Ancient Anorthosites on Mars: Widespread in the Lower Crust

We report extensive detections of feldspathic, likely anorthositic, lithologies within outcrops uplifted by the Argyre basin-forming impact on Mars. The radial position of feldspathic outcrops from the basin center indicates that they are stratigraphically above olivine-bearing material and below low-calcium pyroxene-bearing rocks, and the inferred depths suggest that feldspathic lithologies are present in the lower martian crust in the circum-Argyre region. This stratigraphy bears striking resemblance to that observed at the Hellas basin rim and indicates that feldspathic rocks could be a pervasive component of the lower crust across the entire southern highlands of Mars. These results are consistent with evidence from geophysical data suggesting a lower-density lower crust, but likely not sufficient to fully account for the discrepancy in density. Importantly, these feldspathic rocks are present alongside pyroxene- and olivine-bearing units, a suite of compositions commonly associated with layered mafic intrusions (LMI). We suggest that a process early in Mars' history produced large volumes of melt that cooled slowly to form LMI-like complexes and that these complexes represent an aspect of primary martian crust. The melt-producing process ostensibly affected the whole southern hemisphere, and possibly the globe. These observations offer direct evidence to support and refine existing models of early crust formation, addressing key questions regarding crustal density variations and formation mechanisms that have previously lacked widespread observational constraints.