Isaac J. Onyett^a, Martin Schiller^a, Mikael Stokholm^a, Jean Bollard^a, Martin Bizzarro^a,b
Earth and Planetary Science Letters 118986 Link to Article [https://doi.org/10.1016/j.epsl.2024.118985]
^aCentre for Star and Planet Formation, Globe Institute, University of Copenhagen, Copenhagen, Denmark
^bInstitut de Physique du Globe de Paris, Université de Paris Cité, Paris, France
Copyright Elevier

Chondrules, the principal high-temperature component of chondritic meteorites, may represent the fundamental building blocks of the terrestrial planets. The mass-independent isotope compositions of chondrules can be used to investigate their origins, as well as their subsequent transport and storage in the protoplanetary disk, which are weakly constrained. Debate surrounds whether mass-independent variability among chondrules arises from isotopically distinct precursor dust or small-scale addition of anomalous phases such as calcium-aluminium-rich inclusions (CAIs) and ameboid olivine aggregates (AOAs). Previous investigations employed isotope tracers that are concentrated in refractory inclusions (such as Ti), rendering them vulnerable to potential “nugget effects” arising from the presence of these anomalous phases and hindering their effectiveness as tracers of precursor dust compositions. An isotope tracer evenly distributed among silicates and thereby less sensitive to local additions from refractory inclusions, is essential to distinguish precursor dust compositions from minor additions of these phases. To address this challenge, we measured the mass-independent Si isotopic composition of chondrules from the carbonaceous Vigarano-type (CV) chondrites Allende and Leoville. Distinct isotopic signatures are observed in chondrules with different petrographic textures. Non-porphyritic chondrules exhibit ³⁰Si deficits akin to differentiated inner disk planetesimals, suggesting early formation within the inner disk (<1 Myr) before transportation to the CV accretion region in the outer disk. Conversely, porphyritic chondrules display a wide range of silicon isotope compositions, including both non-carbonaceous-like values and those exceeding bulk CV chondrites. Notably, non-porphyritic chondrules with substantial porphyritic igneous rims show compositional variations within individual chondrules, whereby cores retain ³⁰Si-depleted signatures while rims record more positive ³⁰Si compositions. Our findings show that contributions from isotopically anomalous refractory condensates cannot be the primary cause of mass-independent variability among chondrules in CV chondrites. Instead, we find that the observed compositional diversity in porphyritic chondrules results from the recycling of inner disk chondrules following the accretion of CI-like dust from the outer Solar System.

Dijun Guo^a et al. (>10)
Earth and Planetary Science Letters 118986 Link to Article [https://doi.org/10.1016/j.epsl.2024.118986]
^aState Key Laboratory of Space Weather, National Space Science Center, Chinese Academy of Sciences, Beijing, 100190, China
Copyright Elevier

The Apollo basin, located in the northeastern part of the South Pole-Aitken (SPA) basin, represents one of the Moon’s most significant geological features, offering profound insights into the lunar interior structure, the effects of the SPA impact, and the history of lunar crust evolution. This study presents an in-depth geological analysis of the Apollo basin region, revealing the distribution of rock types and compiling a comprehensive geologic map that correlates with the lithologic and geochemical properties of the area. Utilizing the characteristics and compositional provenance of the geologic units, we have constructed schematic cross-sections that elucidate the interior structure and stratigraphic evolution of the Apollo basin region. Despite excavations of the SPA and Apollo impacts, the anorthositic crust of this area was not entirely removed and has been uplifted to shallow depths, making it more susceptible to exposure by subsequent impacts. Additionally, upper mantle material, characterized by ultramafic, low-Ca pyroxene, was excavated by the SPA impact and is present in the impact melt/breccias of the Apollo basin. After the formation of the Apollo basin, multiple mare units were emplaced over a period potentially spanning ∼1.5 billion years, with the oldest of these maria being superposed by substantial postdating basin ejecta. The results of this study strengthen our understanding of the geology and evolution of the Apollo and SPA basins and offer valuable insights for interpreting the exploration and sample analysis results of the Chang’e-6 mission.

James B. Garvin^a, Richard J. Soare^b 
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2024.116303]
^aNASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
^bDept of Geography, Dawson College, Montreal H3Z 1A4, Canada
Copyright Elsevier

No abstract to this preface.