Brine Pools With Round Caps in the Western Qaidam Basin: Application to Iron Mobility and the Evaporation Formation of the Iron Oxide and Sulfate Assemblage on Mars

Abstract

The coexistence of sulfates and iron oxides is widespread on Mars, including at Meridiani Planum, the Opportunity rover's landing site. However, the formation mechanism is still poorly understood. Here, we report evaporitic iron oxyhydroxides and gypsum deposited as round caps in the center of brine pools in a playa area of the western Qaidam Basin in the northern Tibetan Plateau. Geochemical and mineralogical analyses indicate that the gypsum is precipitated in situ by strong evaporation of subsurface percolating brine containing dissolved iron. We suggest that the transport of iron from the anoxic subsurface and the evaporation of upwelling groundwater in an oxidized and hyperarid environment may have led to the co-deposition of sulfates and iron oxides at Meridiani Planum and other similar sites on Mars. Our results also suggest that the subsurface water cycle could be an important source of iron mobility even under an oxidizing atmosphere and cold climatic conditions on Mars. Groundwater evaporation during the desiccation of Mars after the late Noachian may have contributed to the heterogeneous distribution of iron and the widespread coexistence of sulfates and iron oxides on the martian surface.