A WIDESPREAD ULTRAMAFIC SANDSTONE ON MARS
We find that the unit is a clastic rock, with probable origin as an air-fall pyroclastic deposit from the nearby Syrtis Major volcanic complex. The clastic nature of the Circum-Isidis olivine-rich unit strengthens previous spectroscopic arguments that significant regions of Mars’ Noachian bedrock may be clastic deposits rather than lavas or impact melts, and this study provides criteria to constrain the origins of these clastic ultramafic rocks from orbit. Given the relatively high permeability, porosity, and specific surface area of pyroclastic rocks, our work suggests that protolith textures of clastic ultramafic units may partially control the degree of their water-limited aqueous alteration, consistent with in situ observations of carbonate-bearing ultramafic pyroclasts at Columbia Hills and the absence of orbitally detected alteration minerals in the olivine-rich ejecta from the Argyre and Hellas impact basins. Finally, our revised volumetric estimates of the carbonate-bearing olivine-rich unit indicate that it has sequestered approximately ~0.03 mbar equivalent of atmospheric CO2, an order of magnitude less than the previously suggested minimum for the unit and less than most of Mars’ modern geological reservoirs.