Thermal Emission Spectroscopy of Silicate Glasses and Melts: Applications to Remote Sensing of Glassy Volcanic Environments

摘要

Thermal infrared (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 TIR spectroscopy for compositional determination have focused primarily on crystalline minerals, as minerals possess high-order molecular structure, and unique and identifiable TIR spectral features. Although ubiquitous in both hazardous and non-hazardous volcanic terranes, silicate glasses are poorly studied spectrally. This is primarily due to the amorphous and structurally-disordered nature of glasses, which causes them to display very similar spectral features regardless of composition. Because of this, glasses are difficult to distinguish spectrally in the TIR, especially at lower spectral resolutions. Furthermore, spectral features change 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 in thermal emission spectra as a broadening of the main Si-O spectral feature, and an increase in its wavelength position. A micro glass-melting furnace has been developed specifically for use with a Nexus 670 FTIR spectrometer in order to collect in-situ thermal emission spectra of actively melting and cooling synthetic silicate glasses of dacitic and rhyolitic composition. Changes in the wavelength position, the 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. Differences in spectral behavior and morphology are observed between a glass in a solid state, and its molten counterpart. Furthermore, the approximate temperature range over which the glass transition takes place is also identified. This growing library of laboratory-acquired solid glass and melt spectra will be used in conjunction with TIR airborne and field-based remote sensing instrumentation to more definitively characterize the ever-changing composition and physical state of active silicic lava domes and flows. This, in turn, will contribute to improvement of mapping and hazard assessment of volcanic environments.