ORIGIN OF JEZERO CRATER’S MARGIN UNIT: TEXTURAL AND CHEMICAL CONSTRAINTS FROM THE PIXL INSTRUMENT ON M2020

Rocks rich in olivine and carbonate minerals, based on CRISM reflectance spectra, are common across the Nili Fossae region of Mars, including Jezero Crater. Outside the crater, the olivine-carbonate unit drapes pre-existing topography, suggesting that it is pyroclastic – e.g., an ash-flow deposit. Jezero’s Margin Unit is a prominent exposure of olivine-carbonate rock is along the crater’s western interior edge adjacent to the Jezero delta, and is cut by the Neretva Vallis channel. The Perseverance rover has traversed and explored the Margin unit for ~250 sols, and obtained detailed (Proximity Science) data on four abraded surfaces and three natural surfaces. The PIXL instrument (Planetary Instrument for X-ray Lithochemistry) acquired maps of these surfaces in X-ray fluorescence and diffraction, and in UV-NIR reflectance. Examined rocks of the Margin Unit are composed primarily of Fe-Mg silicates (olivine, serpentine, smectites), Fe-Mg carbonates, and silica (ignoring dust and coatings on natural surfaces); minor minerals include Ti-chromite, feldspar, augite, Ca-phosphate, and Mg- and Ca-sulfates. In these rocks, the Fe-Mg silicates and carbonates form a porous connected network of mm-sized rounded and elongate domains. Domain cores commonly contain olivine, which is cut and surrounded by serpentine, other phyllosilicates, and Fe-Mg carbonate. Pores in the network of Fe-Mg domains are filled with nearly pure silica, likely hydrated (NIR and Raman spectra from the SuperCam and SHERLOC instruments). These data are not obviously consistent with a pyroclastic origin for the Margin Unit. PIXL scans and V-NIR images of Margin Unit rocks do not show characteristic textures of pyroclastic rocks, e.g., angular shapes like glass shards, pumice, or lithic clasts. Likewise, PIXL chemical compositions of Margin Unit rocks, with Al2O3 at 0.2-0.6% wt, and CaO at 0.3-0.8% wt, are unlike those of eruptive igneous rocks (at least on Earth). The PIXL results are, however, consistent with the Margin Unit having been a well-sorted coarse sandstone, with grains mostly of olivine and carbonate. After deposition, diagenetic alteration produced hydrous silicates, more carbonate, and filled the Fe-Mg network’s pores with hydrous silica. A synthesis of these results, larger-scale compositional trends, structures, and regional constraints is in progress.