GEOCHEMICAL EVIDENCE FOR CLOSED- AND OPEN-SYSTEM HYDROLOGY ON MARS

The sedimentary record of Mars records evidence for both closed- (or nearly closed) and open-system hydrological processes. The geochemistry of modern soils and weathered rock surfaces is well explained by the former, yielding evidence for a weathering environment in which low degrees of mineral dissolution are accompanied by the precipitation of secondary sulfate salts, Fe-oxide, and silica that are not physically separated from their host rock. These samples provide an important example that can help guide the search for evidence of closed system weathering processes in the ancient geologic record of Mars. On the other hand, the ancient (~3.5-3.7Ga) sedimentary rocks of the Burns Formation at Meridiani Planum preserve clear geochemical evidence for open-system hydrologic processes. We present an updated interpretation of the provenance of Burns Formation, indicating that the fine-grained siliciclastic component of these “dirty” sulfate sandstones is consistent with a lithology having a chemical composition similar to that of modern basaltic soil and dust. This siliciclastic component was cemented by sulfate salts that were derived from evaporation of a Fe²⁺-, Mg²⁺-, Ca²⁺-, SiO₂ (aq)-, SO₄^2--rich groundwater. The geochemistry of the Burns Formation requires that the groundwater-derived salt component was derived from a fluid that interacted significantly with the basaltic crust of Mars under open-system hydrological conditions, leading to a chemically fractionated, rock-buffered, anoxic fluid composition. This groundwater was subsequently acidified by oxidation and Fe³⁺-mineral precipitation at the Martian surface as a result of interaction with atmospheric oxidants. Our interpretation of the Burns Formation is entirely consistent with a depositional model in which sandstones were formed by cementation of siliciclastic grains by groundwater-derived salts in a playa-dune-interdune environment. These examples from the Mars Exploration Rover landing sites enable us to understand the chemical fractionations induced by weathering and alteration under a broad spectrum of hydrologic conditions, and will provide important examples against which analyses by the Mars Science Laboratory Curiosity can be compared as she begins her exploration of the sedimentary section at Gale Crater.