¹Francis Nimmo, ²Thorsten Kleine, ³Alessandro Morbidelli, ⁴David Nesvorny
Earth and Planetary Science Letters 648, 119112 Link to Article [https://doi.org/10.1016/j.epsl.2024.119112]
¹Dept. Earth & Planetary Sciences, University of California Santa Cruz, Santa Cruz CA 95064, United States
²Max Planck Institute for Solar System Research, Gottingen 37077, Germany
³College de France, Paris Cedex 05 75 231, France
⁴Dept. Space Studies, Southwest Research Institute, Boulder CO 80302, United States
Copyright: Elsevier

The nucleosynthetic isotope signatures of meteorites and the bulk silicate Earth (BSE) indicate that Earth consists of a mixture of “carbonaceous” (CC) and “non-carbonaceous” (NC) materials. We show that the fraction of CC material recorded in the isotopic composition of the BSE varies for different elements, and depends on the element’s tendency to partition into metal and its volatility. The observed behavior indicates that the majority of material accreted to the Earth was NC-dominated, but that CC-dominated material enriched in moderately volatile elements by a factor of ∼10 was delivered during the last ∼2–10% of Earth’s accretion. The late delivery of CC material to Earth contrasts with dynamical evidence for the early implantation of CC objects into the inner solar system during the growth and migration of the giant planets. This, together with the NC-dominated nature of both Earth’s late veneer and bulk Mars, suggests that material scattered inwards had the bulk of its mass concentrated in a few, large CC embryos rather than in smaller planetesimals. We propose that Earth accreted a few of these CC embryos while Mars did not, and that at least one of the CC embryos impacted Earth relatively late (when accretion was 90–98% complete). This scenario is consistent with the subsequent Moon-forming impact of a large NC body, as long as this impact did not re-homogenize the entire Earth’s mantle.

^1,2Yuki Masuda, ²Martin Schiller, ²Martin Bizzarro, ¹Tetsuya Yokoyama
Geochimica et Cosmochimica Acta (in Press) Open Access Link to Article [https://doi.org/10.1016/j.gca.2024.11.010]
¹Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan
² Centre for Star and Planet Formation, Globe Institute, University of Copenhagen, DK-1350 Copenhagen, Denmark
Copyright Elsevier

Calcium-aluminum rich inclusions (CAIs) are the oldest condensates in the Solar System. Previous studies have revealed that moderately heavy and trace element isotope anomalies (e.g., Ti, Sr, Mo, and Nd) in CAIs record large nucleosynthetic isotope variations compared to bulk meteorites. Calcium is a major element in CAIs that has six stable isotopes with multiple nucleosynthetic origins. As such, Ca isotopes in CAIs have been an important target of isotopic analysis since the 1970s. However, the Ca isotope compositions of CAIs measured by previous-generation mass spectrometers are less precise than recent isotopic data of heavy elements, which complicates their direct comparisons. Obtaining high-precision Ca isotopic data provides a stronger link between CAI-formation processes from nebular gas and the origin of their source materials.
In this study, we report high-precision Ca isotopic compositions of CAIs, amoeboid olivine aggregates, and an Al-rich chondrule from Vigarano-type carbonaceous chondrites. The obtained µ43Ca and µ48Ca values range from +5.8 ± 1.4 to +40.2 ± 5.2 and +181.2 ± 44.8 to +743.1 ± 8.3 ppm, respectively (µXCa represents the mass bias corrected relative deviation in the XCa/44Ca ratio of the sample from a standard material in parts per million). The improved precision of our measurements reveals that the Ca isotopic compositions of CAIs vary over a narrower range than previously thought. Our precise data also show that µ43Ca and µ48Ca values in CAIs are anti-correlated, which cannot be explained by analytical artifacts such as matrix effects. Additionally, the µ43Ca and µ48Ca values of CAIs increase and decrease, respectively, with increasing Ca abundances of the inclusions. These observations suggest the presence of two distinct gaseous reservoirs from which CAIs condensed, one of which was more enriched in 43Ca but depleted in 48Ca, while the other reservoir was more depleted in 43Ca but enriched in 48Ca. Given the distinct nucleosynthetic sources of 43Ca and 48Ca, this change in isotopic signature is best understood if the two reservoirs inherited material derived from distinct nucleosynthetic sites. As such, our results suggest the presence of more than two compositionally distinct gas reservoirs for Ca isotopes in the early Solar System. If correct, this suggests that the infalling material contributing to the CAI-forming reservoirs was not fully mixed.