Evolution of the Lunar Crust as told by Feldspathic Meteorites

Image credit: NASA/Goddard Space Flight Center Scientific Visualization Studio

The lunar magma ocean (LMO) hypothesis describes the early differentiation and resulting compositional structure of the Moon’s interior, including the formation of a primary flotation crust consisting of anorthositic rocks. However, the crystallization ages of primary crustal anorthosite samples overlap with the ages of other samples thought to represent episodes of secondary magmatism, which should post-date the formation of the primary crust. An international collaboration of researchers led by Xiaoqing Xu from Nanjing University (China) has examined ten lunar feldspathic meteorites that represent lunar crust from areas outside of the Apollo and Luna landing sites. Using scanning electron microscopy, electron microprobe, and laser ablation inductively coupled plasma mass spectrometry, Xu et al. determined the mineralogy, major element compositions, and trace element characteristics for 51 anorthositic clasts from these 10 lunar feldspathic meteorites. Their results suggest that simple models of LMO differentiation cannot account for the rare earth element contents of the anorthitic plagioclase that is the main mineral in the anorthosites. In order to explain the chemical characteristics and overlapping ages of primary crust and secondary igneous samples, the authors propose a model in which the primary flotation crust experiences metasomatism by mantle-derived partial melts. If this model is correct, then secondary heating associated with the metasomatic events could have reset isotopic systems and could explain the apparent overlap in formation ages. READ MORE