Chemical Alteration of Basalt in the Amazonian: OMEGA, TES, and GRS Integrated Datasets

A central question in many OMEGA and TES studies is whether large expanses of Acidalia Planitia represent the spectral signature of a high-silica primary volcanic lithology or the effects of aqueous chemical alteration on basalt. GRS results show Acidalia Planitia materials have higher-abundances of Fe, K, and Th compared to other low-albedo surfaces, while Si and K/Th do not show spatial variations. The relatively constant Si and K/Th abundances across Mars do not appear consistent with evolved volcanic compositions or chemical alteration. The favored GRS model is thus initial bulk differentiation processes producing compositionally distinct magma source regions in the mantle, resulting in basaltic materials with high K and Th abundances, but with a relatively constant K/Th ratio. This GRS model, however, is not consistent with the spectral signatures of silica-rich phases (primary or secondary) observed with OMEGA and TES data. A possible solution to help reconcile these different geologic models is to account for the different sampling depths of each instrument. OMEGA and TES measure the mineralogical composition of the upper 10s to 100 microns of the surface while GRS measures the chemical composition of the upper few 10s of centimeters. OMEGA, TES, and GRS thus provide unique and complementary insights into martian surface compositions and a single geologic interpretation across datasets may not necessarily be expected. By accounting for different sampling depths, results from this investigation show that integrated OMEGA, TES, and GRS observations of Acidalia Planitia support 1) A distinct magma source region and basaltic compositions with 2) thin secondary coatings or rinds of amorphous high-silica rich phases 10s of microns thick. The implication of this work is that limited aqueous chemical alteration of basalt has occurred on Mars throughout the Amazonian and is not constrained to the Noachian and Hesperian geologic time periods.