VARIATIONS IN ELEMENTAL COMPOSITION AND TEXTURE OF SEDIMENTARY ROCKS AT GALE CRATER, MARS
Concentrations of Na, K, and Al are 2 – 4 times higher in coarse-grained, unsorted rocks than in fine-grained, well-sorted samples. Si follows the same pattern but its trend is not as pronounced as those from Na, K, and Al. In contrast, concentrations of Fe, Mg, Mn, Ni, Ti, Zn, and P are 2 – 4 times higher in fine-grained, well sorted samples than in coarse-grained, poorly-sorted rocks. The concentration of Ca does not vary with grain size and sorting.
Variations of elemental composition with texture in sedimentary rocks of Gale crater do not follow trends observed in well-constrained studies of Earth-based physical and chemical weathering of a single source. Two explanations are proposed for Gale crater. The first is that observed variations are controlled by bimodal provenances. In this model, elemental composition – grain size trends are related to two different source regions with distinct chemical composition and little to no influence from chemical weathering. The second interpretation suggests that the observed trends in sedimentary rocks of Gale crater formed by a combination of physical and chemical weathering and elemental mobility at the time these sediments were deposited. In this scenario, reducing of grain size by physical weathering was accompanied by chemical weathering processes and early diagenesis resulting in elemental enrichment or depletion.