Using THEMIS to resolve the discrepancy between modeled phyllosilicate abundances and to assess the origin of phyllosilicate-bearing materials in Mawrth Vallis

We examine the proposed explanations for the discrepancy that exists between phyllosilicate detection in visible/near-infrared vs. thermal infrared datasets -i.e., the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) and the Observatoire pour la Mineralogie, l'Eau, les Glaces et l'Activité (OMEGA) vs. the Thermal Emission Spectrometer (TES). We test the effects of surface temperature discrepancies, physical (intimate) mixing of clay and non-clay bearing materials, and sub-pixel (checkerboard) mixing of clay and non-clay bearing material, all as a function of spatial resolution. Results indicate that a spatial resolution of 100 m/pixel (the resolution of data from the Mars Odyssey Thermal Emission Imaging System (THEMIS) instrument), significantly improves the likelihood of phyllosilicate detection for all of the tested effects, even with its limited spectral resolution. We use spectral unit mapping techniques and the THEMIS dataset to provide the first phyllosilicate abundance maps of the Mawrth Vallis phyllosilicate-bearing units. With the limited spectral resolution, successful unmixing with a few endmembers can be achieved by carefully selecting the appropriate mineral endmembers based on the endmembers identified by both OMEGA/CRISM and TES. Derived abundances are then converted to major element oxides by assuming a representative mineral formula for each endmember present. The molar proportions of these oxides are used to evaluate the proposed formation mechanisms for clay in Mawrth Vallis. Our results indicate that the mineralogy is consistent with a model that requires the chemical weathering and leaching of primary volcanic material. The molar data reveal a trend suggesting the loss of feldspar (CaO + Na2O + K2O) and accumulation of insoluble clays (Al2O3), similar to that of terrestrial weathering of basaltic materials. The further accumulation of clays and loss of ferromagnesian minerals (FeO* + MgO) is only observed in advanced terrestrial weathering, and is not consistent with the THEMIS derived mineral abundances (* indicates all iron is reported as FeO). Regardless of the formation mechanism of the units in Mawrth Vallis, THEMIS modeling indicates phyllosilicate abundances of >50% with heterogeneous spatial distributions, which likely accounts for the discrepancy in detection between datasets. Furthermore, the molar proportions of oxides determined from these abundances are consistent with intermediate alteration of mafic primary material on Mars. This may be indicative of chemical weathering under relatively "dry" conditions (as compared to terrestrial weathering), as erosion and removal of weathering products soon after they are produced does not allow for the development of advanced weathering of those secondary products. With higher resolution mineral abundance modeling, those areas that preserve accumulation of phases characteristic of extreme weathering (e.g., oxides and hydroxides) zones may be revealed. Therefore, the current assessment of phyllosilicate formation in Mawrth Vallis may be a lower limit of the degree of chemical weathering preserved.