Diagenetic, Nonevaporative Origin for Calcium Sulfate Salts at Gale Crater

Sulfate salts are deposited most commonly as evaporites on Earth; however, this is not their only origin. Pyrite oxidation during subsurface weathering is another common process on Earth that also produces a suite of sulfate salts—including the acidic phases jarosite and alunite—in sedimentary deposits. On Mars, the occurrence of sedimentary (non-vein-forming) sulfate salts has typically been interpreted as the result of an environmental evaporitic process. While these processes certainly occurred on Mars, we observed several specific occurrences of crystal pseudomorphs in the sedimentary strata preserved in Gale Crater that are likely nonevaporitic. These occurrences are not associated with bedded precipitates that might indicate a primary origin, nor are they associated with diagnostic sedimentary structures, including desiccation cracks, breccias, and tepee structures, that might indicate an early diagenetic origin. The sporadicity of these crystals implies a localized post-depositional process, which we propose to be the oxidation of sulfide-bearing minerals such as pyrite and pyrrhotite. If post-depositional pyrite and pyrrhotite oxidation took place in Gale Crater strata as it does perennially in terrestrial examples, it would have generated a significant amount of acidity that could dissolve a large proportion of any carbonates in contact with the same fluids.