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

摘要

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 mu m 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 (An(25)-An(67)). 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. Plain Language Summary Plagioclase feldspar is a common mineral of terrestrial rocks that has recently been detected on the surface of Mars with reflectance spectroscopy. In geology, this method allows to non-quantitatively assess the mineralogical composition of rocks by studying the spectrum of the sunlight reflected by them. Previous laboratory studies of plagioclase powders demonstrated that its spectral signature is quickly masked when mixed with more iron-rich minerals and that 90% plagioclase content may be needed to observe their diagnostic absorption band around 1.3 mu m. In the present study, five terrestrial samples of uncrushed rocks of varied nature, which contain plagioclase, were analyzed with instruments for petrographic and spectral characterization. A diverse set of instruments allowed us to obtain the total rock spectrum, its mineralogy, the spectrum of each mineral group, and the chemical composition of plagioclase for each sample. Our results show that plagioclase signature is present in the spectrum of all five rocks despite them having different compositions, textures, and amounts of plagioclase. We further demonstrate that the position of the plagioclase absorption band varies depending on multiple factors. Our findings have strong implications for the interpretation of feldspar signatures on Mars, which could correspond to a range of feldspar-bearing rocks.