Sorting out Compositional Trends in Sedimentary Rocks of the Bradbury Group (Aeolus Palus), Gale Crater, Mars

1,2K. L. Siebach, 2M. B. Baker, 2J. P. Grotzinger, 1S. M. McLennan, 3R. Gellert, 4L. M. Thompson, 1J. A. Hurowitz
Journal of Geophysical Research Planets (in Press) Link to Article [DOI: 10.1002/2016JE005195]
1Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
2Present address: Department of Geosciences, SUNY Stony Brook, Stony Brook, NY, USA
3Department of Physics, University of Guelph, Guelph, Ontario, Canada
4Planetary and Space Science Centre, University of New Brunswick, Fredericton, Canada
Published by arrangement with John Wiley & Sons

Sedimentary rocks are composed of detrital grains derived from source rocks, which are altered by chemical weathering, sorted during transport, and cemented during diagenesis. Fluvio-lacustrine sedimentary rocks of the Bradbury group, observed on the floor of Gale crater by the Curiosity rover during its first 860 sols, show trends in bulk chemistry that are consistent with sorting of mineral grains during transport. The Bradbury group rocks are uniquely suited for sedimentary provenance analysis because they appear to have experienced negligible cation-loss (i.e., open-system chemical weathering) at the scale of the Alpha Particle X-ray Spectrometer bulk chemistry analyses based on low Chemical Index of Alteration values and successful modeling of >90% of the (volatile-free) targets as mixtures of primary igneous minerals. Significant compositional variability between targets is instead correlated to grain size and textural characteristics of the rocks; the coarsest-grained targets are enriched in Al2O3, SiO2, and Na2O, whereas the finer-grained targets are enriched in mafic components. This is consistent with geochemical and mineralogical modeling of the segregation of coarse-grained plagioclase from finer-grained mafic minerals (e.g., olivine and pyroxenes), which would be expected from hydrodynamic sorting of the detritus from mechanical breakdown of subalkaline plagioclase-phyric basalts. While the presence of a distinctive K2O-rich stratigraphic interval shows that input from at least one distinctive alkali-feldspar-rich protolith contributed to basin fill, the dominant compositional trends in the Bradbury group are consistent with sorting of detrital minerals during transport from relatively homogeneous plagioclase-phyric basalts.

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