Lithium Content in Sedimentary Rocks in Gale Crater, Aeolis Mons, Measured by ChemCam as a Tracer for Aqueous Alteration and Source Rock Geochemistry

Lithium behaves uniquely in different geological environments, making it an excellent tracer element. It is moderately incompatible and is most prominent in highly evolved pegmatites and granites. However, its small ionic radius makes it susceptible to substitute, typically for Mg, and incorporate in a range of major rock-forming minerals and secondary phyllosilicates. Moreover, Li is highly soluble and can concentrate in late-stage brines and rare Li salts, and rocks typically preserve Li signatures related to the latest fluid alterations. The ChemCam instrument onboard NASA’s Mars Science Laboratory Curiosity rover uses laser-induced breakdown spectroscopy (LIBS) to quantify Li concentration through a dedicated Li calibration. It is currently one of only three science instruments (the others being the LIBS instruments SuperCam on the Perseverance rover and MarSCoDe on the Zhurong rover) on Mars able to do so. The Curiosity rover landed at the Bradbury Rise landing site in the ~155 km diameter impact crater, Gale, in August 2012 to search for past habitable environments in the more than 5 km tall Mount Sharp composed of sedimentary rocks. Since then, Curiosity has traversed more than 33 km through fluvio-deltaic sandstone and conglomerates, lacustrine mudstones, lake-margin sandstone, and aeolian dunes. We present Li concentrations where a majority of the stratigraphic members are enriched relative to terrestrial and martian basalts (~5 ppm in mid-ocean ridge basalts and ~3 ppm in shergottites) and with local enrichments up to 158 ppm. Furthermore, Li abundance and the correlations between Li and other elements detected by ChemCam vary systematically between the main chemostratigraphic groups encountered in Gale crater, alluding to the fact that Li is likely hosted in various mineral phases and that these vary between groups. The lowermost Bradbury group rocks have slightly elevated Li abundances relative to basaltic compositions (8-14 ppm, 25th-75th percentiles) with local enrichments up to 118 ppm and most likely reflect an igneous signature with Li hosted in multiple mineral phases such as feldspar, mica, and pyroxene. The lower Murray formation and the orbitally defined clay-rich Glen Torridon region are both enriched in Li (10-20 ppm and 11-18 ppm, respectively), which is best explained by Li uptake in secondary phyllosilicates as variations in Li content in these areas mirror the detected abundances of secondary clay minerals. This relationship breaks down in the clay-sulfate transition region, which is very poor in phyllosilicates but retains elevated Li concentrations (10-18 ppm), though Li decreases with increasing member elevation as Mg-sulfates become increasingly pervasive. This is best interpreted as an igneous source rock signature, more evolved than a typical basalt alining with geochemical and mineralogical evidence of dry deposition and a minimal amount of late aqueous alteration. The sulfate unit continues the trend of decreasing Li with increasing elevation observed in the clay-sulfate transition region, which demonstrates that Li is not associated with Mg-sulfates in the region. The younger Stimson formation exhibits slightly enriched Li abundances with local enrichments up to 158 ppm. It is interpreted as a primarily igneous signature potentially affected by post-depositional fluid alteration.