The Temporal Relationship Between the Lunar Magma Ocean and Mg-Suite Rocks

Credit:  Zhang et al., 2021.

Schematic of the early evolution of the lunar interior. (a) Mafic components sink to form the mantle and the segregation of buoyant ferroan anorthosites (FANs). (b) The primordial lunar magma ocean (LMO) differentiates upon cooling [mafic cumulates, FANs, and urKREEP reservoir (ur = primeval; potassium, rare-earth elements, and phosphorus)]. (c) After primordial differentiation, dense ilmenite-bearing cumulates are transposed and inversed with the overlying mafic cumulates, triggering the so-called lunar mantle overturn. Some Ti-rich melts remain in the mantle due to buoyancy, while denser melts penetrate the mantle and reach the mantle-core boundary. The volatile-rich ilmenite-bearing cumulates form a partial melt layer between the lunar mantle and core. Mafic melts ascend and intrude the crust. These intrusions are likely the source of the Mg-suite parental magmas. (d) Subsequent volcanism and impacts modify the morphology of the Moon. The schematic size of each object or unit may not be scaled to its actual size. Credit:  Zhang et al., 2021.

The magma ocean hypothesis is a leading explanation for how the Moon differentiated from its innermost mantle layers to its outermost crustal layer. Igneous samples returned from the surface of the Moon provide a unique opportunity to study its interior — specifically the magnesian-suite samples (Mg-suite), which are believed to have a source region in the lower mantle of the Moon. Therefore, the timing of the formation of the Mg-suite rocks is critical to understanding the early evolution of the Moon.

A new study led by Bidong Zhang of the University of Western Ontario conducted in situ secondary ion mass spectrometry uranium-lead analyses to date a norite sample of the Mg-suite returned by Apollo 17. Zhang and co-authors targeted the minerals zircon and baddeleyite and determined the crystallization age of the sample to be 4332 ± 18 million years old. This crystallization age is only about 30 million years younger than the most precisely dated sample of the lunar crust (4360 ± 3 million years old). Supported by previous geochronological studies on other Mg-suite samples, the results from this work bolster the hypothesis that lunar magma ocean solidification and secondary Mg-suite magmatism occurred near-contemporaneously. According to the results from this study, whatever mechanism was responsible for the generation of Mg-suite magmas on the Moon must have been operating within tens of millions of years of magma ocean solidification that formed the lunar crust.

These results highlight the importance of gathering further chronological data on lunar Mg-suite rocks and putative magma ocean products to place important constraints on models of Mg-suite magmatism. It is clear that constraining the time of Mg-suite formation will help inform models of the early evolution of the Moon. READ MORE