A WIDESPREAD ULTRAMAFIC SANDSTONE ON MARS

The origins of olivine-rich ultramafic rocks exposed on the surface of Mars remain only partially constrained, with proposed emplacement as intrusions, lava flows, impact debris and melt, aeolian sand sea deposits, and volcanic ash. The uncertain origins of these rock units obscure the contexts of their aqueous alteration, potential habitability, and relationships with rocks of other igneous compositions. We evaluate the origin of one of the planet’s most widely outcropping ultramafic rocks, the Circum-Isidis olivine-rich unit, which is ~3.7 Ga in age and locally altered to a diverse suite of alteration minerals, including carbonates and phyllosilicates. Our work synthesizes 1:50,000 scale geomorphic mapping and detailed stratigraphic and geomorphic analyses of >100 unit outcrops with previous spectroscopic modeling to constrain the unit’s origin.

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.