Thermal emission spectra of terrestrial alkaline volcanic rocks: Applications to Martian remote sensing

The long-standing assessment that Martian igneous compositions are strictly subalkaline has recently been brought into question by the detection of alkaline rocks at Gusev crater. In this study, we assess the utility of thermal emission spectra (5-25 μm at 10 cm^-1 spectral sampling) in identifying and classifying terrestrial alkalic volcanic rocks, and then apply our results to MGS/ TES data. Using spectral linear deconvolution, we determined modal mineralogies of a suite of alkaline rocks (basalts, trachyandesites, trachytes, rhyolites). We then compared modeled data to modes measured using an electron microprobe mapping technique. Most phases modeled well (within 10 vol%). Modeled plagioclase abundances, however, were too high, reflecting the presence of devitrified glass and the absence of appropriate glass end-members in the spectral library. Systematic inaccuracies in bulk-rock chemistries, derived by combining modeled mineral end-member compositions in proportion to their abundances, reflect the overdetermination of plagioclase and resulting mass balance problems. Nevertheless, modeled mineralogies and derived bulk-rock chemistries clearly reveal the alkaline character of these rocks, and many rocks are correctly classified petrologically. To determine if alkaline rocks can be detected in global and regional TES spectra, we deconvolved ST1, ST2, and four compositionally distinct regional spectral classes using our end-member library. Our results suggest that alkaline rocks are not readily detected in most regionally- and globally-derived TES spectra. We do not rule out the possibility that areas of regional-scale alkalic volcanism exist, although they are more likely to be identified by in situ surface investigations.