Investigation of the Role of New Glass Compositions in Remotely-sensed Martian

MARTIAN LITHOLOGIES. M.E. Minitti, V.E. Hamilton and M.B. Wyatt, Center for Meteorite Studies, Arizona State University, Tempe, AZ 85287-1404 ([email protected]), Hawai'i Institute of Geophysics and Planetology, University of Hawai'i, Honolulu, HI 96822, Department of Geological Sciences, Arizona State University, Tempe, AZ 85287-1404. Introduction: Mineralogical and spectral data from multiple sources indicate that a broad range of igneous products exist on the Martian surface. The Martian meteorites [1], basalts at the Spirit landing site [2], basalt and andesite detected by the Thermal Emission Spectrometer (TES) [3] and granitoid rocks detected by the Thermal Emission Imaging System (THEMIS) and TES [4] represent petrologic diversity that must be considered in efforts to establish the mineralogy of remotely-sensed lithologies on the Martian surface. Currently, diversity is lacking in compositions of glasses available to constrain the mineralogy of remotely sensed lithologies. High-SiO2 glass, a significant endmember of the deconvolved mineralogy of the TES andesite [3], is one of the few glasses available in endmember libraries. It is unclear if this glass is sufficient to represent all glasses potentially associated with the known variety of Martian lithologies. The goal of our work is to obtain spectra of glasses with a range of compositions relevant to Martian igneous lithologies and investigate them as spectral endmembers in deconvolutions of TES spectra. We derived glass compositions from a number of sources. One glass is based on the high-FeO, low Al2O3 basalt composition connected to multiple Martian meteorites [e.g., 5]. Two other glasses are interstitial melt compositions derived from partial crystallization of this Martian meteorite basalt. One glass is based on the interstitial melt composition present after 90% crystallization of the basalt under anhydrous conditions [6]; the composition is andesitic in nature. The other glass is the interstitial melt composition present after 80% crystallization of the basalt under hydrous conditions [6]; the composition is dacitic in nature. We also utilized the derived major element compositions of TES basalt and andesite [7]. Experimental: Each glass composition was created from a mixture of oxide, carbonate and phosphate powders. Powder mixture was fused in either an Fesaturated Pt or AgPd tube sealed under vacuum within a SiO2 glass tube. The Martian meteorite basalt was fused at 1300 °C, the andesitic interstitial glass was fused at 1100 °C and the dacitic interstitial glass was fused at 1130 °C. Experiments on the TES basalt and andesite compositions are ongoing. Because of the large amount of glass desired (~1 g) for spectral analysis, multiple fusion runs were conducted on a single composition to produce sufficient amounts of glass. The average composition from each run was measured separately and in all cases the average compositions of the runs fell within error of each other. With compositional homogeneity ensured, the products of each of the runs were pooled together. The average compositions of individual runs were combined to represent the bulk composition of the glass by calculating a weighted average, weighted by the mass each run contributed to the total mass of glass. Prior to spectral analysis, the glasses were sieved to yield samples with particle sizes ≥500 μm. Analytical: Glass compositions were analyzed by electron microprobe using a JEOL JXA-8600 at Arizona State University and a Cameca SX-100 at Brown University running a Na loss program. Data were obtained using a 15 kV, 10 nA, 10 μm beam. Thermal emission spectra of the glasses were obtained in the thermal emission spectroscopy laboratory at Arizona State University. Data were collected from 200-2000 cm at a 2 cm sampling interval. Each spectrum is the result of 270 scans. Deconvolutions were carried out using an endmember set incorporating standard igneous phases adopted from [7] and the new glass spectra. TES Type 1 and Type 2 spectra and the spectra of eleven spectrally distinct regions identified within Martian dark regions [8,9] were deconvolved between 400 and 1300 cm, exclusive of the region containing the atmospheric CO2 absorption. Results: Glass compositions are contained in Table 1 and their spectra are pictured in Figure 1. The basalt spectrum has broad, featureless absorptions between 800-1200 cm and 400-500 cm. The glasses with the interstitial melt compositions exhibit narrower