SORTING OUT APXS COMPOSITIONAL VARIATIONS IN GALE CRATER SEDIMENTARY ROCKS, MARS

The compositions of sedimentary rocks are defined by the processes by which those rocks are made; their compositions represent an integration of the compositions of detrital grains from throughout the source area filtered by transport processes and supplemented by diagenetic cements. The Curiosity rover Alpha Particle X-Ray Spectrometer (APXS) measured compositions of over 100 rocks as it traversed across Aeolus Palus, the basin floor of Gale Crater, providing a large enough sample set to begin to distinguish between compositional differences due to changing source rock compositions, transport effects, and cements. In this study we develop a textural classification scheme and geochemical and mineralogical mixing models to investigate the key causes for compositional variations within this dataset.

The average composition of the Gale Crater floor rocks is very similar to Mars average crustal basalt and there is minimal evidence for open system chemical weathering or alteration (see McLennan et al. 2014, Science). This general lack of alteration allows modeling of the proportions of igneous phases within the sedimentary rocks and helps define the mineralogical controls on the compositional trends. The principle trend in the APXS dataset is a mixing line between plagioclase-rich compositions in coarser-grained rocks and plagioclase-poor compositions in finer-grained rocks. These correlated trends in texture and plagioclase content can be explained by either (1) hydrodynamic sorting of grains during transport from a source region that has plagioclase phenocrysts and finer or more erodible mafic components or (2) by contributions of a plagioclase-rich source region that is more proximal or tends to contribute larger grains mixed with contributions from a more mafic source region that tends to contribute finer grains, or (3) some combination of (1) and (2). Detailed textural interrogations of the most plagioclase-rich rocks and modeling of potential sources aims to help resolve the cause of this trend. Secondary trends in the dataset show that there is at least one distinctive potassic source region that contributes K₂O to the sedimentary rock compositions, but the relative contribution from this particular source region is not correlated with the paired plagioclase-grain size trend.