Backstay and Irvine: Alkaline Volcanic Rocks from Gusev

Introduction: Study of the Martian crust by orbital spectroscopy and SNC meteorites has so far only recognized subalkaline volcanic rocks. However, Spirit rover measured high alkalis and low silica in picritic basalts on the Gusev plains [1,2], allowing the possibility that alkaline differentiates might occur. Isolated rocks having mildly alkaline compositions and distinctive spectra have now been discovered in the Columbia Hills. Backstay and Irvine are the type specimens for these classes of rock. Both rocks have been characterized by the Athena instruments. They appear to be related to the Gusev plains (Adirondack-class [3]) basalts through fractional crystallization. Field Occurrence: Backstay and other rocks of this class was encountered as float on top of the Cumberland Ridge at elevations >50m above the plains [3]. Irvine-class rocks occurred as aligned rocks on the summit of Husband Hill, suggesting that they might be a dike or sill, although imaging from several vantage points could not confirm that they were actually parts of an outcrop. Although all Backstay-and Irvine-class rocks could be ejecta blocks, their close spatial association and restriction to high elevations in the Hills may suggest a local magmatic source. Both rocks are very dark and featureless except for small surface pits, and microscopic images (Fig. 1) reveal that they are aphanitic without the discernable megacrysts seen in plains basalts [1,2]. Backstay was brushed with the RAT, but Irvine was analyzed only as-is. Mini-TES observations suggest only minor dust cover. Backstay has the lowest albedo of any rock seen in Gusev. Both rocks are apparently unweathered, unlike the Hills outcrops [3]. Spectroscopy and Mineralogy: Pancam spectra of both rocks suggest the presence of significant amounts of orthopyroxene or pigeonite, with less olivine than in plains basalts. Opaque minerals are indicated in brushed Backstay but not in unbrushed Irvine. Mini-TES spectra for these rocks classes are distinctive (Fig. 2) and serve to identify them in the field. However, thermal emission spectra cannot yet be deconvolved because of a dust coating on the pointing mirror. Synthetic spectra were produced from the normative mineralogy (see below), but they are a poor fit to the measured Mini-TES spectra.