FERROUS SMECTITES AND THE REDOX EVOLUTION OF EARLY MARS

Ferric smectite clay minerals are a widely observed weathering product in the Noachian-aged basaltic crust of Mars. Their occurrence may indicate oxidizing surface conditions early in the planet’s history, much different from the redox state of early Earth. However, it is unclear whether these smectites represent the initial phases that formed or are the products of oxidative alteration earlier, non-oxidized weathering products. Ferrous smectites are the predicted clay mineral to form from basalt alteration in the absence of extensive iron oxidation, but these phases have been little studied to date because their occurrences on the modern Earth are limited to subsurface anoxic zones, primarily in the oceanic crust. We have thus synthesized a series of ferrous smectites of a range of compositions and characterized their structural and spectral properties. These materials have X-ray diffraction features that bracket those observed for smectites in a gray mudstone at Gale Crater by the Mars Science Laboratory. The synthetic clays show only partial oxidation via reaction with oxygen on laboratory timescales, but oxidation by hydrogen peroxide is rapid, complete, and structurally disruptive. However, hydrothermal recrystallization resets the smectite structure, allowing further oxidation by oxygen and for both oxidants eventually yields a ferric smectite. Finally, experimental hydrothermal basalt alteration yields ferrous smectites similar to those produced synthetically, indicating that these compositions are representative of those likely to occur in nature. This work shows that the ferric smectites observed on Mars today need not be the original phases that formed and suggests that ferrous smectites may be preserved in the subsurface of Mars. If found, such smectite occurrences would be markers of an earlier period of crustal alteration under anoxic conditions that predates the development of the oxidizing surface environment that persists to today.