Understanding VNIR Plagioclase Signatures on Mars Through Petrographic, Geochemical, and Spectral Characterization of Terrestrial Feldspar‐Bearing Igneous Rocks

摘要

Abstract Plagioclase feldspar is a common mineral in terrestrial rocks and has recently been detected on Mars surface with visible near‐infrared (VNIR) spectroscopy. The presence of plagioclase using this method is determined through the identification of an absorption band centered around 1.3 μm on reflectance spectra, which requires the incorporation of Fe2+ in plagioclase lattice. Previous laboratory studies of powder mixtures showed that plagioclase should only be detectable if present in amounts >90% as its absorption band can be masked by those of mafic minerals. Plagioclase composition, but also the size of the grains and the associated minerals in a rock, influence the spectral signature of plagioclase feldspars. Thus, the analysis of whole, uncrushed rocks appears to be relevant for comparisons with Mars remote sensing observations that have shown plagioclase‐like signatures. In the present work, we performed laboratory measurements on five feldspar‐bearing terrestrial rocks of various nature chosen because they reflect the first order terrestrial magmatic variability. The mineralogical composition of these samples, the chemical composition of feldspar crystals, and total rock spectra as well as spectra of each mineral were determined using microscopy, electron microprobe, and VNIR spectroscopy. Our study shows that plagioclase signature is visible on the spectra of macroscopic rocks containing between 30% and 80% plagioclase of different compositions (An25‐An67). Our findings have strong implications for the interpretation of feldspar signatures on Mars, which can belong to a range of feldspar‐bearing rocks and could thus provide information about the formation of Mars' crust.