Mini-tes Spectra of Mazatzal and Other Adirondack-class Basalts in Gusev Cra- Ter, Mars: Spectral/mineralogical Evidence for Alteration

Objectives: Our two primary objectives are to use Miniature Thermal Emission Spectrometer (Mini-TES) data to determine the mineralogy of the alteration coating on the target Mazatzal [e.g., 1], and to determine if there is mineralogical evidence for sulfate-producing alteration of Adirondack-class rocks. Secondary objectives include identifying any Adirondack-class exotics on the plains or on the West Spur and verifying the assignment of basaltic exotics from the Colombia Hills to the Adirondack class. Background: The rock target informally referred to (as with all names here) as Adirondack is the type example of a class of basaltic rocks that were the first rocks analyzed by the Mars Exploration Rover Spirit in Gusev crater [2]. Mid-infrared (MIR) spectra of Adirondack-class rocks with moderate to low dust cover acquired by the Mini-TES on Spirit are distinguishable from spectra of other rock classes by the presence of an emissivity peak at 430 cm-1 and a minimum near 510 cm-1, which are located in a spectral region least affected by atmospheric components and downwelling radiance and which are characteristic of olivine [3]. This is the only reported spectral class on the plains of Gusev crater. However, spectra exhibiting similar low wavenumber spectral character have been identified along the rover traverse in the Columbia Hills and are referred to as exotics, as they cannot be linked to local outcrops [3]. Using Mini-TES data, [3] derived a mineralogy of approximately 20% plagioclase feldspar, 30% clinopyroxene, 40% olivine, and 10% high-silica glass for an Adirondack-class rock called Sarah. However, chemical data of Adirondackclass rocks acquired by the Alpha Proton X-ray Spectrometer (APXS) exhibit an excess of sulfur [e.g., 4], suggesting the presence of sulfides or sulfates (usually assumed to be the latter; sulfides would need to be non-Fe-bearing to escape detection by the Mössbauer spectrometer, MB). Therefore, Mini-TES spectra of Adirondack-class rocks may contain evidence of sulfate phases, but this has not been studied. Data and Methods: We are examining the ~670 Mini-TES spectra acquired of rock targets on the Gusev plains and in the Columbia Hills through sol 710. Several of these spectra represent repeat views of the same target(s). Our quantitative mineralogical analysis is conducted using a linear least squares approach [3, 5]. We use the spectral library applied to the rock Sarah by [3], augmented with three Mg-Fe olivines [6] that were not available previously and the sulfates used by [3] in models of non-Adirondack-class rocks. We have applied a correction for the presence of dust on the Mini-TES optics to spectra acquired after sol 420 [7]. The spectral effects of additional downwelling radiance, in cases where it is present, are modeled using a provisional correction [3]. Basaltic Exotics: Numerous float rocks first identified at the base of Husband Hill are termed "exotics" because they are surrounded by rocks of differing lithology [3]. "Basaltic exotics" are distinguished by their dark tone and texture, and several of these exhibit a prominent 430 cm-1 peak in the MIR that [3] used to attribute these rocks to the Adirondack class. In MiniTES spectra acquired prior to sol 710, we have identified 27 examples of Adirondack-class exotics. Here, for the first time, we describe two of these exotics on the West Spur: Raster_rock (sol 203) and Chac (sol 258). These show that rocks of the plains superpose and are younger than the rocks of the West Spur, in addition to being younger than the rocks of Husband Hill [8]. The average TIR spectrum of the remaining 25 rocks exhibits a strong resemblance to the average of the 11 best plains Adirondack-class spectra at all wavelengths (Fig. 1), giving us a high degree of confidence in assigning them to this class.