ANCIENT AND RECENT CLAY FORMATION ON MARS AS REVEALED FROM A GLOBAL SURVEY OF HYDRATED MINERALS IN CRATER CENTRAL PEAKS

Clay minerals are the most commonly detected hydrated mineral on Mars and preserve evidence of past clement conditions. Observations of clays in central peaks of impact craters are often presumed uplifted from depth and have contributed to current paradigms of pervasive water-rock interaction during the Noachian period (>3.7 Ga). However, recent mineralogical and morphological analyses of individual craters have shown that some central peak clays may represent post-impact, possibly authigenic, processes rather than uplifted crustal material. In this work we present a global survey of 633 central peak regions to assess their distribution and diversity of hydrated minerals and the prevalence of uplifted, detrital, and authigenic clays. We examine central peak regions using high-resolution CRISM and HiRISE data and place hydrated minerals in their stratigraphic and geologic contexts. We find that Fe/Mg clays (smectite, mixed-layer chlorite/smectite, and chlorite) are the dominant hydrated minerals at central peaks, followed by hydrated silica. A significant fraction of clay and hydrated silica detections are associated with putative impact melt deposits and may represent more recent episodes of aqueous alteration. Over 35% of clay occurrences in central peak regions are not associated with rocks uplifted from depth, highlighting the importance of placing mineral detections in their proper geologic context when inferring crustal compositions from surface central peak mineralogy. Clear examples of uplifted clay-bearing rocks suggest that clays are present at crustal depths of at least 7 km, and we observe evidence for increasing chloritization with depth. Progressive chloritization within the crust may indicate widespread burial diagenesis of surface-formed clays or changes in subsurface clay-forming conditions. In either case, this trend implies the presence of fluids in the upper ~7 km of the martian crust, though the availability of these fluids likely decreased significantly through time as chloritization appears incomplete even at depths of 2-6 km. Crater count age dating of the studied craters are consistent with widespread clay formation during the Noachian and Early Hesperian, but a number of central peak clays are also suggestive of authigenic formation during the Amazonian.