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Jarosite in a Pleistocene East African saline-alkaline paleolacustrine deposit: Implications for Mars aqueous geochemistry

2011; American Geophysical Union; Volume: 116; Issue: E4 Linguagem: Inglês

10.1029/2010je003680

2156-2202

Lindsay J. McHenry, V. F. Chevrier, Christian Schröder,

Geochemistry and Geologic Mapping

Journal of Geophysical Research: PlanetsVolume 116, Issue E4 Free Access Jarosite in a Pleistocene East African saline-alkaline paleolacustrine deposit: Implications for Mars aqueous geochemistry Lindsay J. McHenry, Lindsay J. McHenry [email protected] Department of Geosciences, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin, USASearch for more papers by this authorVincent Chevrier, Vincent Chevrier W. M. Keck Laboratory for Space and Planetary Simulation, Arkansas Center for Space and Planetary Sciences,, University of Arkansas, Fayetteville, Arkansas, USASearch for more papers by this authorChristian Schröder, Christian Schröder Department of Hydrology, University of Bayreuth, Bayreuth, Germany Center for Applied Geoscience, Eberhard Karls Universität, Tuebingen, GermanySearch for more papers by this author Lindsay J. McHenry, Lindsay J. McHenry [email protected] Department of Geosciences, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin, USASearch for more papers by this authorVincent Chevrier, Vincent Chevrier W. M. Keck Laboratory for Space and Planetary Simulation, Arkansas Center for Space and Planetary Sciences,, University of Arkansas, Fayetteville, Arkansas, USASearch for more papers by this authorChristian Schröder, Christian Schröder Department of Hydrology, University of Bayreuth, Bayreuth, Germany Center for Applied Geoscience, Eberhard Karls Universität, Tuebingen, GermanySearch for more papers by this author First published: 06 April 2011 https://doi.org/10.1029/2010JE003680Citations: 16AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract [1] Jarosite occurs within altered tephra from the saline-alkaline paleolake deposits of Pliocene-Pleistocene Olduvai Gorge, Tanzania. Zeolites (mainly phillipsite), authigenic K-feldspar, and Mg/Fe-smectites dominate the mineral assemblage, indicating saline-alkaline diagenetic conditions (pH > 9). As jarosite is ordinarily an indicator of acidic conditions on Earth and Mars, its association with such undisputed high-pH indicators is unexpected. Of 55 altered tephra samples collected from the paleolake basin and margin deposits, eleven contained jarosite detectable by X-ray Diffraction (XRD) (>0.15%). Mössbauer spectroscopy, Fourier Transform Infrared Reflectance (FTIR), Electron Probe Microanalysis (EPMA), X-ray Fluorescence (XRF), and Scanning Electron Microscopy (SEM) analyses confirm the presence and nature of the jarosite. This paper documents this occurrence and presents mechanisms that could produce this unusual and contradictory mineral assemblage. We favor a mechanism by which jarosite formed recently, perhaps as modern ground and meteoric water interacted with and oxidized paleolacustrine pyrite, providing local and temporary acidic conditions. However, local groundwater (at modern springs) has a pH > 9. In recent studies of Mars, the presence of jarosite or other Fe or Mg sulfates is often used to indicate dominantly acidic conditions. Regardless, the current study shows that jarosite can form in sediments dominated by alkaline minerals and solutions. Its coexistence with Mg/Fe smectites in particular makes it relevant to recent observations of Martian paleolakes. 1. Introduction Jarosite on Mars and Earth [2] On Earth and on Mars, the presence of the hydrous iron sulfate mineral jarosite ((K, Na, H3O)Fe3+3(OH)6(SO4)2) most often indicates aqueous, acidic, and oxidizing conditions [e.g., Stoffregen et al., 2000; Elwood Madden et al., 2004; Papike et al., 2006]. Jarosite typically indicates water-limited rock alteration [Elwood Madden et al., 2004] and thus evaporation in acidic environments [e.g., Tosca et al., 2005], acid-sulfate alteration of basalt under solfatara [e.g., Bishop et al., 2007] or hydrothermal [e.g., Morris et al., 1996] conditions or weathering of sulfide-rich deposits in oxidative environments [e.g., Burns and Fisher, 1990; Chevrier et al., 2004, 2006; Fernández-Remolar et al., 2005]. It is rarely observed under conditions of pH > 4, and typically forms at lower pH [e.g., Dutrizac and Jambor, 2000; Stoffregen et al., 2000]. While some life forms on Earth have adapted to acidic conditions, Knoll et al. [2005] argue that such conditions would have posed a challenge to prebiotic reactions that are thought to have played a role in the origin of life. [3] The Mars Exploration Rover (MER) Opportunity detected the presence of jarosite on Mars [Klingelhöfer et al., 2004]. Burns [1986] had long predicted that jarosite or other Fe3+ sulfate minerals would be present in the Martian regolith. Recent data from orbiters have revealed that the layered sulfate deposits of Meridiani Planum are extensive, and that similar deposits are abundant elsewhere on Mars [e.g., Bibring et al., 2007]. Data from Opportunity allow for a detailed analysis of the mineralogical and geochemical context of one occurrence of Martian jarosite. Despite the abundance of jarosite at the Opportunity site (it accounts for 29% of the Fe present in Burns Formation outcrop rocks [Morris et al., 2006b]) it has not yet been detected at this site from orbit [Murchie et al., 2009]. The first orbital detection of jarosite on Mars was near Ius/Melas Chasma in the Valles Marineris region [Milliken et al., 2008], and it has since also been identified in the potential paleolake deposits of Columbus Crater at Terra Sirenum [R. Wray et al., 2009] and within the vicinity of Mawrth Vallis [Farrand et al., 2009]. Jarosite occurs in association with phyllosilicates, which typically form under neutral/high pH conditions, in Ius Chasma [Roach et al., 2010] and in paleolake deposits [R. Wray et al., 2009; Farrand et al., 2009], attesting to variable pH conditions. To form and preserve abundant jarosite following the conventional model, conditions must remain acidic or water-limited over time. This is difficult in an environment dominated by basaltic weathering, as this leads to alkaline conditions by neutralizing acidity. One possible source of acidity is the oxidation of Fe2+ from the original igneous minerals to Fe3+ [Klingelhöfer et al., 2004; Morris et al., 2006b; Tosca et al., 2008]. [4] The jarosite observed at Meridiani and elsewhere today could also be a more recent (perhaps even Amazonian) phenomenon compared to the original late Noachian/early Hesperian time of deposition [e.g., Fairén et al., 2009]. Jarosite, especially in its pulverulent (poorly crystalline) form, is sensitive to changes in pH, temperature, oxidation conditions, and water abundance and therefore might not have been able to withstand the pre-Amazonian environmental changes. If true, jarosite may have formed much later, under more favorable climatic conditions (within the last 2 Ga), as earlier formed rocks and sediments interacted with small volumes of acidic brines [Fairén et al., 2009]. Under these conditions, jarosite needs not have formed as part of an equilibrium assemblage with the other minerals observed, and could reflect different diagenetic conditions. Regardless of when it was formed, the rate at which jarosite dissolves in contact with aqueous fluids limits the amount of time a jarosite-bearing deposit could have been exposed to such conditions since formation [e.g., Elwood Madden et al., 2009]. Elwood Madden et al.'s [2009] experiments suggest that, depending on the temperature and fluid composition, a 10 μm particle of jarosite can last from 1.5 years (in warm, dilute water) to 1 Ma (in cold, NaCl brines). Concretionary jarosite is more resistant to weathering; Miocene-aged concretionary jarosite has been observed in the Rio Tinto area of Spain [Fairén et al., 2009]. [5] Another hypothesis suggests that Martian jarosite formed as a result of the interaction of dust and sulfate aerosols within a large ice deposit, possibly formed at a time of high obliquity [Niles and Michalski, 2009]. This model would allow locally acidic conditions to persist without being neutralized by interaction with the basaltic regolith, and could allow for localized pockets of acidic alteration over a regional scale. [6] The presence of zeolites in certain Martian environments is often considered unlikely because of the alkaline conditions they tend to form in, in contrast to the acidic conditions implied by the Mg and Fe sulfates identified [e.g., Clark et al., 2005; Ming et al., 2006]. However, Ruff [2004] found spectral evidence in support of a zeolite component to Martian dust, and analcime (NaAlSi2O6 · H2O), a zeolite mineral typical of saline-alkaline environments [Langella et al., 2001], has now been identified at Nili Fossae on Mars [Ehlmann et al., 2009]. The abundance and frequency of zeolites on Mars is difficult to assess, in part because of the inability to distinguish between certain zeolites and polyhydrated sulfates in spectral data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on the Mars Reconnaissance Orbiter (MRO) [e.g., J. J. Wray et al., 2009]. [7] A growing number of mineralogically complex and in some cases nonacidic occurrences of jarosite on Earth show that jarosite is not an indicator of a single diagenetic environment. For example, Ashley et al. [2004] found jarosite formed from oxidizing pyrite in a neutral freshwater spring environment in Loboi Swamp, Kenya. Leveille [2007] found jarosite formed in acidic microenvironments in otherwise highly buffered carbonate sediments in a polar desert. Darmody et al. [2007] found jarosite forming (from pyrite weathering) in a cold, arid, and carbonate buffered environment in Swedish Lapland. Benison and LaClair [2003] first suggested the relevance of Western Australian saline lakes as Mars analogs. Gray [2001] and Baldridge et al. [2009] describe jarosite in these laterally variable phyllosilicate-bearing lacustrine environments, where pH and oxidation conditions vary both horizontally and vertically on a scale of meters to tens of meters, allowing the coexistence of neutral and acidic mineral assemblages in a small area. [8] Jarosite is also found in altered tephra in the saline-alkaline Pliocene-Pleistocene paleolake deposits of Olduvai Gorge, Tanzania [Hay, 1973; McHenry, 2009]. At Olduvai, zeolites (mainly phillipsite, (Ca, Na2, K2)3Al6Si10O32 · 12H2O), authigenic K-feldspar (KAlSi3O8), and Fe/Mg smectite dominate the mineral assemblage, indicating saline-alkaline diagenetic conditions. Phillipsite forms in altered volcanic ashes in closed basin environments only once the pH reaches 9–10 [Langella et al., 2001]. A detailed review of the authigenic minerals (including bulk composition) is presented by McHenry [2009, 2010]. All minerals in the altered Tuff IF samples are most likely authigenic or primary volcanic, since the samples come from the interior layers of a volcaniclastic surge and air fall deposit [Stollhofen et al., 2008]. The complete lack of quartz in the samples from the paleolake also limits the possibility of detrital input, as quartz is a major constituent of the detrital influx into the paleolake from the west. No glass was preserved within the paleolake deposit, but tephra can be correlated (using phenocryst composition) between paleolake center and glass-bearing paleolake margin and paleo-freshwater wetlands samples, providing access to prealteration tephra composition [McHenry, 2005]. Despite the abundance of zeolites and other indicators of saline-alkaline conditions (e.g., casts after trona, Na3H(CO3)2 · 2H2O), jarosite is present within the altered tephra at some sites [Hay, 1973; McHenry, 2009]. Objectives [9] The objectives of this study are to (1) document and constrain this unusual occurrence of jarosite in saline-alkaline paleolacustrine sediments at Olduvai, (2) develop a model that can explain this assemblage, and (3) compare this assemblage and model to data and interpretations from missions to Mars. Where possible, this study will employ techniques recently applied to the Martian surface (e.g., Mössbauer, NIR spectroscopy) for more direct comparison, and will compare the results of these methods to the results of other methods (X-ray Diffraction (XRD), Electron Probe Microanalysis (EPMA), Scanning Electron Microscopy (SEM)) to test the consistency of the different methods. 2. Background: The Geology of Olduvai Gorge [10] Olduvai Gorge exposes a Pliocene-Pleistocene lacustrine basin on the shoulder of the East African Rift in northern Tanzania (Figure 1). This basin is adjacent to the Ngorongoro Volcanic Highlands, which supplied the volcanic ash and sediments that make up much of the Olduvai sedimentary record. Deposition in the Olduvai basin began around 2.03 Ma [Walter et al., 1992]. Figure 1Open in figure viewerPowerPoint Maps showing location of Olduvai Gorge and sampled sites. (a) Location of Olduvai Gorge and nearby Ngorongoro Volcanic Highlands volcanic sources. (b) Map of sampled localities and Lower Bed II depositional environments. Maps after Hay [1976]. Copyright 1976 by the Regents of the University of California. [11] The ∼100 m thick Olduvai Formation is divided into a series of beds, including Beds I-IV, Masek, Ndutu, and Naisiusiu, from oldest to youngest. A shallow, saline-alkaline lake occupied the center of the basin during the deposition of Beds I and II, between about 1.9 and 1.7 Ma [Hay, 1976; Walter et al., 1992]. It expanded and contracted over time in response to changes in climate, faulting, and volcanic input (Figure 1b). The presence of abundant zeolites and authigenic K-feldspar, along with casts after trona [Hay, 1976], indicates prolonged periods of saline-alkaline conditions in the closed-basin lake and in pores within the sediments beneath it [McHenry, 2010]. This paleolake can thus be considered both a depositional and a diagenetic environment. [12] During the deposition of Bed I, a series of trachytic, trachyandesitic, and phonolitic tephra were deposited, altered, and preserved in the lake sediments. The major tephra from the lake deposit include Tuffs IA, IB, ID, IE, Ng'eju, and IF [Hay, 1976; McHenry, 2005]. 3. Methods Site Selection and Sampling [13] Samples were initially collected in 1999, 2002, and 2006 as part of a tephra alteration and correlation project [McHenry, 2005, 2009, 2011; McHenry et al., 2008]. The discovery of jarosite prompted more detailed sampling in 2007, 2008, and 2009 to narrow down its occurrence. Using Hay's [1976] maps and facies interpretations and McHenry's [2009, 2010] prior authigenic mineral results, sample sites were selected from different depositional and diagenetic environments across the paleolake deposit. Sites within the central paleolake basin, intermittently dry paleolacustrine zone, proximal and distal paleolake margin, and freshwater wetland paleoenvironments were selected and sampled (Figure 1). Sampling focused on Tuff IF because this tephra layer is thicker and more easily recognizable than the other Olduvai tephra [McHenry, 2005; McHenry et al., 2008], and because jarosite was first recognized in it [McHenry, 2009]. [14] Four central paleolake basin sites (Localities 80, 54, 77, and 78) were sampled to determine the distribution of jarosite within the paleolake deposit. At Localities 80 and 54, all Bed I tephra layers were sampled to see how the presence of jarosite varied with stratigraphic position and with starting tephra composition. At Localities 80, 77, and 78, 2–6 samples per site were collected from the different layers within Tuff IF to determine the distribution of jarosite within individual exposures. At Locality 80, Tuff IF was sampled in detail (2–6 samples per site) at three sections within ∼100 m of each other, to see if the jarosite occurrence is laterally consistent. One specific part of a single layer of Tuff IF at Locality 80 was initially sampled in 2006 and then resampled in 2008 and 2009. A photograph of the main Locality 80 exposure, including sample positions, is provided in Figure 2. All samples were collected in July or August, during Tanzania's dry season. About 200 g per sample were collected. Mineral precipitates were also sampled at a modern spring at Locality 78 (within meters of the sampled tephra) to help constrain the composition of modern groundwater in the area. The water from two springs at Locality 78 was also analyzed for pH and conductivity in the field. Figure 2Open in figure viewerPowerPoint (a) Photograph of Tuff IF at Locality 80, with sample positions labeled. Surrounding the tuff are green, lacustrine claystones. Jarosite-bearing samples 06-T80, 08-T4, and 09-T23 were collected from a light-colored, fine-grained, powdery layer enclosed by layers of coarser, more resistant non-jarosite-bearing tephra above and below. Other paleolake basin Tuff IF sites (Localities 80E, 80W, 77, and 78) have similar layering. (b) Secondary electron (SE) SEM image of unpolished sample 06-T80, showing texturally associated jarosite and phillipsite, identified using crystal shape, qualitative EDS, and comparison to XRD results. (c) SE SEM image of a mass of small, well-defined jarosite crystals in unpolished sample 06-T80. (d) Backscattered electron (BSE) SEM image of associated jarosite (J), smectite (S), and phillipsite (P) in a polished thin section of sample 06-T80. [15] Samples of Tuff IF from three sites (Localities 91, 88, and 46b) in Hay's [1976] intermittently dry lacustrine facies, five sites in the proximal paleolake margin area (Localities 45, 20, 44, 42, and 85), one from the distal paleolake margin (Locality 38), and three sites within the paleo-freshwater wetlands deposit (Localities 39, 40, and 41) were also collected. Tuff IF at Locality 38 (distal paleolake margin) varied laterally in appearance and was thus sampled from multiple exposures. Together these sites represent a transect across the saline-alkaline paleolacustrine deposit, providing samples from the different bands of authigenic minerals. Table 1 lists all samples collected, with their localities, depositional/diagenetic environments, and authigenic mineral assemblages. Table 1. Diagenetic Environments and Authigenic Mineral Assemblages From XRD Locality Tephra Sample Depositional Environmenta Max Intensity cpsb Amorphous Smectite Phillipsite Chabazite Analcite Anorthoclase K-spar Jarosite Calcite Quartz Erionite Unidentified Jarositec (%) Loc 40 IF 02-T103 Wetland 33 XXX + − − − + − − − − − − Loc 41 IF 02-T118 Wetland 42 XXX + − − − + − − − − − − Loc 39 IF 06-T64 Wetland/distal 44 XXX X X − − + − − − − − − Loc 41 IF 02-T117 Wetland 42 XXX + − − − + − − − − − − Loc 99 IF 01-T13 S Margin 88 XXX + − − − X − − − − − − Loc 38 IF 02-T120 Distal margin 180 X XX − + X + − − − − − − Loc 38 IF 02-T124 Distal margin 115 X XXX − − X + − − − − − − Loc 38 IF 02-T123 y Distal margin 142 X XXX X XX X + − − − − − − Loc 38 IF 02-T123 g Distal margin 95 XX XXX X X + + − − − − − − Loc 38 IF 02-T122 Distal margin 145 − XXX XXX XX XX + − − − − − − Loc 45 IF 06-T1 Proximal margin 175 X XX XXX XXX XX + − − − − − − Loc 20 IF 06-T66 Proximal margin 175 X X XXX XX XX + − − XX − − − Loc 44 IF 06-T92 Proximal margin 102 − + XX XX − + − − − − − − Loc 42 IF 06-T4 Proximal margin 235 X X XXX XXX XX + − − − − − − Loc 85 IF 06-T93 Proximal margin 230 − X XXX XXX − + − − − − − − Loc 91 IF 06-T57 Intermittent 340 X X XXX − X + − − − − − − Loc 88 IF 06-T41 Intermittent 320 X XX XXX − X + − − − − − − Loc 46 IF 02-T133 Intermittent 290 − XX XXX X X + − − − − − − Loc 80 IA 99-80-1A Lake 510 − XX XX XX XX + − − XXX − − − Loc 80 IB 99-802-1B Lake 300 − X XXX − − XX XXX X − − − − 2.60 Loc 80 below ID 99-802-22.5 Lake 270 − X XX − + XX XX − XXX − − − Loc 80 ID 99-802-1D Lake 580 − + XXX − + XXX + − − − − − Loc 80 Ng'eju 99-802-1Em Lake 660 − + XX − X XXX XX − − − − − Loc 80 Ng'eju 99-802-1Eb Lake 500 − + XXX − − XX X − − − − − Loc 80 Ng'eju 99-802-1Et Lake 330 − X XXX − − XXX X − − − − − Loc 80 IF 06-T80 Lake 250 − X XXX − X + XX XX − − − − 4.48 Loc 80 below IF 06-T85 Lake 487 − XX + − XXX − X − XXX − − − Loc 80 below IF 06-T86 Lake 245 − XX XX − XXX XX − − − − − Loc 80 IF 08-T1 Lake 300 − X XXX − + + XXX − − − − − Loc 80 IF 08-T2 Lake 340 − X XX − X XXX X − − − − − Loc 80 IF 08-T3b Lake 500 − X XX − X XXX X − − − − − Loc 80 IF 08-T3T Lake 250 − − XXX − + XX X − − − − − Loc 80 IF 08-T4 Lake 250 − X XXX − X + XX + − − − − 0.69 Loc 80 IF 08-T5 Lake 500 − − XX − X XXX XX − − − − − Loc 80 IF 09-T23 Lake 252 − X XXX − X + XX + − − − − 0.29 Loc 80-E IF 08-T6 Lake 410 − X + − + XXX XX X − − − − 2.77 Loc 80-E IF 08-T7 Lake 340 − X xxx − X XXX XXX − − − − − Loc 80-E IF 08-T8 Lake 460 − X XXX − + XXX XX − − − − − Loc 80-E IF 08-T9 Lake 250 − X XXX − + XX X − − − − − Loc 80-E IF 08-T10 Lake 300 − X XXX − + X XX − − − − − Loc 80-W IF 08-T11 Lake 250 − X XXX − + X XX + − − − − 0.39 Loc 80-W IF 08-T12 Lake 460 − − XXX − + XXX XX − − − − − Loc 77 IF 08-T33 Lake 600 − − XXX − − XXX XX − − − − − Loc 77 IF 08-T34 Lake 300 − X XXX − − XXX X + − − − − 0.55 Loc 77 IF 08-T35 Lake 340 − X XX − − XXX XX + − − − − 0.74 Loc 77 IF 08-T36 Lake 310 − x XXX − + XXX XX − − − − − Loc 78 IF 08-T49 Lake 420 − X XXX − X XX XX − − − − − Loc 78 IF 08-T50 Lake 390 − + XXX − + XXX X − − − − − Loc 78 IF 08-T51 Lake 330 − + XXX − X XXX X X − − − − 1.03 Loc 78 IF 08-T52 Lake 480 − X XXX − + XXX XX + − − − − 0.15 Loc 54 IB 02-T9 Lake 317 − + X − + X XXX − − − − − Loc 54 IE 02-T11 Lake 427 − X XX − − XXX XX − − − − XXX Loc 54 Ng'eju 02-T12 Lake 1038 − + XX − − XXX XX − + − X X Loc 54 Ng'eju 02-T13 Lake 1066 − X − − − XXX XXX − − − − XX Loc 54 below IF 02-T14 Lake 1906 − + XXX − − XXX XX − − − − XX Loc 54 IF 02-T15 Lake 365 − X XXX − − XXX XX − − − − − Loc 54 IF 02-T16 Lake 341 − XX + − − XXX XXX − − − − − Loc 54 IF 02-T17 Lake 410 − X XX − X XXX XX + − − − − 0.49 a Depositional environment relates to Pleistocene sediments, and not to modern conditions. Tephra names are after Hay [1976]; tephra IDs are after McHenry [2005]. XXX, abundant; XX, common; X, rare to common; +, rare; −, absent. b Percent jarosite calculated by Rietveld refinement (excluding clay minerals and amorphous); blank, 0%. c Intensity above background of most intense peak. Sample Preparation and X-Ray Diffraction [16] Samples were cleaned, removing any crack fillings or roots, and then crushed and finely powdered in an agate mortar and pestle. The fine powders were then mounted for random powder XRD analysis, and analyzed using a Bruker D8 Focus. Every sample was analyzed using a "quick" routine: 2–60° 2θ, 1 s/step, step size 0.02° (Cu Kα radiation, Sol-X energy dispersive detector). Samples were compared against the ICDD PDF library using Bruker's EVA software and if potential jarosite peaks were present, the sample was rerun at 4 s per 0.01° 2θ to help separate potential minor jarosite peaks from background. Jarosite-bearing XRD patterns were further analyzed by Rietveld refinement using Bruker's TOPAS software and structure library to determine the jarosite concentration. The relative abundances of other minerals were determined qualitatively and categorized as abundant (highest peaks in the XRD pattern), common (prominent peaks, but not the highest), rare to common (easily identified peaks), rare (peaks recognizable), and absent (not identified). X-Ray Fluorescence Analyses [17] Select powdered samples were prepared and analyzed for major elements using a Bruker S4 Pioneer XRF spectrometer. 1.000 g of each sample was combined with 10.000 g of a 50/50 LiT/LiM flux with an integrated nonwetting agent and ∼1 g of ammonium nitrate (oxidizer) and then fused in a Claisse M4 fluxer following the methods of McHenry [2009]. Additionally, 10 g of three jarosite-bearing powdered samples were milled with a wax binder and pressed at 30 tons of pressure for 1 min into 40 mm pressed pellets. The fused beads were analyzed for major elements (except S) using a calibration curve based on 11 USGS rock standards [McHenry, 2009], and the pressed pellets were analyzed for sulfur using a calibration curve based on 6 USGS rock standards. Loss on Ignition was determined by heating a dried, powdered sample in a muffle furnace at 1050°C for 15 min. Electron Probe Microanalysis [18] A thin section for sample 06-T80 (Locality 80) was prepared and analyzed by EPMA (Cameca SX 50) to determine the jarosite's chemical composition. The instrument was operated at 15 kV and 6 nA, with some analyses conducted using a focused beam and others using a beam defocused to 10 μm. The instrument was calibrated using sulfate and silicate standards. The small grain size (<1 μm) made it impossible to quantitatively measure the composition of a single crystal, though the jarosite crystals occur in overlapping clusters. Low totals likely indicate void space between adjacent crystals. Scanning Electron Microscopy [19] The same thin section used for EPMA was also imaged using a Hitachi S-3400 (SEM) to assess the context of the jarosite in relation to the zeolites. Minerals were identified qualitatively using Energy-Dispersive X-ray Spectrometry (EDS). An unpolished butt of sample 06-T80 was also examined using a Hitachi S-570 SEM to observe the crystal shapes and textural relationship between the jarosite and phillipsite. EDS was also used to qualitatively determine the composition of the jarosite. Near-Infrared Methods [20] Fourier Transform Infrared Reflectance (FTIR) spectra were taken using a Nicolet 6700 Smart Diffuse spectrometer with N2 purge gas to remove atmospheric gases (H2O and CO2). NIR spectra were recorded in reflectance mode in the range of 200–12,500 cm−1 (0.8–5.0 μm) with a resolution of 4 cm−1, although we subsampled the 1.0–2.6 μm range so that the data could be more easily compared to Observatoire pour la Mineralogie, l'Eau, les Glaces et l'Activité (OMEGA) and CRISM data available in the literature. The configuration used a quartz-halogen IR source, a CaF2 beam splitter, and a DTGS detector. Background spectra using a Spectralon white standard were systematically taken before each sample's spectra to remove instrumental signal. Mössbauer Methods [21] Powdered samples prepared for XRD analyses were also used for Mössbauer analyses. A total of 15 samples were investigated, including samples from the paleowetland (02-T103); the distal paleolake margin (02-T122; 02-T124), intermittent (06-T41) and proximal paleolake margin (06-T93); and the paleolake center (99-802-1B; 02-T17; 08-T4; 08-T6; 08-T11; 08-T17; 08-T34; 08-T35; 08-T51; 08-T52). All samples identified as jarosite-bearing by XRD were also investigated by Mössbauer spectroscopy (except for 09-T23). Mössbauer spectra were obtained at room temperature with laboratory copies of the MER MIMOS instruments [Klingelhöfer et al., 2003], which are set up in backscattering geometry. The Mössbauer source was 57Co in Rh matrix. Spectra were evaluated analogous to Martian spectra, as described by Morris et al. [2006a, 2006b]. Area ratios are f-factor corrected to account for differences in recoil-free fractions (f(Fe3+)/f(Fe2+) = 1.21). Mössbauer parameters are quoted relative to α-Fe0. Five of the samples were analyzed across a wider velocity (i.e., energy) range of approximately ±11 mm/s to scan for the presence of sextet phases typical for iron oxides such as hematite. Because there was no evidence for such in these five spectra, the remainder of the samples was investigated over a smaller velocity range of approximately±6 mm/s to increase the resolution for the central doublets in the spectra. 4. Results XRD [22] The XRD results for all samples are provided in Table 1. Eleven of the 55 samples analyzed by XRD have jarosite peaks above background. Rietveld refinement yielded concentrations for jarosite between 0.15 and 4.5% for these samples. All jarosite-bearing samples came from altered tephra in the central paleolake basin. The three samples collected from the same part of Tuff IF at Locality 80 in three separate visits (July or August 2006 (06-T80), 2008 (08-T4), and 2009 (09-T23)) all contained jarosite, but in different abundances. [23] The rest of the mineral assemblage for the central paleolake basin samples consisted of anorthoclase (the dominant mineral in the fresh tephra, (Na, K)AlSi3O8), the zeolite mineral phillipsite, authigenic K-feldspar, and minor analcime and smectite. Relative abundances of these phases varied between sites and between samples from the same site, especially in the content of volcanic anorthoclase phenocrysts, the abundance of which varies depending on the specific part of the tephra sampled. Samples outside of the central paleolake basin lack jarosite and authigenic K-feldspar, instead containing an assemblage of phillipsite, chabazite ((Ca, Na2, K2, Mg)Al2Si4O12 · 6H2O), minor analcime, and more abundant smectite. The paleolake margin samples contain more smectite, with minor zeolites and no jarosite. A more detailed account of the authigenic mineralogy across the paleolake basin is provided by McHenry [2009, 2010]. Three XRD plots, for samples 06-T80 (jarosite-rich), 08-T51 (minor jarosite) and