Olivine and carbonate-rich bedrock in Gusev crater and the Nili Fossae region of Mars may be altered ignimbrite deposits

Exposures of bedrock rich in olivine and carbonate link Gusev crater and the Nili Fossae region (NFR), both of which have the highest abundance of olivine yet identified on Mars. They are recognized as possible explosive volcanic tephra deposits, but the nature of their eruption and emplacement is poorly constrained, limiting understanding of what may be a widespread volcanic process on early Mars. The compositional and morphologic similarities of these widely separated olivine and carbonate-rich rocks have not been investigated previously. We examined orbital and in situ thermal infrared spectra and find notably similar compositions between the two locations, with olivine ranging from ~Fo53 to Fo75 and carbonates composed of multiple Mg and Fe-rich phases, although with minimal magnesite in the NFR. A range of morphologic and textural features occur in the deposits of explosive volcanism on Earth that can be used to constrain the origin of those on Mars. We observed features of the olivine-rich bedrock in both locations that resemble those of welded ignimbrite deposits on Earth that formed from pyroclastic density currents in cataclysmic explosive volcanic eruptions. If correct, an ignimbrite interpretation for the olivine-rich bedrock in the NFR and Gusev crater may apply to other occurrences on Mars and indicate a style of volcanism more common in its early history.