¹Daniel Sheikh,²Alex M. Ruzicka,³Melinda L. Hutson
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14298]
Cascadia Meteorite Laboratory, Department of Geology, Portland State University, Portland, Oregon, USA
Published by arrangement with John Wiley & Sons

Pink spinel anorthosite (PSA), a distinctive plagioclase and spinel-rich lithology (spinel >20%) observed on the lunar surface by the Moon Mineralogy Mapper (M3) imaging spectrometer, has sparked considerable interest in understanding magmatic processes on the Moon that cannot be explained by the well-established lunar magma ocean paradigm. Competing ideas on the PSA-forming mechanisms have invoked either (1) impact melting of troctolitic source rocks on the lunar surface or (2) magma–wallrock interactions between anorthositic crust and Mg-suite parental melts, but have been difficult to evaluate given the lack of ground truth samples. Here, we investigate the textures and mineral compositions of seven PSA clasts in lunar meteorite Northwest Africa (NWA) 15500, and the bulk trace element compositions of a PSA clast separate and NWA 15500 host lithologies A and B. Our findings suggest derivation of PSA from an incompatible-element-poor source and are consistent with PSA representing an Mg-suite lithology genetically related to pink spinel troctolites that reflects increased degrees of crustal assimilation during magma–wallrock interactions, and a sourcing of PSA far from the Procellarum KREEP Terrane. Excavation of PSA material was followed by multiple, subsequent localized impact events, resulting in the formation of Lithologies A and B.

¹Glenn J. MacPherson, ²Michail I. Petaev
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2024.12.032]
¹Dept. of Mineral Sciences, U. S. National Museum of Natural History, Smithsonian Institution, Washington, D. C. 20560, United States
²Department of Earth & Planetary Sciences, Harvard University, 20 Oxford St., Hoffman 208, Cambridge, MA 02138, United States
Copyright Elsevier

New full equilibrium condensation calculations for a hot gas of solar composition show that anorthite condenses prior to forsterite at nebular pressures of 10⁻⁶, 10⁻⁵, 10⁻⁴, and 10⁻³ bars. Because of this difference relative to most previous condensation calculations, the predicted bulk composition trend for total condensed solids now more closely matches the trend defined by natural refractory inclusion bulk compositions. Especially this is true for Type B, Type C, and fine-grained spinel-rich inclusions. Some mismatch exists between our (and others’) calculations with respect to the MgO and SiO₂ compositions of natural inclusions. This is likely due to the kinetically-controlled condensation of spinel prior to melilite. We also explored the effects of pyroxene solid solution models and small degrees of fractional condensation, and found no significant effects on the condensation sequence. Although fractional condensation certainly occurred in the pre-solar nebula, our calculations require the degree of such fractionation to have been less than ∼1 %. Finally, although mass-dependent isotopic fractionation in Type B inclusions indicates some evaporative loss of magnesium and silicon during the molten stage of Type B inclusions, our results remove the necessity that such evaporation occurred in order to explain the bulk compositions of Type Bs. Nevertheless, our results are not incompatible with such evaporative loss.

¹Marina Martínez, ¹Adrian J. Brearley
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2024.12.020]
¹Department of Earth & Planetary Sciences, MSC03-2040, 1University of New Mexico, Albuquerque, NM 87131, USA
Copyright Elsevier

Phosphorus-bearing minerals in carbonaceous chondrites record early aqueous alteration effects in the parent asteroid and potentially provide clues on early solar nebular processes. Despite their importance, only a few studies exist dedicated to investigating P-bearing minerals in primitive carbonaceous chondrites and thus, their origins are not well constrained. Work on Ca phosphates around the edges of type IIA chondrules in primitive CR and CM chondrites has shown that Ca phosphates are generally associated with aqueous alteration in the parent body. The present study examines two different Ca phosphate occurrences in one of the least altered CR chondrites known, QUE 99177, by SEM, EPMA, and FIB-TEM techniques to better constrain their origins. The first type consists of elongate, submicron-sized rods of merrillite that occur in regions of mesostasis at the edge of type IIA chondrules adjacent to the surrounding matrix. The second type occurs as nanometer-sized grains around some type IA chondrules that are surrounded by smooth rims. These smooth rims are a type of rim that consists of an amorphous, Fe-rich, hydrous silicate phase that results from low-temperature aqueous alteration of silica in Silica-rich Igneous Rims (SIRs) at the earliest stages of parent body alteration. The Ca phosphates are located within discrete regions at the interface between smooth rims and adjacent matrix, ranging from whitlockite to apatite compositions. We argue that the first type of Ca phosphate has a solar nebular origin, formed by quenching of Ca- and P-bearing melts in chondrules at the final stages of crystallization, whereas the second type has a parent body origin, formed by oxidation of Fe,Ni metal grains in SIRs surrounding chondrules. Therefore, our new data and a reappraisal of previous data demonstrate, for the first time, that Ca phosphates formed by both primary (solar nebular) and secondary (parent body) processes. These results also provide additional insights into the formation conditions of type IIA chondrules in the protoplanetary disk and constrain the earliest stages of aqueous alteration in the CR chondrite parent body.

¹Samuel Ebert, ²Kazuhide Nagashima, ²Alexander N. Krot, ¹Addi Bischoff
Geochimica et Cosmochimica Acta (in Press) Open Access Link to Article [https://doi.org/10.1016/j.gca.2024.12.025]
¹Institut für Planetologie, University of Münster, Münster, Germany
²Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
Copyright Elsevier

The nature of isotopic differences between ‘normal’ Ca,Al-rich inclusions (CAIs) characterized by the canonical initial 26Al/27Al ratio [(26Al/27Al)0] of ∼5 × 10−5 and the anomalous refractory inclusions characterized by the significantly lower (26Al/27Al)0, < ∼3 × 10−6, which include PLACs (platy hibonite crystals), PLAC-like inclusions, and some corundum-, hibonite-, and grossite-rich CAIs, remains controversial. The 26Al-poor inclusions may have formed earlier, prior to ‘normal’ CAIs, and recorded heterogeneous distribution of 26Al in the CAI-forming region, or they may have formed after nearly complete decay of 26Al, ∼ >4 Myr later than the canonical CAIs. Here we present the first high precision multi-isotopic (O, Mg, Ca, and Ti) study of refractory inclusions (RIs) in the EHa3 enstatite chondrite Sahara 97072 using in situ SIMS measurements. Our study revealed the presence of two isotopically distinct populations of CAIs in this meteorite: ‘normal’ CAIs and PLAC-like inclusions. The ‘normal’ CAIs composed of spinel, Al,Ti-diopside, ±hibonite, and secondary minerals, most likely replacing melilite, have solar-like Δ17O of ∼−23 ‰, ∼ the canonical (26Al/27Al)0, and no resolvable nucleosynthetic isotope anomalies in Ca and Ti. The PLAC-like inclusions composed of hibonite, corundum, and ± Al,Ti-pyroxene have Δ17O of ∼−19 ‰, no resolvable excess of radiogenic 26Mg, and large nucleosynthetic isotope anomalies in Ti and Ca: one inclusion has positive anomalies in 50Ti (835ε) and 48Ca (685ε), whereas another one has negative anomalies in 50Ti (−116ε), 46Ti (−112ε), 48Ca (−284ε).
We infer that (i) PLAC-like inclusions formed in an isotopically heterogeneous reservoir in which 48Ca and 50Ti were coupled but both isotopes were decoupled from 46Ti suggesting different carrier phases for 48Ca + 50Ti and 46Ti. (ii) ‘Normal’ CAIs formed in a reservoir with uniform distribution of Ca and Ti isotopes, possibly reflecting increasing homogenization of this region with time due to evaporation/condensation, mixing and aggregation of isotopically anomalous grains present in the protosolar molecular cloud. (iii) The observed differences in Δ17O of ‘normal’ and PLAC-like CAIs indicate their formation in nebular reservoirs with distinct O-isotope compositions, which could have resulted from evaporation of disk regions with different dust/gas ratios, assuming that dust and gas had different Δ17O values, possibly inherited from the protosolar molecular cloud. (iv) The 26Al-poor PLAC-like inclusions predate formation of ‘normal’ CAIs with the canonical (26Al/27Al)0 supporting heterogeneous distribution of 26Al in the CAI-forming region at the earliest stages of the protoplanetary disk evolution. This heterogeneity may have resulted from heterogeneous distribution of 26Al in the protosolar molecular cloud or from thermal processing of presolar grains having different abundances of live 26Al which were present in the molecular cloud with uniform distribution of 26Al/27Al ratio at the canonical level. We conclude that 26Al-26Mg systematics have a limited significance for the chronology of refractory inclusions.

¹Prajkta Mane, ¹Michael E. Zolensky
Nature Astronomy 8, 1508-1509 Link to Article [DOI https://doi.org/10.1038/s41550-024-02438-x]
¹Lunar and Planetary Institute, USRA, Houston, TX, USA
²NASA Johnson Space Center, Houston, TX, USA

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^1,2Toru Matsumoto et al. (>10)
Nature Astronomy 8, 1536-1543 Open Access Link to Article [DOI https://doi.org/10.1038/s41550-024-02418-1]
¹The Hakubi Center for Advanced Research, Kyoto University, Kyoto, Japan
²Division of Earth and Planetary Sciences, Kyoto University, Kyoto, Japan

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During the last days of this year, Cosmochemistry Papers will be on Christmas break. Normal service will commence on January, 2nd, 2025 .

Merry Christmas & Happy New Year to everyone!

¹Elin M. Morton,^1,4Harvey Pickard,²Frank Wombacher,¹Yihang Huang,³Emeliana Palk,¹Rayssa Martins,⁵Sven Kuthning,⁶Maria Schönbächler,¹Mark Rehkämper
The Astrophysical Journal 977, 53 Open Access Link to Article [DOI 10.3847/1538-4357/ad87ed]
¹Department of Earth Science & Engineering, Imperial College London, London SW7 2AZ, UK
²Institut für Geologie und Mineralogie, Universität zu Köln, Zülpicher Str. 49b, 50674 Köln, Germany
³School of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK
⁴National Environmental Isotope Facility, British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK
⁵Geologischer Dienst NRW—Landesbetrieb, De-Greiff-Straße 195, 47803 Krefeld, Germany
⁶Department of Earth Sciences, ETH Zürich, Clausiusstrasse 25, 8092 Zürich, Switzerland

The origin of volatile depletion in the solar system remains a topic of intense debate. To further inform our understanding of the mechanisms involved, this study characterized the mass-dependent Zn, Cd, and Te isotope compositions and concentrations of a comprehensive suite of carbonaceous chondrites (CCs). In accord with previous studies, Zn and Te display covariations between light isotope enrichments and elemental depletions. Observed here for the first time, Cd shows a similar trend. These correlations are consistent with the interpretation that the primary volatile element budgets of CCs were established by mixing of a volatile-rich CI-like matrix and a volatile-depleted non-matrix endmember (NME) in the solar nebula. All three elements display minor isotopic variations in CI and CM chondrites, as a consequence of aqueous alteration at low temperatures. In contrast, Cd and Te isotope compositions and concentrations are highly variable in CV and CO (Cd) and CK chondrites (Te). This reflects mobilization of the elements during thermal metamorphism at distinct redox conditions. The data of this study show that the NME has Zn, Cd, and Te concentrations that are depleted to an identical level of 0.12 ± 0.03 × CI chondrites, and it is characterized by mass-dependent isotope compositions for all three elements that are fractionated to light isotope values relative to CIs by a similar extent. In conjunction with literature data, this suggests that the concentrations and isotope compositions of NME volatiles record the same depletion processes, and that the NME volatile inventory is likely hosted predominantly in chondrules.

¹Lauren E. McGraw,¹Cristina A. Thomas,²Tim A. Lister,³Becky J. Williams,⁴Andy S. Rivkin,⁵Bryan Holler,⁶Leslie A. Young
The Astrophysical Journal 977, L25 Open Access Link to Article [DOI 10.3847/2041-8213/ad9728]
¹Northern Arizona University, Flagstaff, AZ 86011, USA
²Las Cumbres Observatory, Goleta, CA 93117, USA
³University of Virginia, Charlottesville, VA 22904, USA
⁴JHU/APL, Laurel, MD 20146, USA
⁵Space Telescope Science Institute, Baltimore, MD 21218, USA
⁶Southwest Research Institute, Boulder, CO 80302, USA

Near-Earth object 2024 MK was discovered on 2024 June 16, less than 2 weeks before it made a sub-lunar-distance close approach. This close approach provided an ideal opportunity to determine how planetary encounters affect asteroid surfaces in preparation for the numerous missions to (99942) Apophis during its close approach in 2029. We collected spectroscopic data before and after its close approach to determine if planetary encounters induce spectral changes due to surface refreshing. We used NASA’s Infrared Telescope Facility’s (IRTF) near-infrared spectrometer SpeX prism mode (0.7–2.5 μm) to observe 2024 MK pre and postapproach. We also observed the asteroid before its close approach using Las Cumbres Observatory’s FLOYDS visible spectrometer and after its close approach using IRTF’s SpeX long-wavelength cross-dispersed short grating mode, resulting in full spectral coverage from 0.32 to 4.2 μm. 2024 MK is an S-type asteroid that is compositionally most analogous to an L-ordinary chondrite. Spectral analysis of the 3 μm region indicates no surficial water or hydroxide within the level of noise. Band parameter analysis of the pre and postapproach data shows the planetary encounter did not induce any significant spectral changes, suggesting that surface refreshing did not occur on a measurable scale. Similar studies of other targets at smaller encounter distances are required to determine if the lack of spectral changes on 2024 MK indicates it was not close enough to Earth to affect its surface or if the spectral similarity pre and postapproach instead indicates planetary encounters do not cause surface refreshing.

^1,2M. Galinier1, M. Delbo,^2,1C. Avdellidou,¹L. Galluccio
Astronomy & Astrophysics 683, L3 Open Access Link to Article [DOI https://doi.org/10.1051/0004-6361/202349057]
¹Université Côte d’Azur, CNRS–Lagrange, Observatoire de la Côte d’Azur, CS 34229, 06304 Nice Cedex 4, France
²University of Leicester, School of Physics and Astronomy, University Road, LE1 7RH Leicester, UK

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