VNIR Spectral Properties of Felsic Rocks: Implications for Mars Detections

Felsic (higher silica) rocks are known or hypothesized to be present on the Earth, Moon, Mars, and Venus. However, the spectral character of felsic rocks in the visible to near infrared (VNIR) wavelength range is poorly understood. VNIR spectra are sensitive to iron in mafic minerals, which most felsic rocks do not contain. Fe-substitution in plagioclase can cause an absorption band, but this band is weak compared to mafic absorptions, and thus can be easily obscured unless the material is plagioclase dominated or contains abundant spectrally neutral constituents. In this work we aim to understand how the VNIR spectra of felsic rocks change based on factors like composition and crystallization history.

We obtained VNIR spectra from an initial suite of 14 samples of intermediate to felsic rocks including diorite, granodiorite, weathered rhyolite and granite pegmatite. Overall, the most spectral variability occurs between weathered and unweathered samples. While previous work asserts that plagioclase absorption features should be rare in felsic rocks, we find that most samples have a broad absorption feature centered near 1.2-1.3 µm, consistent with feldspar, and larger plagioclase grain sizes tend to cause deeper absorption features, particularly in diorites. The feldspar band is obscured in oxidized and/or weathered samples. In diorite and some rhyolites, complex absorption features between 2.2 and 2.4 µm are also observed, likely due to mica.

Several studies recently have identified possible feldspar bands in orbital spectra from a variety of sites across Mars, and have variably attributed these signatures to coarse plagioclase in basaltic andesite to andesitic lava flows or high plagioclase content in more evolved felsic rocks (Rogers & Nevaskil, 2015; Rogers & Farrand, 2022). These results support that both interpretations could be correct, and that felsic rocks may be more detectable in VNIR spectra than previously recognized.