure decrease, and the number of non-bridging oxygens (NBO) increase. These structural changes are reflected as a broadening of the main Si-O feature and a decrease in its wavenumber position. In order to quantify these changes and better understand the spectra of melts, a micro glass-melting furnace has been developed for use with an FTIR laboratory emission spectrometer. Changes in the wavenumber position, overall emissivity, and the width of the laboratory emission spectra have been observed as the glasses transition from a fully molten to a completely solid state. This growing library of high silica glass/melt spectra will be used in conjunction with future TIR spaceborne and airborne remote sensing data to more accurately determine the changing composition, physical state and eruption rate of active lava domes.