Abstract
Members of the alunite group are precipitated at low pH (<1 to ~4) in oxidizing environments, are unstable in circumneutral conditions, and are widespread on Mars. At Mollies Nipple in Kane County, Utah, USA, jarosite and alunite are abundant as diagenetic cements in Jurassic sandstones. This research characterizes the jarosite and alunite cements with the goal of determining their origin, and tests the hypothesis that jarosite and alunite may be more stable than the current understanding indicates is possible. Previous studies have placed the jarosite- and alunite-bearing caprock at Mollies Nipple in the Navajo Sandstone, but the presence of water-lain deposits, volcanic ash, volcanic clasts, and peloids show that it is one of the overlying Middle Jurassic units that records sea level transgressions and regressions. A paragenetic timing, established from petrographic methods, shows that much of the cement was precipitated early in a marginal marine to coastal dune depositional environment with a fluctuating groundwater table that drove ferrolysis and evolved the groundwater to a low pH. Microbial interaction was likely a large contributor to the evolution of this acidity. Jarosite and alunite are clearly more stable in natural environments than is predicted by laboratory experiments, and therefore, the Martian environments that have been interpreted as largely acidic and/or dry over geologic time may have been more habitable than previously thought.
Highlights
Introduction and BackgroundExamination of the Martian surface mineralogy over the last couple of decades has revealed the widespread occurrence of sulfate minerals in a variety of geologic settings [1,2,3,4]
Halfway between the base of the butte and the contact with the caprock, iron oxide concretionary cement is present in some locations, and this cement is confined to subhorizontal lenses (Figure 4)
Diagenetic jarosite and alunite cements at Mollies Nipple present some perplexing problems: (1) How did the early diagenetic fluids become so sufficiently acidic as to precipitate jarosite? and (2) Why are these unstable minerals phases still present after exposure to circumneutral meteoric water for millions of years? Here, we present a model for the acidification of early diagenetic fluid first, and propose some possible scenarios for the long-term stabilization of these minerals
Summary
Introduction and BackgroundExamination of the Martian surface mineralogy over the last couple of decades has revealed the widespread occurrence of sulfate minerals (including abundant minerals from the alunite group) in a variety of geologic settings [1,2,3,4]. Jarosite and alunite have been identified using spaceborne spectroscopy at a number of locations around the planet [8,9,10,11,12,13,14,15]. In several cases, these minerals are found in layered deposits indicative of deposition by sedimentary processes. Members of the alunite mineral group are of particular interest for studies of Mars because they have the potential to place definitive constraints on aqueous conditions. The presence of these minerals constrains formation conditions as well as subsequent fluid/rock interactions and leads to interpretations that sites where these minerals are
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
