^1,2Yankun Di, ²Magdalena H. Huyskens, ²Qing-Zhu Yin, ^1,3Yuri Amelin
Geochimcia et Cosmochimica Acta (in Press) Open Access Link to Article [10.1016/j.gca.2026.06.017]
¹Research School of Earth Sciences, Australian National University, Acton, ACT 2601, Australia
²Department of Earth and Planetary Sciences, University of California Davis, Davis, CA 95616, USA
³State Key Laboratory of Deep Earth Processes and Resources, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
Copyright Elsevier

Compared to the Earth and inner Solar System planets, calcium–aluminium-rich inclusions (CAIs) possess nucleosynthetic Sr isotope anomalies manifested as elevated ⁸⁴Sr/⁸⁶Sr after internal normalisation using ⁸⁸Sr/⁸⁶Sr. These anomalies can be generated by heterogeneous incorporation of s-, r-, or p-process nucleosynthesis components. Accurately distinguishing between the enrichment or depletion in those components is critical for correctly understanding the timing of planetary volatile depletion, as they predict very different anomalies in ⁸⁷Sr/⁸⁶Sr and lead to disparate interpretations of ⁸⁷Rb–⁸⁷Sr chronology. Here, we constrain the origin of Sr isotope anomalies in the original Mason and Taylor (1982) set of Allende CAIs by examining their nucleosynthetic Sr and Nd isotope systems. Most CAIs analysed exhibit positive μ⁸⁴Sr and negative μ^145,148,150Nd anomalies (μ-values are defined as part-per-million deviations of isotopic ratios relative to terrestrial standards) in agreement with previous studies, but we also detected significant isotope heterogeneities among them, including discovery of CAIs with Sr and Nd isotope anomalies in directions opposite to the majority. The Nd isotope heterogeneity among CAIs is predominantly consistent with variations in the abundance of the s-process component, with a minor but clearly resolved p-process deficit on μ¹⁴²Nd_corr (μ¹⁴²Nd_corr is μ¹⁴²Nd corrected for ¹⁴⁶Sm decay). The less steep μ¹⁴²Nd_corr vs. μ¹⁴⁸Nd slope defined by the CAIs compared to that predicted by stellar models supports the recent suggestion that the accessible Earth has a small radiogenic excess in μ¹⁴²Nd relative to chondrites. Correlated Sr and Nd isotope anomalies in the CAIs suggest that (1) they formed from at least two isotopically distinct reservoirs, one with and the other without p-process Sr excesses relative to Earth, (2) the majority of CAIs formed in the p-process-Sr-enriched reservoir with additional s-process excesses, and (3) variations in r-process Sr and Nd are not observed among the CAIs. The s-process-induced ⁸⁷Sr/⁸⁶Sr anomalies in CAIs (relative to the inner Solar System) predicted based on Nd isotopes are below the typical measurement precision, negating the need for nucleosynthetic correction on CAIs’ ⁸⁷Sr/⁸⁶Sr in chronological interpretations

¹Pipasa Layak, ^1,2Nachiketa Rai, ³Kuljeet Kaur Marhas, ^4,5Hilary Downes
Geochemistry (Chemie der Erde) (in Press) Link to Article [https://doi.org/10.1016/j.chemer.2026.126426]
¹Department of Earth Sciences, Indian Institute of Technology Roorkee, 247667, India
²Centre for Space Science and Technology, Indian Institute of Technology Roorkee, 247667, India
³Planetary Science Division, Physical Research Laboratory, Ahmedabad, 380009, India
⁴School of Natural Sciences, Birkbeck University of London, Malet Street, London, WC1E 7HX, UK
⁵Natural History Museum, Cromwell Road, London, SW7 5BD, UK
Copyright Elsevier

This study models the compositional evolution of a chondritic starting material representative of the Mesosiderite Parent Body (MSPB) under relevant pressure-temperature-redox conditions (1 bar, 1800–500 °C, fO2 = IW + 1.8), focusing exclusively on the evolution of the silicate portion of the system and not on the origin or evolution of the metallic component. The modelling framework assumes crystallization within a silicate magma ocean, and explores crystallization pathways involving varying degrees of equilibrium crystallization (EC) and fractional crystallization (FC). In addition, we present new mineral-chemistry, and phase data from two mesosiderite specimens, Estherville and Mincy.
Modelling results indicate that mesosiderite silicate mineralogy can be derived from a chondritic composition through an efficient three-stage cooling sequence: 40–50% EC, followed by FC down to 1395 °C, and then final EC of the remaining melt to 914 °C, at which point crystallization is complete. The predicted modal abundances—69 wt% pyroxenes, 26 wt% plagioclase, 1.9 wt% tridymite, and 1.6 wt% whitlockite—closely match the observed proportions in Estherville and Mincy. In both meteorites, pyroxenes and tridymites serve as robust geothermometers, stable across 870–1470 °C. The strong agreement between modelled Mg#, Fe#, density, and fO2 with published mesosiderite values further supports a chondritic starting composition of the MSPB.
The model suggests that the MSPB mantle consisted of olivine-orthopyroxene cumulates (dunitic in character), while the lower crust was dominated by pigeonite and hypersthene, forming a pyroxenitic lithology. The upper crust was enriched in plagioclase and pyroxene, reflecting a basaltic composition. Following differentiation, the MSPB likely underwent a collisional encounter with a differentiated impactor, leading to excavation of its silicate crust, followed by brecciation, remelting, and clast metamorphism. These processes ultimately produced mesosiderite meteorites as composite breccias of MSPB-derived silicates intermixed with metallic phases contributed by the impacting body.