Diviner observations of crystalline plagioclase-rich regions on the Moon as observed by the Spectral Profiler and Moon Mineralogy Mapper

Near infrared observations from the SELENE Spectral Profiler (SP) and the Chandrayaan-1 Moon Mineralogy Mapper (M3) have been used to identify iron-bearing crystalline plagioclase in the central peaks of several highland craters and the Inner and Outer Rook mountains of Orientale Basin. The crystalline plagioclase-rich regions are identified by a diagnostic absorption band at 1.2 μm and a lack of mafic absorptions in near infrared spectra. These observations are significant because they validate the near-infrared identification of plagioclase on the Moon. Shocked plagioclase had been previously inferred from a lack of Fe2+ absorptions in near-infrared telescopic and Clementine measurements of high albedo locations on the lunar surface. The Diviner Lunar Radiometer Experiment (DLRE) on the Lunar Reconnaissance Orbiter (LRO) will provide the first global coverage maps of thermal infrared derived compositions of the lunar surface. Extensive thermal infrared laboratory studies of plagioclase minerals and plagioclase-rich rocks show that both plagioclase abundances and compositions can be determined by linear deconvolution of mixed spectra. Diviner has three mineralogy spectral channels centered at 7.81, 8.28, and 8.55 μm chosen specifically to identify the location of the Christiansen Feature (CF). The CF is an emission maximum and is diagnostic of compositions as the wavelength position changes with the change in bond strength and molecular geometry associated with varying mineralogy. Recent studies of laboratory emissivity spectra convolved to Diviner’s three spectral band pass filters indicate that the CF position and simple band ratios can be used to distinguish between: (1) individual minerals (plagioclase, clinopyroxene, orthopyroxene, and olivine), (2) lunar lithologies (mare basalt vs. anorthositic highlands), and (3) Ca-rich, intermediate and Na-rich plagioclase compositions. Here we focus on crystalline plagioclase-rich regions on the Moon to investigate the applicability of Diviner data to constrain solid solution compositions. Near infrared data from SP and M3 show these regions are essentially monomineralic, allowing pure plagioclase spectral systematics to be applied to interpretation of thermal infrared Diviner data. Apollo samples and lunar meteorites indicate that the plagioclase associated with high albedo regions on the lunar surface is high in anorthite content (An90-96). Diviner spectral measurements over plagioclase-rich regions will be analyzed to determine the wavelength range of the CF position and will be compared to laboratory emissivity spectra of the plagioclase solid solution series to constrain lunar plagioclase compositions. Variations in plagioclase compositions across the lunar surface will provide insight into how representative Apollo samples and meteorites are of high albedo regions. This will also have implications for formation models of the plagioclase-rich crust from a lunar magma ocean.