Lunar and Planetary Science XLVIII (2017) 1237.pdf

JAROSITE IN ANCIENT TERRESTRIAL ROCKS: IMPLICATIONS FOR UNDERSTANDING MARS DIAGENESIS AND HABITABILITY. S.L. Potter-McIntyre1 and T.M. McCollom2, 1Southern Illinois University, Geology Department, Parkinson Lab Mailcode 4324, Carbondale, IL, 62901, [email protected], 2LASP, CU Boulder, 1234 Innovation Drive, Boulder, CO 80303.

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., 1,2,3, 4]. Because these minerals typically precipitate from aqueous solutions and are stable only under acidic conditions, there has been considerable interest in studying their occurrence in martian settings as indicators of depositional and diagenetic conditions on early Mars [e.g., 3]. Many terrestrial occurrences of jarosite and alunite that have been proposed as analogs for these minerals have no clear relevance to the geologic setting where they occur on Mars. In southern Utah, however, Jurassic sand- stones at Mollies Nipple (MN) contain jarosite and alunite cements whose characteristics may be very sim- ilar to the stratified deposits on Mars. In addition, pre- vious studies have indicated these deposits to be spec- trally similar to many of the martian deposits [5,6]. Although the rocks at MN are early to middle Ju- rassic, the diagenetic history that resulted in the precip- itation of the jarosite and alunite cements is not under- stood. The results of the investigation will be used to gain insights into the origin and persistence of mineral from the alunite-jarosite family in martian settings. The principal objective of this study is to understand the origin of the jarosite and alunite cements that are pre- sent in the sandstones at MN and to use this infor- mation to interpret depositional environment and sub- sequent diagenetic conditions that may have affected the stratified rocks containing these minerals on Mars. Geologic Setting: MN is a butte in southern Utah Fig. 1. MN. A. Yellow dashed line delineates jarosite- composed of eolian Navajo Sandstone (Fig.1A). It is alunite-bearing caprock. Black box shows detail in view (B). resistant to erosion due to the presence of a well- B. Black dotted line shows sequestration of jarosite (J) and cemented, finer-grained caprock. The jarosite and alu- alunite (A). These mineral zones crosscut lithology and frac- nite cements are present only in this caprock; however, tures. SEM images of these zones are shown in Fig. 2A, B. float rock from erosion of this caprock gives the entire mountain a jarosite spectral signature in airborne data be an intertonguing of the Page Sandstone and Carmel [5,6]. Formation that overly the Navajo Sandstone in the re- Methods and Results: Field methods are used to de- gion. Jarosite and alunite cements are segregated with- termine the sedimentological properties of the jarosite- in this caprock, but the mineral zones crosscut litholo- bearing rocks. The base of MN is bleached Navajo gy as well as fractures, so no visible lithological or Sandstone where original iron oxide grain coatings structural features control this differentiation of cement would have been reduced and mobilized during infiltra- (Fig.1B). tion of a reducing solution (Fig.1A). Based on our ini- Scanning electron microscopy is used to observe tial investigation of sedimentary structures and miner- euhedral jarosite cubes in the yellow portions of the alogy, the caprock, although mapped as Navajo Sand- rock (Fig. 1B, 2A). Alunite is present with abundant stone in photogeologic maps, may actually kaolinite (an aluminosilicates precipitated in acidic conditions) in the white portions of the rock, but clay Lunar and Planetary Science XLVIII (2017) 1237.pdf

minerals are very sparse in the jarosite-bearing rocks ferrihydrite to hematite. Clays and Clay Minerals, 48(1), (Fig. 1B, 2B). Some jarosite-bearing rocks exhibit red pp.51-56. rims where decomposition of the jarosite leads to hem- atite formation. In these transition zones decomposition of jarosite cubes is visible leaving malformed “shells” filled with fuzzy jarosite (Fig. 2C). Also present are microbial fossils in close proximity to jarosite with both euhedral and unusual habits (Fig. 2D). Conclusions: Likely explanations for the origin of jarosite and alunite cements include: (1) Precipitation from highly acidic pore waters during deposition in an sabhka/eolian depositional environment. (Thick sulfate deposits are present within the Carmel Formation ~20km to the NW), or (2) the sulfates precipitated from acidic fluids during burial or late-stage diagenesis long after the sandstones were deposited (1’s-10’s Ma time scales). In either case, a puzzling aspect of the occurrence of the jarosite-alunite cements is that they have likely been exposed to circumneutral groundwater for at least 10 Ma (in the case of scenario 2) and up to 170 Ma (in the case of scenario 1) since their precipita- tion, which would seemingly contradict experimental studies suggesting that dissolution of these minerals is relatively rapid under such conditions [7,8]. Possible explanations for the recalcitrant jarosite and alunite cements include: 1. Previous laboratory studies underestimate the stability of jarosite-alunite in natural settings and on geologic time scales, or 2. In- teraction with organic polymers [9] or inclusion of trace elements such as silicon [10] renders the crystal structure resistant to transformation to hematite similar to examples of iron oxyhydroxides. It is noted that most crystal morphologies in these ancient sedimentary examples are similar to abiotic examples from volcanic environments, except where they coexist with putative microbial fossils. This study has implications for the interpretation of martian sedimentary settings and their habitability because these environments may have had much more neutral fluids than previously interpreted. References: [1] Klingelhöfer, G.R.D.S. et al. (2004) Sci- ence, 306, 1740-1745. [2] Farrand, W.H. et al. (2009) Icarus, 204, 478-488. [3] Yen, A.S., et al. (2016) Annual Meeting of The Clay Minerals Society. [4] Ehlmann, B.L. et al. (2016) Am. Min., 101, 1527-1542. [5] Bell, J.H. et al. (2010) Re- mote Sensing of Environment, 114, 2259-2270. [6] Bell, J.H. and Bowen, B.B. (2014) Geofluids, 14, 251-265. [7] Mad- den, M.E. et al. (2004) Nature, 431, 821-823. [8] Madden, Fig. 2. SEM images. A. Euhedral jarosite cubes (arrow)on a M.E. et al. (2012) Geochim. et Cosmochim. Acta, 91, 306- quartz grain. B. Small euhedral alunite cubes (solid arrow) 321. [9] Chan, C.S., Fakra, S.C., Edwards, D.C., Emerson, from J zone (in Fig. 1B) and kaolinite (dashed arrow) from A D. and Banfield, J.F., 2009. Iron oxyhydroxide mineraliza- zone. C. Degraded jarosite cubes (arrow) from a transition tion on microbial extracellular polysaccharides. Geochimica zone of jarosite to hematite. D. Apparent microbial fossils et Cosmochimica Acta, 73(13), pp.3807-3818. [10]Glasauer, (solid arrow) in thin section adjacent to jarosite with both S.M., Hug, P., Weidler, P.G. and Gehring, A.U., 2000. Inhi- euhedral (left dashed arrow) and unusual (right dashed ar- bition of sintering by Si during the conversion of Si-rich row) crystal habits.