Chemistry of fracture‐filling raised ridges in Yellowknife Bay, Gale Crater: Window into past aqueous activity and habitability on Mars

Abstract

The ChemCam instrument package on the Curiosity rover was used to characterize distinctive raised ridges in the Sheepbed mudstone, Yellowknife Bay formation, Gale Crater. The multilayered, fracture-filling ridges are more resistant to erosion than the Sheepbed mudstone rock in which they occur. The bulk average composition of the raised ridges is enriched in MgO by 1.2-1.7 times (average of 8.3-11.4 wt %; single-shot maximum of 17.0 wt %) over that of the mudstone. Al 2 O 3 is anticorrelated with MgO, while Li is somewhat enriched where MgO is highest. Some ridges show a variation in composition with different layers on a submillimeter scale. In particular, the McGrath target shows similar high-MgO resistant outer layers and a low-MgO, less resistant inner layer. This is consistent with the interpretation that the raised ridges are isopachous fracture-filling cements with a stratigraphy that likely reveals changes in fluid composition or depositional conditions over time. Overall, the average composition of the raised ridges is close to that of a Mg-and Fe-rich smectite, or saponite, which may also be the main clay mineral constituent of the host mudstone. These analyses provide evidence of diagenesis and aqueous activity in the early postdepositional history of the Yellowknife Bay formation, consistent with a low salinity to brackish fluid at near-neutral or slightly alkaline pH. The fluids that circulated through the fractures likely interacted with the Sheepbed mudstone and (or) other stratigraphically adjacent rock units of basaltic composition and leached Mg from them preferentially.

Diagenesis and clay mineral formation at Gale Crater, Mars

“…After the deposition of the lake bed sediments (most of which are fine, medium, and coarse grained sandstones [ Grotzinger et al ., ]), the first diagenetic features observed to form are raised ridges, nodules, and minibowls (hollows). All models of nodule formation agree on their early diagenetic nature, and hypothesis of formation are gas bubbles [ Leveille et al ., ; Siebach et al ., ; Stack et al ., ]. We note that some of the reactions in our models, for example, the reaction of Ekwir with GPW at 50°C, would produce a water vapor dominated gas phase.…”

ChemCam activities and discoveries during the nominal mission of the Mars Science Laboratory in Gale crater, Mars

“…The occurrence of Ca-sulfate veins is not distributed uniformly through the entire Curiosity path but remains persistent: no other diagenetic feature has been reported so widespread along the traverse. 79 Moreover, the Ca-sulfate veins cross other diagenetic features (such as nodules at the YKB formation, and cluster-like features at Pahrump Hills). 27,80 This suggests the occurrence of a late, quite-widespread precipitation of sulfates from groundwater.…”

Discovering compositional trends in Mars rock targets from ChemCam spectroscopy and remote imaging

“…A number of ChemCam analyses were performed on this target, including both 30-and 150-shot depth profiles. Layers of both high and low magnesium were observed that showed trends with depth [11]. These trends were ascertained by direct examination of the spectra.…”

“…features have been interpreted as fracture-filling cements that record variability in the depositing fluid composition [11].…”