DETERMINING GLASS CONTENT AND ERUPTION STYLE FROM SPECTRAL ANALYSIS OF VOLCANIC TEPHRAS

Possible volcanic deposits containing iron-bearing glass have been identified on the Moon and Mars using orbital spectroscopy. On Earth, both phreatomagmatic and magmatic explosive eruptions create glassy tephras. Phreatomagmatic eruptions form significant abundances of glass by quickly quenching lava through interaction with meteoric water. Magmatic eruptions create less glass through slower cooling in larger pyroclasts or eruption columns. Wall et al. (2014) used X-ray diffraction (XRD) on samples from 17 different eruptions with variable eruption styles to determine their degree of crystallinity. Their results found that there were lower degrees of crystallinity in phreatomagmatic eruptions compared to magmatic eruptions. Unfortunately, planetary XRD is limited to the MSL mission; thus, this study asks the question: is it possible to use spectroscopy to differentiate eruption styles from orbit?

Visible/near-infrared (VNIR) and thermal-infrared (TIR) spectra were collected of the Wall et al. (2014) tephra samples, and were analyzed for trends relating glass content and eruption style. VNIR is sensitive to iron and alteration minerals and comparable to orbital spectrometers such as the Moon Mineralogy Mapper and CRISM (Mars) instruments. TIR is sensitive to most minerals and analogous to measurements taken at DIVINER (Moon) or TES (Mars). Using TIR, mineral and glass abundances can be derived by deconvolving the spectra with a standard spectral library. Our results show that glass could only be definitively detected in VNIR spectra at high abundances, but not for intermediate abundances. We find that the degrees of crystallinity inferred from XRD and TIR data are consistent for glass-rich samples, such as the phreatomagmatic sediments. However, the crystallinity of magmatic material was consistently lower in TIR spectral models by 20-30% compared to XRD. The discrepancy could be a systematic error due to over-modeling of the non-unique shape of glass spectra. A physical mixture study of amorphous and crystalline samples is underway to investigate this hypothesis. Overall, the combination of VNIR and TIR can differentiate phreatomagmatic and magmatic tephras. Increased analysis of eruption styles through spectroscopy may lead to more detailed understanding of volcanic deposits from orbit.