¹D.C. Rubie, ²K.I. Dale, ³G. Nathan, ⁴M. Nakajima, ⁵E.S. Jennings, ¹G.J. Golabek, ³S.A. Jacobson, ^2,6A. Morbidelli
Earth and Planetary Science Letters 651, 119139 Open Access Link to Article [https://doi.org/10.1016/j.epsl.2024.119139]
¹Bayerisches Geoinstitut, University of Bayreuth, Universitätsstrasse 30, 95440 Bayreuth, Germany
²Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Nice, France
³Department of Earth & Environmental Sciences, Michigan State University, 288 Farm Lane, East Lansing, MI 48823, USA
⁴Department of Earth and Environmental Sciences, University of Rochester, 227 Hutchison Hall, Rochester, NY 14627, USA
⁵School of Natural Sciences, Birkbeck, University of London, Malet Street, London WC1E 7HX, UK
⁶Collège de France, CNRS, PSL Univ., Sorbonne Univ., Paris, 75014, France
Copyright Elsevier

The Hf-W isotopic system is the reference chronometer for determining the chronology of Earth’s accretion and differentiation. However, its results depend strongly on uncertain parameters, including the extent of metal-silicate equilibration and the siderophility of tungsten. Here we show that a multistage core-formation model based on N-body accretion simulations, element mass balance and metal-silicate partitioning, largely eliminates these uncertainties. We modified the original model of Rubie et al. (2015) by including (1) smoothed particle hydrodynamics estimates of the depth of melting caused by giant impacts and (2) the isotopic evolution of ¹⁸²W. We applied two metal-silicate fractionation mechanisms: one when the metal delivered by the cores of large impactors equilibrates with only a small fraction of the impact-induced magma pond and the other when metal delivered by small impactors emulsifies in global magma oceans before undergoing progressive segregation. The latter is crucial for fitting the W abundance and ¹⁸²W anomaly of Earth’s mantle. In addition, we show, for the first time, that the duration of magma ocean solidification has a major effect on Earth’s tungsten isotope anomaly. We re-evaluate the six Grand Tack N-body simulations of Rubie et al. (2015). Only one reproduces ε¹⁸²W=1.9 ± 0.1 of Earth’s mantle, otherwise accretion is either too fast or too slow. Depending on the characteristics of the giant impacts, results predict that the Moon formed either 143–183 Myr or 53–62 Myr after the start of the solar system. Thus, independent evaluations of the Moon’s age provide an additional constraint on the validity of accretion simulations.

^1,2W. Abouchami, ¹F. Wombacher, ²S.J.G. Galer
Geochimica et Cosmochimica (in Press) Open Access Link to Article [https://doi.org/10.1016/j.gca.2024.12.001]
¹Institute of Geology and Mineralogy, University of Cologne, Köln, Germany
²Max Planck Institute for Chemistry, Climate Geochemistry Department, Mainz, Germany
Copyright Elsevier

Early pioneering studies of Apollo lunar soils revealed a geochemical dichotomy reflecting a dominance of mare and highland lithologies, with variable additions of Procellarum KREEP Terrane material. Here, we use the moderately volatile element cadmium to identify the sources and processes responsible for mass-dependent Cd stable isotope variations in the lunar regolith. In addition, capture of thermal neutrons by 113Cd, resulting from galactic cosmic rays (GCR) impacting the lunar surface, provides a means of reconstructing the exposure history of the regolith.
We report TIMS double spike Cd stable isotope data on samples from the Apollo 12, 16 and 17 missions, consisting of twelve soils of varying maturity, two ferroan anorthosites, and orange glass 74220. Cadmium abundances are generally lower in mare (12 to 79 ng/g) than highland soils (∼70 to 95 ng/g). Cadmium stable isotope compositions, expressed as ε112/110Cd, display a larger range in mare (∼0 to + 106) and highland (+60 to + 97) soils. The two anorthosites exhibit contrasting ε112/110Cd values (−107 vs. + 47) and Cd concentrations similar to those of highland soils. Orange glass 74,220 is Cd-rich (290 ng/g) and has a light Cd isotopic composition (ε112/110Cd = -27) due to condensation of Cd vaporized during lava fountaining.
A broad trend of decreasing Cd abundance and increasing heavy isotope enrichment with increasing maturity is observed for mare soils but is not apparent for the highland soils. These characteristics might arise from space weathering, including micrometeorite bombardment, but simple mass balance indicates that meteoritic addition has a negligible effect on the lunar regolith Cd. Likewise, neutron capture on 113Cd tends to increase with maturity in mare soils while being greater and relatively uniform in highland soils, reflecting a longer exposure history and more extensive reworking of the highland regolith. Neutron capture effects were not resolved for immature mare soils, orange glass and one anorthosite, indicating these samples experienced only short near-surface exposure to GCR.
The relationships between Cd abundances and isotope effects reveal three distinct correlations for the highland soils and the mature and immature mare soils, respectively. These are best explained by simple binary mixing between isotopically distinct components. The “heavy” Cd components of mare and highland soils have variable but overall low Cd contents while the cadmium-rich component is always isotopically “light”, and common, at least, to all mare soils. The strong correlation between Cd stable isotopic composition and neutron capture effects in mare soils constrains the ε112/110Cd of the neutron capture-free component to be −4.9 ± 2.3, that is marginally lighter than that of the Bulk Silicate Earth (0.01 ± 0.94). This component is predominantly found in immature, KREEP-rich soils that were not exposed to GCR. This supports an origin as exhumed material, possibly from the relatively recent Copernicus Crater, and/or as vapor re-distributed over the lunar surface. The ubiquitous presence on the Moon of a cadmium-rich reservoir and its apparent isotopic similarity with the BSE requires further scrutiny for a critical evaluation of its significance and implications for the bulk Moon composition.