^1,2Jonas M. Schneider, ¹Thorsten Kleine
Earth and Planetary Science Letters 669, 119592 Open Access Link to Article [https://doi.org/10.1016/j.epsl.2025.119592]
¹Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
²GEOMAR Helmholtz Center for Ocean Research Kiel, Wischhofstraße 1-3, 24148 Kiel, Germany
Copyright Elsevier

The formation of the Moon by a giant impact of an object called Theia onto proto-Earth marks the end of the main stage of Earth’s accretion. However, the timing of this event is controversial, with estimates ranging between ∼50 and ∼220 million years (Ma) after solar system formation. The 87Rb-87Sr system has the potential to resolve this debate, as formation of the Moon resulted in strong fractionation of rubidium from strontium. To better determine the initial 87Sr/86Sr of the Moon, we obtained Rb-Sr isotope data for several lunar ferroan anorthosites, which define an initial 87Sr/86Sr of 0.6990608±0.0000005 (2 s.e.) at 4.360±0.003 Ga. Modeling the pre-giant impact Rb-Sr isotopic evolution of Theia and the proto-Earth reveals that while in the canonical giant impact model no Rb-Sr model age can be determined, all other current impact models yield a Moon formation age of 4.502±0.020 Ga, or 65±20 Ma after solar system formation. When compared to the chronology of lunar samples, this age implies that solidication of the lunar magma ocean took ∼70 Ma, and that the Moon underwent a global re-melting event ∼150 Ma after its formation.

^1,2Madelyn Sita, ²Marvin Osorio, ²Colin Jackson, ³Sujoy Mukhopadhyay
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2025.08.022]
¹Department of Geology, University of Maryland, 8000 Regents Dr., College Park, 20742, MD, USA
²Department of Earth and Environmental Science, Tulane University, 6823 St. Charles Ave, New Orleans, 70118, LA, USA
³School of Earth and Space Exploration, Arizona State University, 781 E Terrace Mall, Tempe, 85287-6004, AZ, USA
Copyright Elsevier

Ocean island basalts (OIBs) sourced from mantle plumes contain a high ³He/⁴He component, marking the lower mantle as a potential reservoir for primordial, less degassed, material. Some of these same samples have been observed to contain low ¹⁸²W/¹⁸⁴W isotope ratios, which suggest the formation of high ³He/⁴He reservoirs occurred during the early stages of Earth’s formation and point to the core as, potentially, the ultimate source of high ³He/⁴He materials. We developed a computational model to investigate parameters that affect the time-integrated He/(U+Th) ratio present in the core in order to establish the conditions during planetary formation that favor the formation of a high ³He/⁴He reservoir in the core. The parameters investigated are representative of the processes responsible for transporting primordial ³He from the nebular atmosphere and the refractory elements U and Th from the silicate magma ocean into the protoplanets’ differentiated core. The parameters investigated include the radius of the protoplanet, timescale of accretion (), optical opacity of the atmosphere (), amount of Si in the bulk planet (), depth of magma ocean-core equilibration, magma ocean thermal gradient, and the metal-silicate partition coefficient of He (D). The model results, obtained through random sampling of the parameter space, indicated that protoplanets which undergo relatively slow accretion during the lifetime of the solar nebula but still reach sizes larger than 4500 km, protoplanets with optically thin atmospheres, and protoplanets that maintain relatively shallow and cool magma oceans will preferentially develop high ³He/⁴He cores. Overall, Earth’s core could serve as a reservoir for primordial helium, but current parameter space makes the core’s ³He/⁴He ratio highly uncertain.