¹Zoë E. Wilbur (>10)
Earth and Planetary Science Letters, 695, 120296 Open Access Link to Article [DOI: 10.1016/j.epsl.2026.120296]
¹Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, WA, DC, USA
Copyright Elsevier

Exogenous fragments in brecciated planetary materials offer direct evidence of mixing among planetary bodies in the Solar System. Fragments of undifferentiated carbonaceous material are frequently identified in differentiated meteorites. However, differentiated fragments are rarely observed in carbonaceous material. An exception is the exogenous basalt observed by the OSIRIS-REx spacecraft on the carbonaceous asteroid (101955) Bennu, which motivated a search for differentiated material in the returned samples. We report a pyroxene grain in a brecciated Bennu sample that is texturally and compositionally like pyroxene from cumulate eucrites — basaltic meteorites linked to the differentiated asteroid (4) Vesta — but oxygen isotopic measurements indicate that it is likely sourced from a basaltic asteroid not clearly represented among known basaltic achondrites. This grain may have been delivered to a precursor to Bennu in the main asteroid belt. Whether this grain is representative of the pyroxene-rich boulders observed remotely on Bennu’s surface remains unconstrained. Nevertheless, extending spacecraft observations to the grain scale reveals that Bennu serves as an archive for uncharacterized basaltic asteroids, documenting a history of planetary mixing that is otherwise elusive in current meteorite records.

¹Ling-Zhi Hu, ¹Maxence Regnault, ¹Philipp Gleißner, ¹Ninja Braukmüller, ²Ashley J. King, ¹Anne Lindner, ¹Harry Becker
Earth and Planetary Science Letters, 695, 120270 Open Access Link to Article [DOI: 10.1016/j.epsl.2026.120270]
¹Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstrasse 74-100, Berlin, D-12249, Germany
²Planetary Materials Group, Natural History Museum, Cromwell Road, London, SW7 5BD, UK
Copyright Elsevier

Carbonaceous chondrites (CCs) exhibit profound non-systematic heterogeneity in their elemental and isotopic compositions, both within individual meteorite splits and among bulk samples of the same group, providing opportunities for a better understanding of early solar system processes. This variability also poses significant challenges for constraining bulk compositions of the parent bodies. We report new mass-independent Ti and Cr isotope data, integrated with major and trace element abundances for a suite of CC bulk rocks, to better understand the origin of this multiscale heterogeneity. Some bulk CCs (e.g., Yamato chondrites and Allende) exhibit elemental or isotopic compositions consistent with the literature, while others display significant differences.

Simple binary or ternary mixtures, using ε⁵⁰Ti, ε⁵⁴Cr and chemical data from the present study and literature as constraints, cannot reproduce the full range of isotopic and elemental data. The data are best explained by a multi-component mixing model with a minimum of four endmembers, involving CI-like matrix, heterogeneous non-carbonaceous (NC) dust, and two distinct refractory inclusion populations. The latter are similar to the average composition of calcium-aluminium-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs) and a CAI-like component found in some chondrules. Crucially, the model constraints reveal that NC dust, which migrated into the CC region, contained chemically fractionated material, characterized by a significant depletion in siderophile elements. This result suggests that some NC dust in CC may represent silicate-rich debris derived from the collision of early differentiated planetesimals in the inner solar system. Thus, carbonaceous chondrites record a multi-stage history of early transport of variably ⁵⁰Ti- and ⁵⁴Cr-enriched CAIs and subsequent migration of chondritic and differentiated planetary debris into CI chondrite-like icy dust of the outer disk.

¹Nicole X. Nie, ²Damanveer S. Grewal, ¹Zhe J. Zhang
Earth and Planetary Science Letters, 695, 120321 Link to Article [DOI: 10.1016/j.epsl.2026.120321]
¹Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, 02139, USA
²Department of Earth and Planetary Sciences, Yale University, New Haven, CT, 06511, USA
Copyright Elsevier

High-temperature evaporation is a fundamental process shaping the chemical and isotopic signatures of planetary materials, yet the specific physical laws governing isotopic fractionation during evaporation remain debated. We investigated the mass-dependent isotope fractionation of Fe and Ni during vacuum evaporation of metallic melts. The residues exhibit large isotope fractionation, reaching up to 11.4 ‰ for δ^56/54Fe and 2.9 ‰ for δ^60/58Ni. Determining the isotope fractionation factors for Fe and Ni [$\alpha ={\left(m_{i1}/m_{i2}\right)}^{\beta }$] using the Rayleigh distillation model yielded a $\beta$ value of 0.5 for both Fe and Ni. Further, the evaporation coefficients of Fe and Ni were found to be similar (${\gamma }_{\text{Fe}}\simeq {\gamma }_{\text{Ni}}$). These results suggest that Fe and Ni evaporation from metallic melts is ideal and uninhibited by kinetic barriers.

We used high-precision three-isotope plots to constrain the mass fractionation law governing the evaporation process. Both Fe and Ni in the evaporation residue strictly follow the law of $n=-0.5$, distinct from the exponential law ($n=0$) typically assumed for kinetic processes. We propose a theoretical framework to reconcile this observation with kinetic gas theory: while the intrinsic instantaneous isotope fractionation between vapor and melt follows the exponential law, the apparent law observed in the residue is shifted by the Rayleigh distillation process. We derive a general relationship showing that the divergence between the apparent law and the intrinsic law during a Rayleigh distillation process is controlled by the magnitude of isotope fractionation ($\beta$), $n_{\text{appr}}\simeq \left[1+\frac{{\beta }_{i2/i1}}{2}\text{ln}\left(\frac{m_{i1}}{m_{i2}}\right)\right]n_{\text{inst}}-{\beta }_{i2/i1}$. This framework unifies conflicting observations regarding mass fractionation laws in previous evaporation experiments. The implications of these results for the evaporation history of Type-I cosmic spherules and for identifying nucleosynthetic isotopic anomalies are discussed.

^1,2Wei Dai, ¹Julien Siebert, ³Alexandre Corgne, ¹Nicolas Wehr, ⁴Yunke Song, ¹Frederic Moynier
Earth and Planetary Science Letters (in Press) Open Access Link to Article [DOI: 10.1016/j.epsl.2026.120342]
¹Université Paris Cité, Institut de Physique du Globe de Paris, CNRS, 1 rue Jussieu, Paris, 75005, France
²Centre for Star and Planet Formation, Globe Institute, University of Copenhagen, DK-1350 Copenhagen K, Denmark
³Instituto Ciencias de la Tierra, Universidad Austral de Chile, Valdivia, Chile
⁴Bayerisches Geoinstitut, University of Bayreuth, Bayreuth, 95440, Germany
Copyright Elsevier

Considerable isotopic variation between Earth and major chondritic materials implies a possible fractionation in the early differentiate stage of the Earth. However, it is challenging to study isotopic fractionation at the high pressures relevant to the deep Earth due to the lack of natural samples. We present Ca isotopic fractionation (Δ^44/40Ca_pv-melt) between calcium perovskite (Ca-Pv, CaTiO₃) and co-existing silicate glass (melt fraction) from experimental samples. The Δ^44/40Ca_pv-melt varies in experiments with different pressure, yields equilibrium fractionation factor [10³lnα(1000K)] of $0.{04}_{-0.08}^{+0.06}$ at 1 atm and $0.{50}_{-0.07}^{+0.07}$ at 3 GPa, respectively. The variation on Ca isotope fractionation is likely caused by Ca-Pv phase structure transition from orthorhombic/tetragonal (coordination number (CN) = 8) to cubic (CN = 12) with the increase of pressure and temperature. Our model calculation shows that Ca perovskite is enriched in heavy Ca isotopes relative to silicate melts by 0.02 to 0.08 ‰ in the lower mantle. It suggests that the Ca isotope fractionation in the deep magma ocean is less than 0.05 ‰, regardless of the variation of isotopic fractionation factor caused by phase transition. We conclude that no considerable Ca isotope variation occurred in the deep mantle and the Ca isotope composition of upper mantle could be representative of the bulk silicate Earth. Considering the Nd isotopic differences between Earth and Moon, we suggest that the Moon formed primarily from a partly molten proto-Earth with advanced crystallization of perovskites in its lower mantle.