HR: 09:30h
AN: P31A-07    [Abstracts]
TI: TES and GRS Compositions of the Martian Surface: Evidence for Igneous and Secondary Chemical Fractionation Processes.
AU: * Wyatt, M B
EM: michael_wyatt@brown.edu
AF: Brown University, Department of Geological Sciences, Providence, RI 02912, United States
AU: McSween, H Y
EM: mcsween@utk.edu
AF: University of Tennessee, Department of Earth and Planetary Sciences, Knoxville, TN 37996, United States
AB: TES and GRS provide unique and complementary insights into martian surface compositions. TES measures the composition of the upper hundred microns of the surface while GRS measures the upper few tens of centimeters. We examine TES oxide abundances of low-albedo surfaces and compare distributions to GRS element abundances to constrain the relative roles of igneous and alteration processes on Mars. The bulk variability of compositions measured by TES is accounted for by two spectral endmembers (ST1 and ST2). TES oxide abundances (wt. %) (SiO₂, Na₂O, K₂O, CaO, MgO, FeO, Al₂O₃) are calculated by combining compositions of spectral endmembers in proportion to their relative modeled abundances (vol. %). Recent GRS studies report Si, K, Fe, Th, and K/Th for 'regions' dominated by TES ST1 (RT1) and ST2 (RT2) materials. The most significant TES chemical trends are higher abundances of FeO for ST1 (ST1 15.2 % vs. ST2 12.4 %) and higher abundances of SiO₂ for ST2 (ST2 57.9 % vs. ST1 53.9 %). Results from OMEGA are in agreement with TES oxides. OMEGA pyroxene maps are closely correlated with the distribution of ST1 (High FeO) while ST2 materials lack evidence of mafic bands and are consistent with an enrichment of high-silica phases. GRS RT2 chemistries have higher abundances of FeO (RT2 20.1 % vs. RT1 17.6 %), K, and Th compared to RT1. Abundances of SiO2 (RT1 44.7 % and RT2 45.8 %) and K/Th ratio do not show significant spatial variations. Chemical trends from TES and GRS appear to be in disagreement. TES ST1 is enriched in FeO while GRS RT1 is depleted in FeO. TES ST2 is enriched in SiO₂ while GRS RT2 shows no enrichment in SiO₂. One can account for these discrepancies, and constrain igneous and alteration processes, by considering the sampling depth differences between TES and GRS. The constant K/Th ratio across RT1 and RT2 is not consistent with subaqueous or deep subaerial aqueous weathering of basalt as K would fractionate from Th. Fractional crystallization and subduction zone magmatism could enrich K and Th, however GRS does not detect an enrichment of Si as would be expected. The lack of any significant enrichment in SiO₂ between GRS RT1 and RT2 indicates that evolved volcanics (andesites) are not present in high-abundances within the upper few tens of centimeters at global scales. The favored model from the GRS team is thus initial bulk differentiation processes on Mars producing compositionally distinct magma source regions in the mantle. RT1 and RT2 basaltic provinces with distinct trace element compositions could then be produced. However, the differences in SiO₂ between TES ST1 and ST2 must be taken into consideration. Thin coatings or rinds of secondary high-silica phases (tens of microns) significantly affect the shape and position of absorptions in thermal emission spectra of basalt. Such coatings on Mars may form from near-surface ice and/or surface-atmosphere interactions with little to no water penetrating or cycling into the surface. Limited degrees of alteration in only the upper few tens of microns of the surface could affect TES derived chemistries and be undetectable to GRS due to a deep sampling depth. GRS and TES chemistries support: 1) Distinct magma source regions and basaltic compositions for ST1-RT1 and ST2-RT2 and 2) Thin secondary coatings or rinds of amorphous high-silica phases on ST2-RT2 basalt.
DE: 5415 Erosion and weathering
DE: 5464 Remote sensing
DE: 5470 Surface materials and properties
DE: 5494 Instruments and techniques
SC: Planetary Sciences [P]
MN: 2007 Joint Assembly