VEINS IN GLEN TORRIDON, GALE CRATER, MARS: EXPLORING THE POTENTIAL TRANSITION INTO THE SULFATE-BEARING UNIT. Patrick J. Gasda1, D. Das2, M. Nellessen3, E. Dehouck4, W. Rapin5, P.-Y. Meslin6, H. Newsom3, A. Baker3, M. Hoffman3, G. Ganter3, D. Fey7, R. Kronyak8, J. Frydenvang9, R. C. Wiens1, S. Clegg1, S. Maurice6, O. Gasnault6. 1LANL, 2McGill University, 3UNM, 4U. Lyon, 5Caltech, 6IRAP/CNES, 7MSSS, 8U. Tennessee, Knoxville, 9Copenhagen Museum of Natural History.

Introduction: Calcium sulfate veins are remnants

of groundwater that once was present in the subsurface of Mars. Hence these veins, and their major, minor, trace chemistry, hydration, and mineralogy are win-dows into the past subsurface aqueous processes of ancient Mars. The conditions inferred from the study of veins are key to understanding the long-term habitabil-ity of the martian subsurface. Veins are ubiquitous in the Murray formation, phyllosilicate rich lacustrine mudstone deposits in Gale crater. Vein morphology and abundance, hydration, minor chemistry, and min-eralogy has been well-documented [1–9].

Figure 1: Traverse map through Glen Torridon.

As of sol 2600 (Dec 2019), Curiosity has traversed through most of the clay-bearing Glen Torridon (GT) area to Central and Western Buttes (Fig 1). These buttes are interpreted as the edges of the Greenheugh pediment. The strata skirting the pediment and the buttes and strata to the east of the pediment likely rep-resent a transition zone into the sulfate unit identified from orbit [10]. As the rover approaches the sulfate unit of Gale crater, it is important to track the changes in the veins. The sulfate unit may represent a transition, or the onset of the transition, from a relatively “warm and wet” Mars to dryer conditions [10,11].

At least two distinct chemical endmembers and three distinct facies have been observed in GT. [12] discusses the bedrock chemical endmembers observed by ChemCam: a ~54 wt% SiO2 and 6–10 wt% MgO “coherent” endmember, and a ~56 wt% SiO2 and 4–6 wt% MgO “rubbly” endmember. The pebbles observed throughout Glen Torridon match the rubbly endmem-ber composition. Different facies include Jura member bedrock, pebble strewn areas, capping cross-bedded sandstone Knockfarril Hill (KfH) member that include

the Harlaw Rise and Glen Etive sites, and strata that skirts Central and Western Buttes [13].

Methods: We use a combination of ChemCam chemistry and remote micro imager (RMI) data and imaging data from Mastcam and Mars Hand Lens Im-ager (MAHLI) from the NASA Curiosity mission.

Results: Figure 2 summarizes the distribution of the veins along the traverse through GT (sols 2300–2600). Figure 2 shows where veins were observed in RMI data, with large circles indicating the position of ChemCam observation points within a raster that indi-cate a vein target (bottom row), indicate a vein with >20 wt% CaO (2nd row), contained nodules (3rd row), or contained likely cements (top row). Figure 2 shows that veins are typically seen in the coherent endmember targets (Jura strata), and more rarely in rubbly endmember targets (KfH strata). As the rover ap-proached the buttes, the number of veins seen and sampled by ChemCam dramatically increased. Cements and nodules were also observed much more frequently near the buttes. Veins, nodules, and cements were also observed at Harlaw Rise, part of KfH strata. Veins are not observed in pebbles. Veins or patches surrounded by sand are labeled “other” in Figure 2.

Discussion: Generally, veins are less abundant in GT compared to lower Murray strata [cf. 7]. In GT, veins that cross-cut each other and the bedrock strata were observed most frequently in the coherent endmember targets, part of the Jura member. In addi-tion to the cross-cutting veins that occur in rubbly endmember targets, light-toned banding occurs at the base of Central Butte in the target Sourhope (Fig 3A). Light-toned banding follows bedrock bedding and may represent primary evaporite deposits or veins that fol-low bedding due to coarser grain sizes or weaknesses along bedding planes. Similar rock textures with light-toned banding and nodular gypsum have been studied on Earth, including in southern New Mexico, where large inland seas disappeared during the Jurassic peri-od, forming large deposits of gypsum [14].

Nodules have been observed in a few locations [15], primarily in KfH rubbly endmember bedrock, and have compositions rich in Fe, Mn, or P, reminiscent of diagenetic features in the Sutton Island member (strata 100–200m below GT) [16]. Embedded Fe-rich dark-toned surface features have been observed in targets near the buttes in association with the Ca-sulfate rich light-toned veins and surface features (Fig 3B). Veins

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sometimes cross-cut the nodules. Pebbles are frequent-ly pitted; the pits may be remnants after weathering or dissolution of the nodules.

Cements are inferred from chemical composition trends and have been directly observed in the target Gleneagles (Fig 3C). In targets without veins or nod-ules in images, but have >4 wt% CaO, we can infer the presence of sulfate cements. Three points in Gleneagles that are enriched in CaO also show light-toned materi-als within the ChemCam laser pits (Fig 3C) providing further evidence of a cement. Ca-sulfate cements likely formed when Ca sulfate rich fluids permeated the bed-rock while it was still porous, before later fracturing that produced the more typical cross-cutting veins.

The bedrock near the buttes tend to have a chemical composition intermediate between the rubbly and co-herent endmembers. Some KfH strata (e.g., Harlaw Rise) is rich in nodules, similar to the butte skirting units, which may be explained by coarser grain size of the unit. It is unclear why the nodules only occur at one location along the GT traverse.

ChemCam does not observe large changes in vein major oxide composition as compared to previous Murray veins. Veins in GT tend to have very low (<10 ppm) Li and almost no B has been detected, which is

consistent with GT strata as being clay-rich bedrock deposited in a wetter environment.

Implications: As the rover continues to ascend Mt Sharp, this ongoing work will shed light on the transi-tion from clay-rich facies to sulfate-rich facies, which potentially represents the overall drying of the martian climate. Hence, understanding the transitional strata has important implications for understanding Mars geologic and climatic history.

Acknowledgments: NASA Mars Exploration program; CNES, France; Carlsberg Foundation.

References: [1] Nachon et al (2014) JGR:P, 119 (9), 1991–2016. [2] Nachon et al (2016) Icarus, 281, 121–36. [3] Rapin et al (2016) EPSL, 452, 197–205. [4] Schwenzer et al (2016) MAPS, 51(11), 2175–2202. [5] Gasda et al. (2017) GRL, 44(17), 8739–48. [6] L’Haridon et al. (2018) Icarus, 311, 69–86. [7] Nachon et al. (2020) Sedimentology, in press. [8] Das et al. (2020) JGR:P in review. [9] L’Haridon et al. (2020) JGR:P in review. [10] Milliken et al. (2010) GRL, 37(4). [11] Fraeman et al. (2016) JGR:P, 121(9), 1713–36. [12] Dehouck et al. (2020) this meeting. [13] Fedo et al. (2020) this meeting [14] Kirkland et al. (1995) New Mexico Bureau of Geology and Mineral Resources Bulletin 147. [15] Minitti et al. (2020) this meeting [16] Meslin et al. (2018) LPSC 1447.

Figure 2: Summary of ChemCam vein, nodule, and cement observations in GT (sols 2300–2600) for coherent (lavender), rubbly

(gold), and other targets (grey). If the RMI observation includes a vein, there is a point in “total veins” row. The dot’s size repre-sents the number of ChemCam LIBS observations (up to 7) that indicate a vein. Dots with black outlines indicate veins with

>20 wt% CaO.

Figure 3: MAHLI images of A) Sourhope; B) Everbay; C) Gleneagles. Labels: 1) crosscutting veins; 2) light-toned banding; 3)

nodules; 4) light-toned surface features; 5) dark-toned surface features. Circled ChemCam cyan points have elevated CaO.

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