A small volcanic complex ~25x35 km lies at the center of a Lunar Prospector Gamma-Ray Spectrometer (LPGRS) thorium anomaly located at ~ 100°E and 60°N [1,2]. The area at the center of the Th anomaly has elevated topography and high reflectance relative to surroundings. Lunar Reconnaissance Orbiter (LRO) Narrow Angle Camera (NAC) images reveal small domes and cones, and irregular depressions that constitute evidence of a volcanic origin. We refer to this feature as the Compton-Belkovich Volcanic Complex (CBVC).
Models for the Th concentration anomaly yield central Th concentrations of > 20 ppm, and perhaps as high as 40-55 ppm [2,3]. These Th concentrations coupled with low FeO (<5 wt%)  implicate an alkali-suite rock type and, if at the high end of the range, granite or its extrusive equivalent, rhyolite. Lunar samples contain small bits of granite or felsite (fine-grained, alkali-feldspar- and silica-rich material). These materials have intrinsically high reflectance and have high Th concentrations, e.g., 10-66 ppm .
Here we report the first quantitative normalized reflectance associated with the CBVC and compare the reflectance to some other key locales on the Moon. Normalized reflectance (IoF/LS) is reflectance (I/F) measured using NAC images and normalized to the Lommel-Seeliger (LS) function. We compare to Apollo landing sites, for which surface compositions are known, and to several other locations of known silicic composition  including the Gruithuisen Domes, Hansteen Alpha, and bright ejecta of Aristarchus Crater. Because of latitudinal photometric effects and limited NAC coverage at low phase angles at CBVC, we use a photometric model to correct normalized reflectance to a 45° phase angle.
We use NAC images with a variety of illumination conditions and apply a photometric function optimized for areas surrounding the Apollo landers  to fit the normalized I/F data. We normalize reflectance to the Lommel-Seeliger function to mask effects of viewing geometry and fit the data with a Hapke photometric function to determine the single scattering albedo (w) at 45° phase angle. We chose a flat region of interest in the center of the CBVC, areas on top of the Gruithuisen Gamma dome and Hansteen Alpha, and on the southwestern ejecta of Aristarchus Crater.
The CBVC area has the highest normalized reflectance of all the sites chosen for comparison, and the highest w, which correlates broadly with composition and mineralogy based on Apollo landing site analysis  (increases with feldspar content and decreases with FeO-bearing minerals). The normalized reflectance for the CBVC at 45° phase is 0.29, compared to Apollo 16 at 0.17, Hansteen Alpha and Aristarchus ejecta at 0.16, and Gruithuisen Gamma at 0.13. Extrapolation of trends of w and compositional parameters suggests high Si+Al and low FeO at the CBVC, consistent with interpretations of felsic rock types.
References:  Lawrence et al., 2003, JGR 108;  Jolliff et al. 2011, Nat. Geosci. 4;  Wilson et al., 2013, this Conf.;  Wieczorek et al., 2006, New Views of the Moon, RiM-G, 60;  Glotch et al., 2010, Science 329;  Clegg et al., 2013, submitted.