The potassic sedimentary rocks in Gale Crater, Mars, as seen by ChemCam on board Curiosity

Abstract

The Mars Science Laboratory rover Curiosity encountered potassium-rich clastic sedimentary rocks at two sites in Gale Crater, the waypoints Cooperstown and Kimberley. These rocks include several distinct meters thick sedimentary outcrops ranging from fine sandstone to conglomerate, interpreted to record an ancient fluvial or fluvio-deltaic depositional system. From ChemCam Laser-Induced Breakdown Spectroscopy (LIBS) chemical analyses, this suite of sedimentary rocks has an overall mean K 2 O abundance that is more than 5 times higher than that of the average Martian crust. The combined analysis of ChemCam data with stratigraphic and geographic locations reveals that the mean K 2 O abundance increases upward through the stratigraphic section. Chemical analyses across each unit can be represented as mixtures of several distinct chemical components, i.e., mineral phases, including K-bearing minerals, mafic silicates, Fe-oxides, and Fe-hydroxide/oxyhydroxides. Possible K-bearing minerals include alkali feldspar (including anorthoclase and sanidine) and K-bearing phyllosilicate such as illite. Mixtures of different source rocks, including a potassium-rich rock located on the rim and walls of Gale Crater, are the likely origin of observed chemical variations within each unit. Physical sorting may have also played a role in the enrichment in K in the Kimberley formation. The occurrence of these potassic sedimentary rocks provides additional evidence for the chemical diversity of the crust exposed at Gale Crater.

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

“…Chemistry at the Kimberley outcrop is proving to be anomalous in a number of respects. As mentioned previously, the region is especially enriched in potassium, 73 reecting the substantial fraction of K-feldspar (20 wt%) detected by the CheMin instrument. 94 Relatively abundant uorine detections were made, 37 and the apparent fracture-lling material was found to contain high concentrations (>10 wt%) of manganese-oxide minerals.…”

“…83 This enrichment is even more important at the Kimberley formation (Sols 576-632 another drill site anked by foreset beds), located 7 km SW of Shaler, with up to 5.3 wt% K 2 O on average for the Mount Remarkable member. 73 Although it is outside the scope of this review, it can be noted that ChemCam subsequently probed the Murray unit at the Pahrump (Sols 801-919) and Marias_Pass (Sols 992-1072) locations. This unit represents the rst observed material of Mt Sharp's base, and shows a very diverse composition, including areas highly enriched in SiO 2 (75-85 wt%) and TiO 2 , suggesting a stronger alteration.…”

“…This property suggests a genetic link with the potassic sandstones and mudstones observed at the Kimberley outcrop. 73 …”

Observation of > 5 wt % zinc at the Kimberley outcrop, Gale crater, Mars

“…When comparing Zn with other elemental abundances observed at the ChemCam locations, there appears to be few correlations. Overall, the Zn‐rich targets show a general enhancement in alkali (Na and K, which is quite high at Kimberley, from 1.9 to 5.3 wt % on average) [ Le Deit et al , ; L. Le Deit et al, submitted manuscript, 2016]. The most robust correlation between the Zn abundance and other elements detected at Kimberley is with Na 2 O (correlation value of 0.6) and H signal‐to‐background ratio (SBR; 0.2), which is an indicator of the hydrogen content of the target [ Schröder et al , ].…”

“…Chemistry at the Kimberley outcrop proved to be anomalous in a number of respects. The region is especially rich in potassium [ Le Deit et al , ; L. Le Deit et al, submitted manuscript, 2016], reflecting the substantial fraction of K feldspar (20 wt %) detected by the X‐ray diffraction data from the CheMin instrument [ Treiman et al , ]. Relatively abundant fluorine detections were made [ Forni et al , ; O. Forni et al, manuscript in preparation, 2016], and the apparent fracture‐filling material was found to contain high concentrations (>10 wt %) of manganese‐oxide minerals (N. Lanza et al, Oxidation of manganese in an ancient aquifer, Kimberley formation, Gale crater, Mars, submitted to Nature Geosciences , 2016).…”