Uncovering Geochemical Evidence for the Moon's High-energy Giant-impact Origin

Abstract

The development of new-generation mass-spectrometry has enabled precise measurements for mass-dependent stable isotopic fractionation of metal elements. Ca is one of the most abundant elements in the mantle and crust of terrestrial planets and their moons. However, the behavior and extent of Ca stable isotopic fractionation during magmatic differentiation have yet been clearly understood. As a case study, we measured high-precision Ca stable isotopic data for the late-Permian alkaline igneous suite of the Øyangen Caldera, Oslo Rift, Norway. Our data show minimal Ca isotopic variation in the intermediate magma and a marked increase in 44Ca/40Ca ratio in the felsic magma of the Øyangen Caldera. This systematic increase is best explained by equilibrium isotopic fractionation controlled by plagioclase and alkali feldspar crystallization. The observed prominent fractionation demonstrates Ca-isotopes as a robust tracer of magmatic evolution and warrants broader implementations in studying planetary differentiation. We further apply this understanding to exploring the crystallization history of the lunar magma ocean. We made new Ca isotope measurements for 13 lunar rocks and minerals. These results, when combined with isotopic modeling of the lunar magma ocean, show that the Ca isotopic compositions of the bulk silicate Earth and Moon are nearly identical. This implied Earth-Moon isotope equilibration supports the Moon’s high-energy giant-impact origin and is not readily explained by the traditional giant-impact models. Furthermore, the magmatic context provided by our Ca and Mg isotope data suggests that an important sample of lunar primitive crust, 60025, is the end product of the lunar magma ocean solidification. Therefore, its absolute ages recalibrate and refine two possible lunar differentiation timescales of (short-lived) or ~130–150 (long-lived) million years due to its debated ages. These new insights from the stable isotopic data call for better lunar chronological data to pair with for a clearer distinction of the lunar differentiation timeline. Next, we have developed a novel methodology that uses the compositions of pristine lunar anorthosite samples and lunar magma ocean crystallization models to resolve the Moon’s refractory lithophile element compositions, which are key constraints on the Moon formation. The results suggest indistinguishable refractory element compositions of the bulk silicate Moon and Earth. This striking chemical similarity is consistent with the high-energy giant-impact origin and is challenging to reconcile the outcomes of canonical low-energy giant impact conditions. Lastly, we designed a new numerical modeling approach to directly test the compatibility of the observed lunar chemical compositions with the variously proposed Moon-formation models. The data-model comparison shows that the canonical low-energy giant-impact models could hardly explain the Moon’s similar refractory element compositions to the Earth, even when assuming the impactor is a chemical twin of the proto-Earth. The findings presented in the thesis provide new geochemical evidence for the high-energy formation of the Earth-Moon system and highlight the significance of giant impacts in shaping other solar system bodies and exoplanets.