JAROSITE IN ANCIENT TERRESTRIAL SEDIMENTARY ROCKS: IMPLICATIONS FOR UNDERSTANDING MARS DIAGENESIS AND HABITABILITY

Jarosite has been detected at nearly every landing site on Mars and from orbiters in many locations. The existing model is that this mineral precipitates at extremely low pH (1-2) and will rapidly recrystallize to hematite with even limited exposure to fluids of ~pH 5 or more. Recent findings are challenging this paradigm. Jarosite and alunite are present as cements precipitated in the Jurassic Navajo Sandstone at Mollies Nipple, Kane County, UT, USA. Jarosite is also identified as a fracture fill in the upper sandstones of the Jurassic Sundance Formation near Gypsum Creek, MT. At Mollies Nipple, the jarosite and alunite may either be primary cements precipitated from pore fluids during acid saline sabhka deposition of overlying units or burial diagenetic cements. The Gypsum Creek example is likely precipitated during burial or late-stage (uplift) diagenesis and sourced from sulfate and iron present in the Sundance Formation or possibly the underlying Gypsum Springs Formation.

If the jarosite is a primary precipitate, it may have precipitated in highly acidic pore waters; however, the formation has been exposed to 170 Ma of subsequent formational fluid and groundwater flow, which almost certainly would have been more neutral – especially during uplift and exposure when meteoric fluids dominate diagenetic processes. Similarly, if the jarosite precipitated from acidic fluids as a cement during burial or late-stage diagenesis, the formations still would have been exposed to at least 10Ma of meteoric fluids during erosion of the overlying units. Possible explanations for the recalcitrant jarosite and alunite cements include: 1. Previous laboratory studies underestimate the stability of jarosite in natural settings and on geologic time scales, or 2. Biotic influence during precipitation renders the crystal structure resistant to transformation to hematite (similar to examples of amorphous iron oxyhydroxides such as ferrihydrite) – although crystal morphologies in these ancient sedimentary examples are similar to abiotic examples from volcanic environments. This study has important implications for the interpretation of martian sedimentary settings and their habitability because these environments may have had much more neutral fluids than previously interpreted.