45th Lunar and Planetary Science Conference (2014) 1171.pdf

CHEMICAL AND TEXTURAL OBSERVATIONS BY CHEMCAM OF CONGLOMERATES IN GALE CRATER R.C. Wiens1, N. Mangold2, O. Forni3, A. Ollila4, J. Johnson5, V. Sautter6, S. Maurice3, S. Clegg1, D. Blaney7, S. Le Mouelic2, R.B. Anderson8, N. Bridges5, B. Clark9, G. Dromart10, C. D’Uston3, C. Fabre11, O. Gasnault3, K. Herken- hoff8, Y. Langevin12, H. Newsom4, D. Vaniman13, G. Berger3, A. Cousin1, L. Deflores7, N. Lanza1, J. Lasue1, E. Lewin14, P.-Y. Meslin3, P. Pinet3, S. Schröder3, R. Leveille15, M.R. Fisk16, J. Blank17, N. Melikechi18, A. Mezzacap- pa18, J. Grotzinger19, and the MSL Science Team (1LANL; [email protected], LPGN2, IRAP/CNRS3, UNM4, APL/JHU5, MNHN6, JPL/Caltech7, USGS Flagstaff8, SSI9, U. Lyon10, U. Lorraine11, U. Paris-Sud12, PSI13, U. Gre- noble14, CSA15, Oregon State U.16, BAERI17, Caltech19) Overview and Localization: Observations of Conglomerate morphologies are illustrated in conglomerates along the Curiosity rover traverse Fig. 2. They usually appear as cemented agglomerates provide important clues to the sedimentary histo- of subangular to rounded pebbles with sand-size or ry of Gale crater [1]. Differences in clast sizes, finer-grained matrix. They usually lack obvious layer- and context, including imbrication, may indicate ing but often have a flat top due to erosion. Conglom- flow velocities. Clast shapes including rounding, erates often weather leaving loose pebbles on the and compositions may signal different source re- ground whereas finer material is removed by wind. gions and episodes of fluvial activity. The extent Conglomerate pebbles vary from light-toned to dark- of the conglomerate bedding helps define the toned, suggesting variety in the source material. stratigraphic relationships and the time-ordered Whereas many conglomerates are similar to the previ- history of the distal portion of Peace Vallis fan ously-studied clast-supported Link and Hottah targets and the lowest topographic portions of Gale [1], several have distinct morphologies such as poorly crater [2]. Compositions of the cementing materi- cemented breccia and matrix-supported conglomerates, al provide insights into suggesting origins other than the fluid chemistry dur- fluvial are locally possible, ing deposition [1]. including local impact brec- The ChemCam in- cia floats or cemented soil. strument suite provides Detailed studies of textures elemental compositions are ongoing. In contrast, and remote imaging to sandstone-like material (or 40 µrad resolution [3,4]. perhaps siltsone/mudstone, Over the first 360 sols at grain sizes not accessible ChemCam returned with RMI images) are usual- >75,000 laser-induced ly relatively light-toned and breakdown (LIBS) spec- display flat tops and a tra from 2,256 locations smooth texture suggesting on the Mars surface, ac- they are either of sedimen- companied by > 1,000 tary or volcaniclastic origin. images from the Remote Their composition is usually Micro-Imager (RMI). homogeneous and similar to Conglomerates were Bathurst [7] (e.g. Kasegalik, observed from sol 14, at sol 336) and are not dis- the landing site, until cussed here. ~40 [1,5], and then again Conglomerate Com- after sol ~326, upon positions: The conglom- leaving Yellowknife Bay Fig. 1. Names and locations of conglomerates analyzed erates present varied (YKB). The locations of compositions overall, as those analyzed by by ChemCam in the first 360 sols. YKB is to the right. might be expected for a ChemCam are shown in Fig. 1. The coverage 400 µm probe. Fig. 3 shows these compositions suggests that the dominant bedrock composition to be much more felsic than the Sheepbed sedi- over Bradbury rise, comprising most of the rover mentary unit, and in fact, significantly dissimilar traverse to date except for YKB, is a combination to any of the YKB units [9]. It is perhaps instruc- of conglomerates and sandstones. tive to compare the mean of these 61 conglomer- Description of Observations, Classification: ate locations to Jake Matijevic [10], with the ca- The targets sampled by ChemCam have been veat that ChemCam’s 14 points on Jake leave it classified first as likely igneous or sedimentary, underrepresented. The conglomerates are on av- with the igneous classified by grain size and other erage higher in alkalis and Al, and lower in Fe, characteristics [6]. Sedimentary targets are classi- Ti, Mg, and Ca (Fig. 3). A norm calculation sug- fied as conglomerate bedrock, conglomerate gests the conglomerates are on average higher in floats or small outcrops, sandstone-like layered albite, anorthite, and k-feldspar; and substantially floats, and sandstone bedrock. The target Bathurst lower in high-Ca pyroxene. is the only layered float analyzed by APXS [7]; Perhaps more instructive are comparisons ChemCam analyzed these in various locations among the different conglomerate regions, ob- and found them identical in composition [8]. served by independent component analysis (ICA). 45th Lunar and Planetary Science Conference (2014) 1171.pdf

Fig. 4 shows that conglomerates encountered near the mean Sr is 5x higher, and average Rb is 4x the landing site form an alkali ratio trend parallel higher. This may be evidence for differing ori- to Jake (which was not a conglomerate [10]), gins between the two types of conglomerates, and while those encountered after sol 350 are richer similar provenances within each group. in Na, with floats being the highest in alkalis. Conclusions: At more than 60 locations Link (sol 27; [1]) remains a high K/Na outlier. across Bradbury Rise, traversed prior to sol 40 Trace elements: Link also contains the highest and after sol 326, conglomerate analyses confirm abundances of Sr (to 1670±160 ppm) and Rb (to that alkali-rich compositions first observed at 190±30 ppm) in any target analyzed in the first conglomerate Link [1] are prevalent throughout 360 sols [1,11]. Point 4 in Kenwood_River has a the local area. Differences, particularly in Mg and similar Rb abundance to the lowest Rb in Link alkalis (Li, Na, K, Rb), suggest different source (~140 ppm) and this point is associated with high regions for the clasts. Si, Al, Na, and K, similar to Link. Ba is present Acknowledgement: This work was supported by in several locations, up to ~500 ppm and is gen- NASA’s Mars Program Office and by CNES. erally associated with high Si. For comparison, References: [1] R.M.E. Williams et al. (2013) Sci- the highest Ba observed in the first 360 sols is ence 340, 1068-1072, DOI: 10.1126/science.1237317. found in the light-toned rock called Chakonipau [2] J.P. Grotzinger et al. (2013) Sciencexpress, 9 De- (~2000 ppm, Sol 338). Li is low in the majority cember, DOI:10.1126/science.1242777 [3] S. Maurice of these targets. The highest Li is found in the et al. (2012) SSR 170, 95-166, DOI 10.1007/s11214- 5th point of Kenwood_River, which has ~40 012-9912-2. [4] R.C. Wiens et al. (2012) SSR 170, ppm. This point also has high TiO2 and FeOT 167-227, doi 10.1007/S11214-012-9902-4. [5] A. (1.8, 20 wt. % resp.) relative to the other 4 loca- Yingst et al. (2013) JGR 118, 2361–2380, tions on this target. Interestingly, when the con- doi:10.1002/2013JE004435. [6] Sautter et al., this glomerates are divided into bedrock-type (n=19 meeting. [7] M.E. Schmidt et al. (2013) JGR, in press. points) and float/small outcrop-type conglomer- [8] N. Mangold et al. (2014) submitted. [9] S.M. ates (n=21 points), the latter generally show in- McLennan et al. (2013) Sciencexpress, 9 December, creased trace elements, consistent with Fig. 4. For DOI:10.1126/science.1244734. [10] E.M. Stolper et al. example, the average Ba abundance is nominally (2013) Science 341, DOI: 10.1126/science.1239463. higher in the float/small outcrop conglomerates, [11] Ollila et al. (2013) JGR, in press. Fig. 2. Flat-lying conglomerate Red Wine, sol 339, Mastcam and ChemCam images (left); Float conglomerate Bird River, sol 359 (rt). FOV = 7.4 cm.

Fig. 3. Mean of 61 conglomerate locations (dashed line), rati- Fig. 4. Conglomerates display varying oed to Sheepbed, show that conglomerates are, overall, more Na/K ratios corresponding to location on depleted in Fe, Ti, Mg and enriched in Al, Na, and K than the Fig. 1. Targets early in the mission trend YKB units, and even more than Jake [10]. with Jake [10], shown for reference.