Lunar Mare Basalts as Analogues for Martian Volcanic Compositions: Evidence from Visible, Near-ir, and Thermal Emission Spectroscopy

Introduction: The lunar mare basalts potentially provide a unique sample suite for understanding the nature of basalts on the martian surface. Our current knowledge of the mineralogical and chemical composition of the basaltic material on Mars comes from studies of the basaltic martian meteorites and from orbital and surface remote sensing observations. Petrographic observations of basaltic martian meteorites (e.g., Shergotty, Zagami, and EETA79001) show that the dominant phases are pyroxene (primarily pigeonite and augite), maskelynite (a diaplectic glass formed from plagioclase by shock), and olivine [1,2]. Pigeonite, a low calcium pyroxene, is generally not found in abundance in terrestrial basalts, but does often occur on the Moon [3]. Lunar samples thus provide a means to examine a variety of pigeonite-rich basalts that also have bulk elemental compositions (particularly low-Ti Apollo 15 mare basalts) that are comparable to basaltic SNC meteorites [4,5]. Furthermore, lunar basalts may be mineralogically better suited as analogues of the martian surface basalts than the basaltic martian meteorites because the plagioclase feldspar in the basaltic Martian meteorites, but not in the lunar surface basalts, is largely present as maskelynite [1,2]. Analysis of lunar mare basalts my also lead to additional endmember spectra for spectral libraries. This is particularly important analysis of martian thermal emission spectra, because the spectral library apparently contains a single pigeonite spectrum derived from a synthetic sample [6]. Samples and Methods: We were allocated ~1.0 g of 15 lunar mare basalts. We concentrated our selection on large basaltic rocks (total initial mass > 40 g [5]) that are pigeonite-rich on the basis of petrographic observations and/or have spectral characteristics in the visible and near-IR that are similar to that are similar to Martian reflectivity spectra (i.e., a local reflectivity maxima near 760 nm and a ferrous iron band minimum near 950 nm). The rock samples were crushed with an alumina mortar and pestle and the resulting powder was sieved to yield 500-1000 µm and <45 µm size separates fractions. The coarse size fraction was washed with ethanol to remove adhering particles. The <45 µm size fraction is used for analytical procedures where a fine powder is needed and to give a size fraction that is more representative of Martian dust. Sample characterization was preformed by visible, near-IR spectroscopy (VNIR) from 0.35 to 2.10 µm,