DIAGENESIS ON MARS AND INDICATIONS OF PROLONGED WATER-ROCK INTERACTION

Clay minerals and hydrated silica phases have been detected in numerous locations across the surface of Mars using orbital and in situ techniques. On Earth, these phases typically experience diagenetic maturation upon prolonged interaction with water, often at elevated temperatures during burial diagenesis. Detailed examination of orbital spectral reflectance data (CRISM) at near-infrared wavelengths reveals that many reported martian smectite deposits are more consistent with mixed-layer chlorite-smectite. Indeed, ‘pure’ smectite deposits are apparently rare and are often associated with Fe varieties such as nontronite. This indicates that many clays on Mars may represent the products of diagenesis and record interaction with crustal or basinal fluids, as is common on Earth. Intriguingly, two locations where smectite persists include Mawrth Vallis (some of the oldest terrain on Mars) and strata at the base of Mt. Sharp in Gale Crater (underneath several kilometers of overburden), two settings that would tend to favor conversion to mixed-layer phases or chlorite. Similarly, hydrated silica phases such as opaline silica typically convert to more crystalline phases upon continued exposure to water (e.g., opal-A transitions to opal-C, CT and microcrystalline quartz). Reanalysis of CRISM spectra indicates that many hydrated silica exposures on Mars are more consistent with opal-A than opal-C or CT. However, in some cases distinctly different hydrated silica phases occur in close proximity, indicating a complex local history of water-rock or water-sediment interaction. These observations suggest that many hydrated phases on Mars record different degrees of water-rock interaction through time. Older clay-bearing terrains may have experienced prolonged water-rock interaction during burial, later excavated by impact processes and eolian erosion. In contrast, hydrated silica in Hesperian terrains may record quite limited interaction between sediments and water. Together, these hydrated mineral deposits represent an important record of fluid interactions on Mars through geologic time.