THERMAL EMISSION ANALYSIS OF MINERAL GLASSES: APPLICATION TO REMOTE SENSING STUDIES OF HOLOCENE SILICIC LAVA FLOWS

RAMSEY, M.S., FINK, J.H., and CHRISTENSEN, P.R., Department of Geology, Arizona State University, Tempe, AZ., 85287-1404, U.S.A.

Energy emitted by a material at a specific wavelength in the thermal infrared region of the E-M spectrum (5-50 microns) is both a function of the temperature and emissivity. The emissivity of any material can be directly related to its atomic structure. Absorption bands in the emission spectrum of a mineral are the result of the asymmetric vibrations of the atomic bonds. In silicate minerals, for example, the primary absorption band shifts to longer wavelengths with decreasing tetrahedral polymerization. This ability to discern mineralogy remotely makes thermal emission spectroscopy ideal for answering certain geological questions.

Emission from lava surfaces is complicated by the presence of a glassy groundmass and/or coating. A glass by definition contains no crystalline long range order and hence is, to some degree, amorphous. This effect is evidenced when comparing the spectrum of an aphyric obsidian to a numerically-formed spectrum of its normative (CIPW) mineralogy. Changes in band morphology could incorrectly be interpreted as changes in mineralogy. An understanding of this effect is important for accurate interpretation of remote sensing data of glassy volcanic flows and domes such as those found in the western United States.

High resolution emission spectra of decreasing degrees of crystallinity were acquired for synthetically formed glasses of quartz and mineral assemblages approximating obsidian. The percent crystallinity, determined by XRD analysis, produces a systematic change in the spectral features. Application of this study to remotely gathered Thermal Infrared Multispectral Scanner (TIMS) data of rhyolite domes in the western U.S. enables the identification of obsidian exposed on the flow surface. In addition, through linear deconvolution of the spectra, the glass percentage within the other more pumiceous flow units can be mapped. The ability to determine mineralogy and degree of vitrification remotely provides key insights into the cooling rate, internal structure and source magma of these glassy lavas.

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Presented at: IAVCEI General Assembly Meeting
Date: 1993