INTERPRETING MARS SURFACE FLUID HISTORY USING MINOR AND TRACE ELEMENTS IN JAROSITE: AN EXAMPLE FROM POST PIT, NEVADA. P.V. Burger1, J.J. Papike1, C.K. Shearer1 , and J.M. Karner1, 1Institute of Meteoritics, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131 (pvburger@unm.edu). Introduction Martian jarosite was first identified in a jarosite/hematite rich outcrop at Meridiani Planum by the Opportunity Rover [1]. The utility of jarosite as a recorder of rock-fluid interaction has been discussed by Papike et al. [2,3,4]. Understanding the trace element crystal chemistry of terrestrial jarosite will further our understanding of martian near-surface processes and our ability to better interpret current martian surface data sets. Here, we discuss the morphological characteristics, and major and minor/trace element chemistry of jarosite from Post Pit, NV, and demonstrate its applicability to the martian surface. Major Element Crystal Chemistry of Jarosite The general formula for jarosite is AB3(XO4)2(OH)6 [2] where A is a 12-fold coordinated site that can contain mono-valent cations such as K, Na, and Rb, divalent cations such as Ca, Pb, Ba, and Sr, and trivalent cations such as the REE. The B position represents an octahedrally coordinated site that usually contains trivalent Fe and Al. The X position represents the tetrahedrally coordinated site and contains many elements including S, P, As, and Mo. The major ele-ment chemistry of terrestrial jarosite can be represented within the compositional space defined by alunite (KAl3(SO4)2(OH)6), natroalunite (NaAl3(SO4)2(OH)6), jarosite (KFe3+

3(SO4)2(OH)6) and natrojarosite (Na Fe3+

3(SO4)2(OH)6). Analytical Parameters Electron Microprobe – The sample was initially documented using backscattered electron imaging (BSE) at an accelerat-ing voltage of 15 kV and a beam current of 10 nA, using UNM’s JEOL 8200 electron microprobe. Wavelength dis-persive (WDS) maps were conducted in regions with promi-nent growth zones, as determined by BSE. Quantitative analyses were conducted using two analytical packages, one for major elements (Fe, K, Na, Al, S) using a 5 µm beam, and a 1 nA beam current to minimize volatilization, and a second package, using a 1 µm beam, with a 10 nA beam current for minor and trace elements (Ba, Sr, P, V, As, Pb, Cr). Analyses were conducted as traverses across euhedral jarosite grains, parallel to one another, and perpendicular to growth zones, so as to be directly comparable to one another. The remainder of the trace element data discussed here was collected by SIMS and is described by Burger et al. [5]. Results Sample Description – The Post Pit jarosite thin section is predominantly characterized by a shale host-rock, on which there is fine grained overgrowth of jarosite (Fig. 1a). Several larger (>100 µm) barite crystals are interspersed throughout this matte. An unidentified Fe-oxyhydroxide occurs at the exterior of the fine grained jarosite. A large, contiguous band of euhedral, zoned jarosite occurs above the Fe-bearing phase. Zoning bands occur on the sub-micron scale. Major Elements in Jarosite – The average major element composition of Post Pit jarosite falls into the pure endmem-ber jarosite composition field. A representative, stoichiomet-ric analysis is presented in Table 1. Sulfur concentration

Figure 1. (a) False color BSE of the Post Pit jarosite sample, where purple = shale, blue/green = jarosite, red = barite, yellow = Fe-oxyhydroxide. Figure 1b WDS map outlined in red. (b) WDS map of Ba concentration. Warmer colors indicate higher concentration. Red arrow indicates compositional profile shown in Fig. 2. averages 1.85 afu, K 0.95 afu, and little (0.01 afu) Na. Fe is the predominant B-site cation, with an average of 2.76 afu, relative to Al, with 0.05 afu. As growth zones occur on the sub-micron scale, major element compositional variation is attenuated by the larger spot size. Nevertheless, major ele-ment concentration do show limited variability through the profiles (Fig. 2a). WDS elemental mapping of jarosite crys-tals reveal subtle differences in the major element composi-tion throughout the jarosite grains. Minor and Trace Elements in Jarosite – P and Ba have the highest average concentrations, among the minor and trace elements, with 0.08 afu and 0.05 afu, respectively. There is also significant V (0.01 afu) and As (0.01 afu). Average Sr concentration is very low (0.001 afu), but spikes to a high of

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0.027 afu. Chromium and Pb are generally below the limit of detection for electron microprobe in this sample.

Oxide Oxide Oxide K2O 9.378 Fe2O3 46.871 SO3 32.010 Na2O 0.028 Al2O3 0.636 P2O5 0.258 BaO 0.557 V2O3 0.235 As2O5 0.000

SrO 0.000

Atoms / Formula Unit Atoms / Formula Unit Atoms / Formula Unit K 0.991 Fe 2.921 S 1.990 Na 0.005 Al 0.062 P 0.019 Ba 0.019 V 0.016 As 0.000 Sr 0.000Total: 1.015 Total: 2.999 Total: 2.009

A Site B Site X Site

Table 1. A representative, stoichiometric analysis of Post Pit jarosite. Discussion It is apparent from the observations that fluid chemistry plays an important role in determining the trace element composition of jarosite. Crystal morphology alone suggests changing fluid history, as the precipitation of fine grained jarosite and barite is followed by the deposition of Fe-oxyhydroxide. Later conditions again favor jarosite precipi-tation, but are still dynamic, resulting in the development of growth zones, as seen in the WDS map and composition profiles (Fig. 1b,2). Typically, growth zones in jarosite re-flect a change in precipitation from the (K) jarosite endmem-ber to the (Na) natrojarosite endmember [3,4]. Growth zones in Post Pit jarosite are characterized instead by changes in minor and trace element chemistry. This is seen as warmer colors in the Ba WDS map (Fig. 1b) and correspond to a drop in S and K, and an increase in P, Ba, and to a lesser extent, Sr, as seen in the compositional profile (Fig. 2). It appears likely that this occurs through a coupled substitu-tion:

Ba2+ (or Sr2+) + P5+ (or As5+) = K+ + S6+ This coupled substitution may be the result of changing fluid chemistry, temperature and/or possibly fO2. The current data set suggests that V occupies the B crystallographic site, with a valence of 3+. This implies a more reducing environment than is typical for jarosite deposition. Trace element analy-ses by [5] suggested a negative Ce anomaly in most jarosite samples where Ce is likely 4+ and not easily incorporated into the jarosite structure (Fig. 3); this is not the case with jarosite from Post Pit. Vanadium valence may be a particu-larly useful tool in recording fluid history if it occupies dif-ferent crystallographic sites in jarosite, depending on its valence state. Relevance to Mars Surface Processes – This study illustrates the potential of trace elements in jarosite for fingerprinting the evolution of martian fluids. In addition, Post Pit jarosite reinforces the need for a sample return mission to Mars, as the measurements required to characterize minor and trace element behavior in jarosite are not feasible using current remote sensing or robotic mission techniques.

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Figure 2. (a) Major element profile conducted across the grain seen in Fig. 1b. A decrease in S and K concentration corre-sponds to an increase in P, and Ba. (b) Minor element concentra-tion across the same traverse.

Figure 3. Rare earth element diagram for jarosite from Post Pit and three other locations, after [5]. Acknowledgments This research was supported by a NASA Cosmochemistry Grant to J.J.P. References [1] Klingelhöfer et al. (2004) Science, 306, 1740. [2] Papike et al. (2006) GCA, 70, 1309. [3] Papike et al. (2006) AmMin, 91, 1197. [4] Papike et al. (2007) AmMin, 92, 444. [5] Burger et al. (2007) 7th Inter. Conf. Mars. Abst. #3244.

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