Figure 2. (A) Jarosite- and Alunite/Kaolinite-Cemented Sandstones Near the Top of Mollies Nipple
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Fourth Conference on Early Mars 2017 (LPI Contrib. No. 2014) 3009.pdf JAROSITE AND ALUNITE CEMENTS IN JURRASIC SANDSTONES OF UTAH AND NEVADA, A POTENTIAL ANALOG FOR STRATIFIED SULFATE DEPOSITS ON EARLY MARS. T. M. McCollom1 and S. L. Potter-McIntyre2, 1LASP, University of Colorado, Boulder ([email protected]), 2Department of Geology, Southern Illinois University. Introduction: Sulfate minerals of the alunite- jarosite family have been identified in stratified depos- its at numerous locations across Mars, including two of the rover landing sites [e.g., 1-4]. Because these min- erals precipitate from acidic aqueous solutions, there has been considerable interest in studying their occur- rence in martian settings as indicators of depositional and diagenetic conditions on early Mars. In many martian deposits, the occurrence on minerals from the alunite-jarosite family within stratified formations sug- gests that the minerals may have been deposited during emplacement of the strata as sediments, or during dia- Figure 1. Overview of Mollies Nipple. Red line genetic alteration of the strata. However, occurrences delineates the approximate base of caprocks cement- of jarosite and alunite in sedimentary settings have ed by jarosite or alunite plus kaolinite. Jarosite- and received only limited study as martian analogs, with alunite-bearing float rocks eroded from the caprock most attention focused on weathering of sulfide miner- cover much of the lower slopes of the butte. White als or volcanic hydrothermal environments [e.g., 5-7]. exposed areas are bleached Navajo Sandstone. In southern Utah, Jurassic sandstones at Mollies Geologic setting: Mollies Nipple is a prominent Nipple (MN) contain abundant jarosite and alunite butte located in southern Utah that rises ~200 m above cements [8,9] whose depositional characteristics and the surrounding landscape (Fig. 1). The base of the sedimentary setting may be analogous to stratified de- butte is composed of eolian Navajo Sandstone, a fine- posits on Mars. Similar occurrences are also found in grained quartz arenite. The butte persists because the sandstone of the same age in western Nevada [10]. We sandstone caprock is resistant to erosion owing to the are currently investigating the formation and persis- presence of cements dominated by jarosite, alunite, and tence of these deposits in an effort to provide insight kaolinite (Fig. 2). Curiously, most of the cemented into occurrences of jarosite and alunite on Mars. Figure 2. (a) Jarosite- and alunite/kaolinite-cemented sandstones near the top of Mollies Nipple. Yellow-toned rocks labeled “J” have cement dominated by jarosite while the white-toned rocks la- beled “A” have cements dominated by alunite plus kaolinite. (b,c) Back-scattered electron images of jarosite-cemented sandstone. Bright, psuedocubic crystals are jarosite (Jar). Qtz = quartz. Fourth Conference on Early Mars 2017 (LPI Contrib. No. 2014) 3009.pdf rocks contain either jarosite or alunite plus kaolinite, fates have likely persisted in the sandstones for many but not both (Fig. 2a). The sulfates occur as pseudo- millions of years. cubic crystals up to 20 µm in diameter (Fig. 2b,c). Implications for Mars: Because jarosite is found Although the cements constitute only a few percent of to dissolve fairly rapidly in laboratory experiments, it the sandstone, the visible-near infrared spectra of the has been suggested that sites where it occurs on Mars rocks are dominated by jarosite or alunite plus kaolin- were not in contact with neutral aqueous fluids for ite since the quartz matrix has no spectral signal [5,6]. more than a few thousand years after the jarosite Origin of the jarosite/alunite: The origin of the formed [11-13]. Yet, the jarosite at MN has persisted sulfate-rich cements is the subject of ongoing investi- in highly porous sandstones with an active aquifer for gation. Preliminary analysis indicates that the sand- for probably several million years or longer. The per- stones are matrix-supported, indicating the cements sistence of jarosite and alunite in these rocks indicates formed after original deposition of the sediments. Two that they can persist for much longer periods than indi- of the most likely scenarios for precipitation of the cated by laboratory dissolution experiments. sulfate-rich cements are illustrated in Fig. 3. Although We anticipate that continued study of the jaro- the Navajo Sandstone is an eolian erg deposit, the site/alunite cements at MN and resolution of their overlying Carmel and Page Formations were deposited origin will provide additional insights that will be use- on the margins of an inland sea that formed to the west ful for assessing depositional environments and persis- of MN after the Navajo Sandstone was emplaced. One tence of these minerals in stratified deposits on Mars. possibility, therefore, is that the sulfate cements precip- References: [1] Klingelhöfer G. et al. (2004) Science itated during early diagenesis in an interdune, sabhka- 306, 1740. [2] Thollot P. et al. (2012) JGR 117, E00J06. [3] like environment on the margin of the inland sea, con- Ehlmann B. et al. (2016) Am. Mineral. 101, 1527. [4] John- temporaneous with deposition of the overlying Car- son J.R. et al. (2016) Am. Mineral. 101, 1501. [5] Morris mel/Page Formations (left panel, Fig. 3). Another pos- R.V. et al. (1996) in Mineral Spectroscopy: A Tribute to sibility is that the sulfates are a burial diagenetic fea- Roger G. Burns, pp. 327-336. [6] Fernández-Remolar D. C., ture formed by interaction of migrating acidic, sulfate- et al. (2010) EPSL 240, 149. [7] Papike J.J., et al. (2006) rich groundwaters with K-feldspar and hematite pre- Geochimica Cosmochimica Acta, 70, 1309. [8] Bell J. H. et sent in the original sandstone (right panel, Fig. 3). We al. (2010) Remote Sens. Environ. 114, 2259. [9] Bell J. H. are currently testing these and other alternatives and Bowen B. B. (2014) Geofluids 14, 251. [10] Eichhubl P. through fieldwork and geochemical models. et al. (2004) GSA Bull. 116, 1120. [11] Elwood Madden M. If the first scenario is correct, it would imply that E. et al. (2004) Nature 431, 821. [12] Elwood Madden M. E. the Fe- and Al-sulfates precipitated in the middle Ju- et al. (2009) Geology 37, 635. [13] Miller J. L. et al. (2016) rassic (~170 Ma), while the second scenario would Geochim. Cosmochim. Acta 172, 93. allow a younger age that would most likely postdate the Laramide orogeny (~50Ma) but is older than recent uplift and exposure (>10Ma). In either case, the sul- Figure 3. Schematic illustration of working scenarios for deposition of jarosite and alunite/kaolinite cements at Mollies Nipple. (left) Deposition as shallow pore-filling cements from upwardly migrating reducing fluids be- neath sabkha environment on margin of inland seaway. (right) Deposition from acidic, sulfur-rich fluids migrat- ing along fractures (dashed organic lines) in deeper subsurface. .