Paper No. 6
Presentation Time: 9:25 AM


GORDON, Suzanne R.1, NEWSOM, Horton E.1, OLLILA, Ann M.1, WIENS, Roger C.2, ANDERSON, Ryan B.3, CLEGG, Sam M.2, MAURICE, Sylvestre4, GASNAULT, Olivier4, MANGOLD, Nicolas5 and MSL SCIENCE TEAM, The, (1)Institute of Meteoritics, University of New Mexico, Albuquerque, NM 87131, (2)Los Alamos National Laboratory, P.O. Box 1663, MS J565, Los Alamos, NM 87545, (3)USGS, Flagstaff, AZ 86001, (4)Institut de Recherche en Astrophysique et Planetologie, Toulouse, 31028, France, (5)Laboratoire de Planetologie et Geodynamique de Nantes, University of Nantes, France, Nantes, 44322, France,

MSL Curiosity investigated the Yellowknife Bay formation in Gale crater between sols 55 and 269. Orbiter observations and MSL images show visible differences in the stratigraphy of the area. Four main members classified from low to high elevation are Sheepbed, Gillespie Lake, Point Lake, and Shaler. The most notable physical boundary is between Sheepbed and Gillespie Lake. ChemCam has observed targets in all four members, and these data can be compared to see if a change in chemical makeup accompanies the physical changes among members. Alpha Particle X-ray Spectrometer (APXS) data from Yellowknife Bay can also be compared.

The 4.75-m-thick assemblage consists of the Sheepbed mudstone and Gillespie Lake sandstone members whose chemistry resembles the bulk upper crust. The uppermost Shaler member is dominated by a more alkali-rich composition and the Point Lake member has an intermediate composition; it is fine-grained with lighttoned veins and nodules – possible sulfates. Oxide ratio plots (K2O/Al2O3, Al2O3/ TiO2, and FeO/MgO) best show trends in the chemistry of the members as they are less affected by the compositional variability of the targets. The Point Lake member has similar K2O/Al2O3 and FeO/MgO to the Shaler member as well as similar Al2O3/TiO2 to the Gillespie Lake member, all within a 95% confidence interval. FeO/MgO mirror K2O/Al2O3, with Sheepbed and Gillespie Lake having similar but lower ratios than Point Lake and Shaler. ChemCam and APXS data agree for K2O/Al2O3 and FeO/MgO, but the ChemCam Al2O3 generally show higher abundances than APXS data. Major oxides show little variation within a 95% confidence interval. The most notable chemical boundary is between Gillespie Lake and Point Lake, seen in the differing SiO2, MgO, CaO, Na2O, and K2O. TiO2 and FeO(T) also differ at the Sheepbed/Gillespie Lake boundary. An increase in alkaline components from the lower to upper two members is clear. This trend is loosely anticorrelated with Al2O3. Trace element abundances of Rb, Sr and Li between these members show that Shaler, Point Lake and Gillespie Lake are more diverse than Sheepbed. Sheepbed is particularly low in Li relative to the other members. These differences in chemistry may reflect differences in provenance of each member.