Evidence for explosive silicic volcanism on the Moon from the extended distribution of thorium near the Compton-Belkovich Volcanic Complex

We reconstruct the abundance of thorium near the Compton-Belkovich Volcanic Complex on the Moon, using data from the Lunar Prospector Gamma Ray Spectrometer. We enhance the resolution via a pixon image reconstruction technique, and find that the thorium is distributed over a larger ($40 mathrm{km}times 75$ km) area than the ($25 mathrm{km}times 35$ km) high albedo region normally associated with Compton-Belkovich. Our reconstructions show that inside this region, the thorium concentration is $14!-!26$ ppm. We also find additional thorium, spread up to $300$ km eastward of the complex at $sim!2$ ppm. The thorium must have been deposited during the formation of the volcanic complex, because subsequent lateral transport mechanisms, such as small impacts, are unable to move sufficient material. The morphology of the feature is consistent with pyroclastic dispersal and we conclude that the present distribution of thorium was likely created by the explosive eruption of silicic magma.