Evidence for a high‐Th, evolved lithology on the Moon at Hansteen Alpha

We have revisited a previous interpretation of lunar Th abundances at Hansteen Alpha, which is thought to be a nonmare volcanic construct. Previous studies concluded that while Th abundances were higher than the surrounding mare basalts, Th abundances at Hansteen Alpha were not high enough to be indicative of known, evolved rock types. However, a forward modeling analysis of the Lunar Prospector gamma‐ray data provides evidence that Hansteen Alpha may have Th abundances up to 25 μg/g. This indicates Hansteen Alpha may be composed of a rhyolitic, volcanic lithology. We propose that a possible formation process for Hansteen Alpha is underplating where basaltic magma intrudes into and partially melts the overlying crust. The resulting partial melt is compositionally and thermally buoyant and can be erupted to produce rhyolite. Based on Earth‐analogues, underplating could explain both the composition and the relatively large volume (∼100 km3) of material represented by Hansteen Alpha.