Bedrock geochemistry measured by ChemCam along a 2-km eastward traverse in the Glen Torridon region, Gale crater, Mars

International audienceIn January 2019, the Mars Science Laboratory (MSL) rover Curiosity started exploring the GlenTorridon (GT) region of Gale crater, which corresponds to the topographic trough between the VeraRubin ridge and the Greenheugh pediment (Fig. 1). From orbit, this region stands out due to therelatively strong near-infrared signatures of clay minerals [1,2]. The in-situ data collected so far byCuriosity have allowed the MSL team to confirm and quantify the presence of these clay minerals[3], to characterize their organic content [4], and to document in detail their geochemical andsedimentological settings [5].In particular, the bedrock chemistry measured by ChemCam shows elevated values of the ChemicalIndex of Alteration (CIA) throughout most of Glen Torridon [6,7], which indicates an open-systemtype of alteration [8]. In addition, the ChemCam observations suggest the presence of illite in thefine-grained rocks [9] and reveal a possible relationship between grain size and geochemicalvariations [10]. Finally, they show that the light-toned rocks located just below the unconformity atthe base of the Greenheugh pediment (Fig. 1) are associated with “anomalous” bedrockcompositions that may be related to a late-stage diagenetic event at Gale [7].Here, we report the findings of the ChemCam instrument along a ~2-km eastward traverse withinthe upper member – named Glasgow – of the Glen Torridon region (Fig. 1). This traverse allowedCuriosity to investigate the lateral variability of this member, while also getting gradually closer tothe transition into the overlying sulfate-bearing unit [11].DatasetChemCam measures the chemical composition of targeted rocks and soils within a few meters of therover using laser-induced breakdown spectroscopy (LIBS). Quantification of eight major rockformingoxides is performed routinely [12]. Each ChemCam analysis consists of a raster of severalpoints a few millimeters apart from each other, and each point itself consists of a series of lasershots (typically 30). For bedrock targets, this approach allows removal of unwanted contributionsfrom Ca-sulfate veins or soil material by discarding the corresponding points [e.g., 8].On sol 3007, ChemCam experienced a technical issue, which required halting the use of the laserduring the time of the investigation, while imaging and passive spectroscopy activities continued.LIBS analyses on Mars targets resumed on sol 3107, in the Mont Mercou area (Fig. 1), ~600 m tothe east and ~19 m higher in elevation. In addition, data acquired between sols 2816 and 2956 arenot used here, since they correspond to a downsection excursion into the underlying Knockfarril Hillmember [13].ResultsVariations in MgO, K2O and FeOT abundances along the rover path within the Glasgow member areshown in Figures 2, 3 and 4 (respectively). The bedrock composition is overall homogeneous,although small variations are observed for some oxides. In particular, the mean MgO abundancewas well below 5 wt% in the western part of the traverse, but reached nearly 6 wt% after thedownsection excursion (Fig. 2). This increase in MgO is accompanied by slightly lower Al2O3abundances (not shown here).In contrast, the K2O content does not show a clear change after the excursion (Fig. 3); however, itdecreases slightly during the exploration of an area with rougher terrain (previously mapped as “fIUrubbly” [14]), between sols 2973 and 3000.Regarding FeOT, large variations have been observed at small scale (i.e., within a given LIBS raster)in a group of targets characterized by the presence of dark nodules, likely of diagenetic origin[15,16]. These nodules have high FeOT (up to ~50 wt%), whereas the points in between the nodulesshow low FeOT (down to ~12.5 wt%). However, at the scale of the whole Glasgow member, themean FeOT content is quite stable, with only a subtle increase in the eastern part of the traverse (Fig. 4).As mentioned above, the most recent data (last bin in Fig. 2-4) were acquired after a gap of ~600 mlaterally and ~19 m vertically. Despite this gap, the bedrock compositions appear remarkably similarto those measured earlier in the Glasgow member, especially after the downsection excursion. Thissuggests that no major change of environment occurred during the time when LIBS was unavailable.Discussion and conclusionCuriosity’s eastward traverse within the Glasgow member is a rare opportunity to investigate thelateral variability of Gale sedimentary strata at the kilometer scale. The compositions appear overallhomogeneous, but some variations are observed. As previously seen in Glen Torridon [6,7,10], Mgand K are the two most varying elements; however, their variations are not always well correlated,as illustrated by the trends observed after the downsection excursion (Fig. 2-3). In addition, whileFeOT displays significant variations within some LIBS rasters due to the presence of dark nodules inthe bedrock, it is stable at the scale of the whole Glasgow member, which suggests that diageneticremobilization of iron occurred at small scale only.Finally, the latest data acquired in the Mont Mercou area show that the bedrock compositions arestill remarkably similar to those measured ~2 km to the west, when Curiosity first encountered theGlasgow member. As the rover continues to progress uphill, ChemCam will continue surveying thebedrock in search of the first chemical indications of the clay-to-sulfate transition