Ninth International Conference on Mars 2019 (LPI Contrib. No. 2089) 6390.pdf

CLAY MINERALS OF GLEN TORRIDON, MOUNT SHARP, GALE CRATER, MARS. T. F. Bristow1, E. B. Rampe2, J. P. Grotzinger3, V. K. Fox3, K. A. Bennett4, A. S. Yen5, A. R. Vasavada5, D. T. Vaniman6, V. Tu2, A. H. Treiman7, M. T. Thorpe2, S. M. Morrison8, R. V. Morris2, D. W. Ming2, A. C. McAdam9, C. A. Malespin9, P. R. Mahaffy9, R. M. Hazen8, S. Gupta9, R. T. Downs10, G. W. Downs10, D. J. Des Marais1, J. A. Crisp5, P. I. Craig6, S. J. Chipera11, N. Castle7, D. F. Blake1, and C. N. Achilles8, 1NASA Ames Research Center, Moffett Field, CA (thomas.f.bristow@nasa.gov), 2Johnson Space Center, Houston, TX, 3California Institute of Technology, Pasadena, CA, 4USGS Astrogeology Science Center, Flagstaff, AZ, 5JPL/Caltech, Pasadena, CA, 6Planetary Science Institute, Tuscon, AZ, 7Lunar and Planetary Institute, USRA, Houston, TX, 8Carnegie Institute, Washington DC, 8Goddard Space Flight Center, Greenbelt, MD. 9Imperial College, London, UK. 10University of Arizona, Tuscon, AZ, 11Chesapeake Energy, Oklahoma City, OK.

Introduction: Phyllosilicates are common compo- habitability and organic preservation potential to be as- nents of Noachian and Early Hesperian age (>3.5 Ga) sessed [5]. Combined with in situ examination of the terrains on Mars, providing evidence of a larger water terrain, mineralogical data will provide important inventory earlier in the planet’s history [1]. The end of ground-truth for orbital data and new information on the phyllosilicate formation appears to coincide with plane- factors that influence the detectability of clay minerals tary aridification, also signaled by an increasing abun- from orbit. dance of sulfate and Fe-oxide minerals [2]. This miner- In this contribution we present preliminary CheMin alogical motif has been identified by orbital spectrome- XRD mineral analyses from two sets of drill cuttings re- try in stratified deposits of Aeolis Mons (Mount Sharp), cently collected from GT. These drill targets are called a 5 km tall mountain in the center of Gale crater, with Aberlady and Kilmarie. distinct clay-bearing strata overlain by sulfate-bearing Sample context: MSLs’ first foray into GT in- units [3]. The Mars Science Laboratory (MSL) Rover, volved surveying strata along the contact between Vera Curiosity, was sent to study these units and examine the Rubin ridge (VRR) and GT (Fig. 2). Aberlady and Kil- possibility that they record a global change in environ- marie were drilled in laminated mudstone bedrock ex- mental conditions [4]. posed in a topographic low (GT) adjacent to VRR. This area is characterized by strong orbital phyllosilicate sig- natures. Based on the recessive nature of GT and the rel- ative ease of drilling, Aberlady and Kilmarie are com- prised of weaker lithologies than VRR rocks, which may indicate variations in bulk mineralogy.

Fig. 1 – HiRISE view of the MSL traverse area, with orbital CRISM detections of CBU Fe/Mg smectite over- lain in blue.

Glen Torridon and the clay-bearing unit: Earlier Fig. 2 – HiRISE view of the Aberlady and Kilmarie drill this year MSL began investigating Glen Torridon (GT) location showing adjacent rock units. [5] – a geomorphic feature that encompasses the clay- bearing unit (CBU) originally identified from orbit (Fig. Glen Torridon mineralogy: Quantitative mineral 1) [3]. The campaign aims to document the nature, abundances derived from CheMin XRD patterns (Fig. abundance and origin of mineral assemblages, including 3) show that Aberlady and Kilmarie are the most clay the clay minerals of GT. This will help constrain strati- mineral-rich samples analyzed by CheMin in Gale graphic models of sedimentary deposits at Gale, im- crater to date (>30 wt.%). These samples also contain prove our understanding of how environmental condi- significant amounts (>10 wt.%) of plagioclase feldspar, tions changed in Gale crater, and allow ancient an X-ray amorphous component, and the Ca-sulfates Ninth International Conference on Mars 2019 (LPI Contrib. No. 2089) 6390.pdf

basanite and anhydrite. Minor constituents (<5 wt.%) lacustrine conditions and processes support formation confidently identified at this stage include hematite and close to the time of deposition [10,11,13]. pyroxene. The most Fe-rich smectites appear within the rocks The position and breadth of XRD peaks attributed to of VRR. Although the mechanical strength of VRR and clay minerals indicate that they include contributions GT are different, they appear to be stratigraphically from smectite group minerals. More detailed infor- equivalent. Sedimentary facies observed in both GT and mation about the chemistry of these smectites, and other VRR are similar to 100’s m of primarily lacustrine de- possible phyllosilicate components, is under investiga- posits of the underlying Murray formation. No evidence tion using a combination XRD, SAM evolved gas anal- of a depositional break or tectonic contact between GT ysis, and geochemical data from APXS and ChemCam and VRR is observed. The similarities in sedimentary [6,7] . facies suggest comparable depositional environments. Despite sharing a common depositional history, VRR and GT have notable differences in mineral makeup. The smectite abundance of VRR drill samples analyzed by CheMin is less than half of GT rocks [15]. Additionally, VRR samples contain significant amounts of Fe-oxide and oxyhydroxides, hematite and akagene- ite, whereas hematite is a minor component of GT [15]. Textural, geochemical, and mineralogical evidence in- dicates that VRR was subject to enhanced diagenetic al- teration [16]. Together, this suggests that the geo- morphic expression of the VRR and transition from VRR to GT represents a diagenetic front, with GT con- taining the better preserved lacustrine sediments. This may have implications for the preservation potential of organic molecules. In addition, comparison of the min- eral assemblages of VRR and GT may provide new in- sights into the conditions and nature of diagenetic fluids that modified the Murray fm. References: [1] Carter J. et al. (2013) JGR Planets, 118, 831-851. [2] Bibring J-P et al. (2006) Science, 312, 400-404. [3] Milliken R. E. et al. (2010) GRL, 37, Fig. 3 – 2-dimensional CheMin XRD pattern of the Kil- L04201. [4] Grotzinger J. P. et al., (2012) Space Sci. marie drill sample. Rev. 170, 5-56. [5] Fox V. K. et al. (2019) LPSC, 50. [6] McAdam, A. C. et al. (2019) this conf. [7] Dehouck, E. Comparative clay mineralogy: An important as- et al. (2019) this conf. [8] Grotzinger J. P. et al. (2015) pect of the GT campaign involves comparing the char- Science, 350, aac7575. [9] Fedo C. M. et al. (2018) acteristics and origins of GT clay minerals with clays LPSC 49. [10] Vaniman D. T. (2014) Science, 343, previously documented during the mission. Since land- 1243480. [11] Bristow T. F. et al. (2015) American Min- ing in 2012, MSL has traversed almost 400 m of vertical eralogist, 100, 824-836. [12] Rampe E. B. et al. (2017) stratigraphy consisting of fluvial-lacustrine sedi- Earth Planet. Sci. Lett., 471, 172-185. [13] Bristow T. ments/sedimentary rocks of the Bradbury and Mount F. et al. (2018) Science Advances, 4, eaar3330. [14] Sharp Groups, deposited ~3.5 Ga [8,9]. Although orbital Schieber J. (2017) Sedimentology, 64, 311-358. [15] phyllosilicate signatures are absent or muted along trav- Bristow, T. F. et al. (2018) AGU annual meeting. [16] erse to GT, the majority of the drill samples collected Fraeman, A. A. et al. (2019) this conf. from these units by MSL, contain clay minerals, com- prising up to ~28 wt. % of the bulk rock [10-13]. Smec- tite clay minerals are dominant in all the clay-bearing samples analyzed so far. The smectites include ferrian Mg-rich trioctahedral, Al-rich dioctahedral, and Fe3+- rich dioctahedral varieties, indicating a range in paleo- aqueous alteration conditions [13]. Detrital sources for the clays have been proposed [14], but coincidental changes in the occurrence of Mg-rich trioctahedral and Al-rich dioctahedral smectites with sedimentological, mineralogical, and geochemical indicators of changing