T. Mark Harrison^a, Bidong Zhang^a,^b, Andrew F. Parisi^a, Elizabeth A. Bell^a 
Earth and Planetary Science Letters 118933 Link to Article [https://doi.org/10.1016/j.epsl.2024.118943]
^aState Department of Earth, Planetary and Space Sciences, University of California Los Angeles, Los Angeles, CA 90095, USA
^bDepartment of Earth, Environmental and Planetary Sciences, Rice University, Houston, TX 77005, USA
Copyright Elevier

Investigations of Apollo-returned samples radically altered our understanding of lunar history which has important implications for terrestrial habitability and Solar System evolution. Radiometric dating of those samples inspired the hypothesis that Moon experienced a Late Heavy Bombardment (LHB) at ∼3.9 Ga. The LHB concept has come under several recent challenges, including the concern that ⁴⁰Ar/³⁹Ar step-heating dates of Apollo impactites had been misinterpreted. Ultraviolet laser ablation (UVLAMP) ⁴⁰Ar/³⁹Ar dates – with their capacity for much higher spatial resolution and thus potential to avoid dating near-ubiquitous clasts in impact melt rocks – should in principle provide more interpretable results. Here we compare new ion microprobe ²⁰⁷Pb/²⁰⁶Pb accessory mineral dates for two Apollo 17 impactites for which UVLAMP ⁴⁰Ar/³⁹Ar dates had been previously obtained. Our results are consistent with a single accessory phase growth event for each sample, though the two samples yielded statistically different mean ages of ca. 3.974±0.013 and 3.928±0.003 Ga. Both can reasonably be interpreted as dating an impact event, but the ²⁰⁷Pb/²⁰⁶Pb dates are older than the associated ⁴⁰Ar/³⁹Ar dates by several hundred million years. We interpret that the age differences result from subsequent thermal disturbances. The discordancy between impact ages inferred from lunar impactites using two different radiometric systems suggests caution in acceptance of the LHB hypothesis without the benefit of both larger lunar datasets and more multichronometric studies. Even with such information, our capacity to know the lunar bombardment history is likely limited by compositional and thermal effects which appear to restrict growth of impact-produced accessory minerals to a small fraction of the lunar surface. Using currently available datasets, the LHB hypothesis may be effectively untestable.

Uta Beyersdorf-Kuis^a,b, Ulrich Ott^a,b,c, Mario Trieloff^a
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2024.09.012]
^aMax-Planck Institute for Chemistry, Hahn-Meitner-Weg 1, D-55128 Mainz, Germany
^bUniversity of Heidelberg, Im Neuenheimer Feld 234-236, D-69120 Heidelberg, Germany
^cInstitute for Nuclear Research Atomki, Bem tér 18/c, H-4026 Debrecen, Hungary
^dKlaus-Tschira-Labor für Kosmochemie, D-69120 Heidelberg, Germany de Lorraine, CNRS, CRPG, UMR 7358, Nancy, 
Copyright Elsevier

Excesses of cosmic-ray produced nuclei in individual components of meteorites indicate “pre-irradiation”, either in the surface region of their parent bodies or as free-floating small particles in the early Solar System. We expand on our earlier work (Beyersdorf-Kuis et al., 2015) and report a study of cosmic-ray produced He and Ne in chondrules and “matrix” (i.e., matrix-dominated) material of several CR2 and CV meteorites as well as the highly primitive, unique, carbonaceous chondrite Acfer 094. In accordance with previous work, no evidence for pre-irradiation was found for CV3 Allende, while for CV3 Vigarano evidence for pre-irradiation is marginal at best. Also, the single chondrule from unique Acfer 094 that we studied has a cosmic ray exposure indistinguishable from the one we found for “matrix” material. Chondrules from Acfer 082 (CV) exhibit both excesses and deficits relative to “matrix”, which points to pre-irradiation of not only chondrules, but also matrix material. A similar case may be Renazzo (CR2), where, however, the identification is complicated by the presence of abundant pre-solar Ne-E. A large number of chondrules (ten) were studied from CR2 El Djouf 001, which yielded slightly variable, small but consistent, excesses relative to “matrix”, corresponding to “nominal” (i.e., irradiation by galactic cosmic rays in 4π geometry) excess ages of 1 to 2 Ma. Modelling suggests contributions from irradiation in the parent body regolith by solar cosmic rays (SCR) as well as galactic cosmic rays (GCR), where the latter dominates. Reevaluating the large variations previously identified in chondrules from QUE 99177, we suggest either a very different regolith history compared to that of El Djouf 001 or, more likely, pre-irradiation by, primarily, GCR in the early solar system as suggested previously. The case of solar-wind-rich NWA 852 (CR2) shows similarity to El Djouf 001 except for a much larger size of the effects. We suggest that the situation may be common among meteorites with a regolith origin. With independent information on the cosmic ray exposure age, which could be obtained by the study of cosmic-ray produced radionuclides, the individual parent body contributions may be disentangled, providing constraints on regolith dynamics.

Kees C. WELTEN¹, Marc W. CAFFEE² , Monika E. KRESS^1,7, Marlene D. GISCARD⁴ , A. J. Timothy JULL⁴, Ralph P. HARVEY⁵ , and John SCHUTT^5,6
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14264]
¹Space Sciences Laboratory, University of California, Berkeley, California, USA
²Department of Physics, Purdue University, West Lafayette, Indiana, USA
³Department of Physics & Astronomy, San Jose State University, San Jose, California, USA
⁴NSF Arizona AMS Laboratory, University of Arizona, Tucson, Arizona, USA
⁵Department of Geology, Case Western Reserve University, Cleveland, Ohio, USA
⁶Geology & Geophysics, University of Utah, Salt Lake City, Utah, USA
⁷Stanford University, Stanford, California, USA

The US Antarctic Search for Meteorites (ANSMET) discovered a dense cluster of 88 ordinary chondrites with a total mass of more than 100 kg on a blue ice area (BIA) of 1.6 × 0.3 km² near the Otway Massif, Grosvenor Mountains, Antarctica. The larger masses (weighing up to 29 kg) were found at one end of an oval-shaped pattern and the smaller masses (50–200 g) at the other end. We measured concentrations of the cosmogenic radionuclides ¹⁰Be (half-life—1.36 × 10⁶ year) and ³⁶Cl (3.01 × 10⁵ year) in the metal fraction of 17 H chondrites, including 14 fragments of this cluster, to verify the hypothesis that this meteorite cluster on the Otway Massif BIA represents a meteorite strewn field produced by the atmospheric breakup of a single meteoroid. The ¹⁰Be and ³⁶Cl concentrations confirm that 10 out of 14 H chondrites from different locations within this small area are paired fragments of the same meteorite fall, while the four other H chondrites represent two additional—smaller—falls. The radionuclides suggest a pre-atmospheric mass of 200–400 kg for the large pairing group, suggesting that 25%–50% of the meteoroid survived atmospheric entry. Based on the distribution of the paired H chondrites and evidence of their common cosmic-ray exposure history in space, we conclude that most of the 88 meteorites within this small area represent a meteorite strewn field. The small size of the strewn field suggests that the meteoroid entered at a steep angle (>60°), while the low amount of fusion crust on most meteorite surfaces most likely indicates atmospheric break up at low altitude, while additional fragmentation of a large surviving fragment may have occurred during impact on the ice. This well-documented strewn field provides a good opportunity to apply model simulations of the atmospheric fragmentation of this object as a function of entry angle, velocity, and meteoroid strength. Cosmogenic ¹⁴C analyses in two members of the Otway Massif pairing group yield a terrestrial age of 15.5 ± 1.5 kyr, which represents the time elapsed since this meteorite fell on Earth. The excellent preservation of an Antarctic meteorite strewn field suggests that the Otway Massif BIA represents a relatively stagnant blue ice field.