The effects of magmatic evolution, crystallinity, and microtexture on the visible/near-infrared and thermal-infrared spectra of volcanic rocks

Abstract The natural chemical and physical variations that occur within volcanic rocks (petrology) provide critical insights into mantle and crust conditions on terrestrial bodies. Visible/near-infrared (VNIR; 0.3–2.5 μm) and thermal infrared (TIR; 5–50 μm) spectroscopy are the main tools available to remotely characterize these materials from satellites in orbit. However, the accuracy of petrologic information that can be gained from spectra when rocks exhibit complex variations in mineralogy, crystallinity, and microtexture occurring together in natural settings is not well constrained. Here, we compare the spectra of a suite of volcanic planetary analog rocks from the Three Sisters Volcanic Complex, Oregon to their mineralogy, chemistry, and microtexture from X-ray diffraction, X-ray fluorescence, and electron microprobe analysis. Our results indicate that TIR spectroscopy is an effective petrologic tool in such rocks for modeling bulk mineralogy, crystallinity, and mineral chemistry. Given a library with appropriate glass endmembers, TIR modeling can derive glass abundance with similar accuracy as other major mineral groups and provide first-order estimates of glass wt.% SiO2 in glass-rich samples, but cannot effectively detect variations in microtexture and minor oxide minerals. In contrast, VNIR spectra often yield non-unique mineralogic interpretations due to overlapping absorption bands from olivine, glass, and Fe-bearing plagioclase. In addition, we find that sub-micron oxides hosted in transparent matrix material that are common in fine-grained extrusive rocks can lower albedo and suppress mafic absorption bands, leading to very different VNIR spectra in rocks with the same mineralogy and chemistry. Mineralogical interpretations from VNIR spectra should not be treated as rigorous petrologic indicators, but can supplement TIR-based petrology by providing unique constraints on oxide minerals, microtexture, and alteration processes.