Nd isotope variation between the Earth–Moon system and enstatite chondrites

Shelby Johnston, Alan Brandon, Claire McLeod, Kai Rankenburg, Harry Becker & Peter Copeland

Published: 06 October 2022


“Reconstructing the building blocks that made Earth and the Moon is critical to constrain their formation and compositional evolution to the present. Neodymium (Nd) isotopes identify these building blocks by fingerprinting nucleosynthetic components. In addition, the 146Sm–142Nd and 147Sm–143Nd decay systems, with half-lives of 103 million years and 108 billion years, respectively, track potential differences in their samarium (Sm)/Nd ratios. The difference in Earth’s present-day 142Nd/144Nd ratio compared with chondrites, and in particular enstatite chondrites, is interpreted as nucleosynthetic isotope variation in the protoplanetary disk. This necessitates that chondrite parent bodies have the same Sm/Nd ratio as Earth’s precursor materials. Here we show that Earth and the Moon instead had a Sm/Nd ratio approximately 2.4 ± 0.5 per cent higher than the average for chondrites and that the initial 142Nd/144Nd ratio of Earth’s precursor materials is more similar to that of enstatite chondrites than previously proposed. The difference in the Sm/Nd ratio between Earth and chondrites probably reflects the mineralogical distribution owing to mixing processes within the inner protoplanetary disk. This observation simplifies lunar differentiation to a single stage from formation to solidification of a lunar magma ocean. This also indicates that no Sm/Nd fractionation occurred between the materials that made Earth and the Moon in the Moon-forming giant impact.”