Feldspathic rocks on Mars: VNIR spectral properties and formation mechanisms from terrestrial analogues and Mars remote sensing data analyses”
Abstract :
The detection of plagioclase feldspars on the surface of Mars, using visible near-infrared (VNIR) reflectance spectroscopy, raises questions about the nature of the rocks involved and the magmatic processes responsible for their formation. These detections could indicate the presence of anorthositic rocks, mainly composed of calcic plagioclase feldspars, or they could correspond to the signature of effusive rocks with andesitic to basaltic composition. In parallel, in-situ analyses conducted with the Curiosity rover have highlighted the presence of felsic rocks, also observed in the Martian meteorite NWA 7034 and its pairs, potentially akin to an evolved continental crust. The interpretation of plagioclase spectral signatures has thus significant implications for the nature and origin of the Martian crust. However, the spectral signature of plagioclase feldspars remains poorly understood due to the lack of reference data suitable for the study of the Martian surface, as most previous studies were conducted on powders and grains rather than whole rocks.
This thesis investigates and compares VNIR reflectance spectra of whole terrestrial rocks containing plagioclase feldspars to Martian detections in order to determine the geological context and nature of the rocks involved in these detections. It combines two complementary aspects: the first relates to the petrographic, geochemical, and spectral characterization of terrestrial feldspathic rocks. Macroscopic samples studied in the laboratory come from various magmatic rocks, volcanic and plutonic (e.g TTG, anorthosites, granitoids, gabbros, andesites, basalts), and are composed of plagioclases. The data collected in this thesis show that plagioclase crystals of various chemical compositions (An27 to An67) are responsible for the presence of an absorption band centered around 1.3 microns in the reflectance spectrum of these rocks. The position, width, and depth of this characteristic plagioclase absorption band depend on the plagioclase chemical composition, grain size, and degree of alteration. The plagioclase content in the rock does not seem to be a determinant factor for the presence of the plagioclase spectral signature on whole rock spectra. Our measurements, conducted on whole rocks, rather than powders, demonstrate that only ~15% of plagioclase crystals in a rock are required to reproduce the signatures observed on Mars, and that plagioclase crystals grain size and the nature of associated minerals are essential factors in the resulting spectral signature of whole rocks. The second aspect of this thesis relates to comparing VNIR spectra of whole rocks acquired in the laboratory with Martian remote sensing data. Plagioclases detected on Mars, using the CRISM instrument, can thus be associated with rocks of varied nature, including plutonic rocks such as granites and anorthosites, and volcanic rocks with plagioclase phenocrysts, such as basalts and dacites. The results of this thesis provide new reference data contributing to the understanding of the geological history of Mars, at a time when several rovers equipped with VNIR spectrometers are exploring the surface of the red planet.
Membres du jury :
Directeurs de thèse :
Jessica Flahaut : Centre de Recherches Pétrographiques et Géochimiques (CRPG), Université de Lorraine, CNRS UMR7358
Rapahël Pik : Centre de Recherches Pétrographiques et Géochimiques (CRPG), Université de Lorraine, CNRS UMR7358
Rapporteurs :
Violaine Sautter : Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), Université de la Sorbonne, CNRS UMR7590
Bernard Schmitt : Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) , Université Grenoble-Alpes, CNRS UMR5274
Examinateurs :
Sylvain Bouley : Laboratoire Géosciences Paris Saclay (GEOPS), Université Paris-Saclay, CNRS UMR8148
John Carter : Institut d’Astrophysique Spatiale (IAS), Université Paris-Saclay, CNRS UMR8617 et Laboratoire d’Astrophysique de Marseille (LAM), Université Aix-Marseille, CNRS UMR7326
Invités :
Pascal Allemand : Laboratoire de Géologie de Lyon : Terre, Planètes, Environnement (LGL-TPE), Université Claude Bernard Lyon 1, CNRS UMR5276
Martin Guitreau : Laboratoire Magmas et Volcans (LMV), Université Clermont-Auvergne, CNRS UMR6524