^1,2,3D. P. Moriarty III,¹N. E. Petro
Journal of Geophysical Research (Planets)(in Press) Open Access Link to Article [https://doi.org/10.1029/2023JE008266]
¹NASA GSFC, Greenbelt, MD, USA
²University of Maryland, College Park, MD, USA
³Center for Research and Exploration in Space Science and Technology, College Park, MD, USA
Published by arrangement with John Wiley & Sons

The lunar south pole is a region of focused scientific and exploration interest, with several crewed and robotic missions to this region planned within the next decade. Understanding the mineralogy of the region is essential to inform landing site characterization and selection and provides the key context for interpreting samples and in situ observations. At high latitudes, extreme illumination conditions (high phase angles) can negatively impact the data quality of orbital instruments. This is especially true for passive near-infrared spectrometers such as the Moon Mineralogy Mapper (M3) and the Kaguya Spectral Profiler, which measure the spectral properties of the surface using reflected sunlight. Using Moon Mineralogy Mapper data, we observed that the south polar region is associated with a detectable mafic signature consistent with the presence of pyroxenes. The strongest mafic signatures are associated with the South Pole—Aitken Basin, suggesting that impact melt and basin ejecta from the lower crust and upper mantle are present within this region. This observation is validated in several ways: (a) comparisons between M3 data acquired during different mission phases, (b) comparisons between multiple spectral parameters sensitive to the presence of mafic minerals, (c) comparisons between the north and south lunar polar regions, and (d) comparisons with publicly available Kaguya polar mineralogy maps and Lunar Prospector elemental abundances. We also investigate the nature of an anomalous high-albedo region within 2–3° of the south pole observed in Lunar Orbiter Laser Altimeter reflectance data exhibiting a spatially conflicting apparent FeO abundance pattern between several data sets.

¹Tarunika Ramprasad,^1,2Venkateswara Rao Manga,²Laura B. Seifert,³Prajkta Mane,^1,2Thomas J. Zega
Geochimica et Cosmochmica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2024.04.028]
¹Department of Materials Science and Engineering, University of Arizona, 1235 E. James E. Rogers Way, Tucson, AZ 85721, United States
²Lunar and Planetary Laboratory, University of Arizona, 1629 E. University Blvd., Tucson, AZ 85721, United States
³Lunar and Planetary Institute (USRA), 3600 Bay Area Blvd., Houston, TX 77058, United States
Copyright Elsevier

Calcium-aluminum-rich inclusions (CAIs) are the first formed solids in our solar system. Information regarding their formation and alteration is imprinted within their crystal structures, and so analysis of CAIs can provide insight into the early stages of solar system formation. Here we report on micrometer-sized metal grains that occur inside of fluffy type A (FTA) CAIs in the NWA 8323 and Leoville CV3 chondrites. Transmission electron microscopy (TEM) shows that the ultra-refractory metal assemblages contain subhedral grains of alloys of Pt, Os, Ir, Ru, Fe, Ni, and Mo with minor amounts of oxides and silicates inclusions and are crystalline. These assemblages occur in melilite and are surrounded by or adjacent to spinel and perovskite. TEM analysis shows that the majority of the alloys present in the assemblages are significantly enriched in Pt-group elements, with compositions of 75 wt % Pt in some Fe-Ni-Pt grains, and >90 wt % Pt-group elements in Os-Ir-Ru grains. Electron diffraction shows that the alloys occur predominantly in a hexagonal (HCP) structure, with a minority of the grains exhibiting cubic (FCC) and tetragonal lattices. To support these findings, we present a thermodynamic model for the formation of hexagonal (HCP) and cubic (BCC and FCC) ultra-refractory alloys. We use an Fe-Os-Ir ternary system to approximate the various compositions and crystal structures observed in the metal grains. Modeling results indicate a condensation temperature for the alloys as high as 1831 K (HCP, 10−4 bar), placing them well above those predicted for the major CAI phases that surround them. Based on the spatial relationships of the refractory metal grains to their host CAIs, our thermodynamic predictions, and prevailing astrophysical models of the solar protoplanetary disk, the data imply that the grains could have formed inward of the regions where CAI materials condensed. We hypothesize that the refractory metal grains were transported radially outward to the part of the disk where CAIs formed and provided a nucleation site for the condensation of CAI phases such as melilite, hibonite, perovskite, and spinel.

¹Fabre, Sébastien,²Bêche, Eric,³Esvan, Jérôme,³Thébault, Yannick,¹Munsch, Pascal,¹Quitté, Ghylaine
ACS Earth and Space Chemistry 8, 174, 193 Link to Article [DOI 10.1021/acsearthspacechem.3c00083]
¹IRAP, Université Paul Sabatier, CNRS, Observatoire Midi-Pyrénées, 14 Av. Edouard Belin, Toulouse, 31400, France
²PROMES, CNRS, Centre du Four Solaire Félix Trombe, Font-Romeu, 66120, France
³CIRIMAT, CNRS-UPS-INPT, ENSIACET, 4 Allée Emile Monso, Toulouse, 31030, France

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¹Yang, Jing,²Zheng, Dewen,²Wu, Ying,¹Chen, Hong,^1,3Yang, Li,⁴Zhang, Bin
Science China Earth Sciences 67, 641-656 Link to Article [DOI 10.1007/s11430-023-1245-8]
¹Key Laboratory of Active Tectonics and Geological Safety, Ministry of Natural Resources, Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing, 100081, China
²State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing, 100029, China
³Institute of Earth Sciences, China University of Geosciences, Beijing, 100083, China
⁴Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China

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¹Neeraja S. Chinchalkar,¹Gordon R. Osinski,²Timmons M. Erickson,³Cyril Cayron
Earth and Planetary Science Letters 636, 118714 Open Access Link to Article [https://doi.org/10.1016/j.epsl.2024.118714]
¹Department of Earth Sciences, University of Western Ontario, 1151 Richmond St, London, ON N6A 3K7, Canada
²Jacobs-JETS II, Astromaterials Research and Exploration Science Division, NASA Johnson Space Center, Mailcode XI3, Houston, TX 77058, USA
³Laboratory of ThermoMechanical Metallurgy (LMTM), PX Group Chair, École Polytechnique Fédérale de Lausanne (EPFL), Rue de la Maladière 71b, 2000 Neuchâtel, Switzerland
Copyright Elsevier

Evolution of impact melt in terms of initial melt temperatures, melt transport, and cooling history, is a process that remains to be fully understood. Theoretical predictions had suggested that impact melts can experience temperatures far exceeding those in endogenous igneous settings. Direct evidence of the hottest temperatures recorded in impactites was observed recently at the Mistastin Lake impact structure, Canada. The former presence of cubic zirconia, a polymorph of ZrO2 that forms at >2370 °C, was documented within impact glass. In this work, we investigated the zircon and zirconia microstructures and crystallographic orientation relationships with electron backscatter diffraction in two impact glass samples from West Clearwater Lake impact structure in Quebec, Canada. Here we present the first report of the former presence of cubic zirconia, indicating a superheated melt temperature of >2370 °C in one of two impact glass samples analysed. Our results make West Clearwater Lake impact structure the second terrestrial structure with confirmed evidence of former cubic zirconia. Furthermore, we found evidence of melt superheating to temperature of 1673 °C in the other impact glass sample. We also document the first occurrence of former reidite in granular neoblastic (FRIGN) zircon grains in the two impact glass samples analysed in this work, giving us a minimum shock pressure estimate of 20 GPa. This study highlights the heterogeneous thermodynamic (high temperature/low pressure, high pressure, and low temperature/ low pressure) conditions recorded within impact glass from West Clearwater Lake impact structure.

¹Samuel Ebert,²Kazuhide Nagashima,²Alexander N. Krot,¹Markus Patzek,¹Addi Bischoff
The Astrophysical Journal 966, 10 Open Access Link to Article [DOI 10.3847/1538-4357/ad2ea8]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany,
²Hawai’i Institute of Geophysics and Planetology, University of Hawai’i at Mānoa, Honolulu, HI 96822, USA

Calcium, aluminum-rich inclusions (CAIs) are the oldest solids dated that formed in the solar system. Most CAIs in unmetamorphosed chondritic meteorites (chondrites; petrologic type ≤3.0) have uniform solar-like ¹⁶O-rich compositions (Δ¹⁷O ∼ −24‰) and a high initial ²⁶Al/²⁷Al ratio [(²⁶Al/²⁷Al)₀] of ∼(4–5) × 10⁻⁵, consistent with their origin in a gas of approximately solar composition during a brief (<0.3 Ma) epoch at the earliest stage of our solar system. The nature of O-isotope heterogeneity in CAIs (Δ¹⁷O range from ∼−24 up to ∼+5‰) from weakly metamorphosed chondrites (petrologic type >3.0) remains an open issue. This heterogeneity could have recorded fluctuations of O-isotope composition of nebular gas in the CAI-forming region and/or postcrystallization O-isotope exchange of CAI minerals with aqueous fluids on the chondrite parent asteroids. To obtain insights into possible processes resulting in this heterogeneity, we investigated the mineralogy, rare-earth element abundances, and O- and Mg-isotope compositions of a CAI from the CO3.1 chondrite Dar al Gani 083. This concentrically zoned inclusion has a Zn-hercynite core surrounded by layers of (from core to edge) grossite, spinel, melilite, and Al-diopside. The various phases have heterogeneous Δ¹⁷O (from core to edge): −2.2 ± 0.6‰, −0.9 ± 2.1‰, −13.7 ± 2.1‰, −2.6 ± 2.3‰, and −22.6 ± 2.1‰, respectively. Magnesium-isotope compositions of grossite, spinel, melilite, and Al-diopside define an undisturbed internal Al–Mg isochron with (²⁶Al/²⁷Al)₀ of (2.60 ± 0.29) × 10⁻⁶. We conclude that the variations in Δ¹⁷O of spinel and diopside recorded fluctuations in O-isotope composition of nebular gas in the CAI-forming region prior to injection and/or homogenization of ²⁶Al at the canonical level. The ¹⁶O depletion of grossite and melilite resulted from O-isotope exchange with asteroidal fluid, which did not disturb Al–Mg isotope systematics of the CAI primary minerals.

^1,2Ai-Cheng Zhang,¹Hao-Xuan Sun,¹Tian-ran Trina Du,¹Jia-Ni Chen,³Li-Xin Gu
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2024.04.025]
¹State Key Laboratory for Mineral Deposits Research and School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China
²CAS Center for Excellence in Comparative Planetology, China
³Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
Copyright Elsevier

The behaviors of radiogenic Pb in Zr-minerals are critical for reconstructing the chronological framework for the evolutionary history of our Earth and other planetary bodies. Previous investigations have revealed the presence of Pb nanograins in some terrestrial zircons and attributed it to radiation decay of U and mobilization and accumulation in zircon and a subsequent thermal metamorphic event. However, whether impact, a ubiquitous and fundamental process for the evolution of materials on planetary surfaces, can directly produce Pb nanograins in Zr-minerals remains unknown. Here, we report the discovery of abundant metallic Pb nanograins in zirconolite polycrystalline aggregates in the brecciated lunar meteorite Northwest Africa 8182. We propose that the metallic Pb nanograins and their host zirconolite polycrystalline aggregates formed during shock lithification of the host meteorite, which had a significant impact on micro-scale U-Pb isotopic chronology of shocked Zr-minerals. The formation of metallic Pb nanograins also indicates that a reduction of PbO took place during shock metamorphism.

¹Wei Dai,¹Frederic Moynier,¹Julien Siebert
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2024.04.022]
¹Universite Paris Cité, Institut de Physique du Globe de Paris, CNRS, 1 rue Jussieu, Paris 75005, France
Copyright Elsevier

In order to understand the origin of oldhamite (CaS) in enstatite meteorites, we report Ca isotopic compositions (δ44/40Ca) of oldhamite (obtained from water leachate of bulk chondrites and aubrites and mineral separates from the Norton County aubrite) and silicate minerals from different types of enstatite chondrites and aubrite. The δ44/40Ca of the bulk enstatite chondrites range from 1.05 ‰ to 1.24 ‰, with an average of 1.13 ± 0.12 ‰, higher than that of the estimate of the bulk silicate earth (∼0.94 ‰). Major and trace element analyses show that the water leachates of enstatite chondrites are mainly composed of oldhamite, and they take over 20.6–68.5 % Ca of the bulk meteorite Ca budget. The Ca isotope fractionation between oldhamite and residual silicate (Δ44/40Caoldhamite-silicate) for the studied enstatite chondrites is minimum (−0.44 ‰) for Abee (impact-melt breccia) and maximum (+0.16) for St.Marks (EH5). The Ca isotope fractionation between oldhamite (individual mineral grains and leachate) and silicates in Norton County varies from −0.47 ‰ to −0.31 ‰ with an average of −0.41 ‰. These Δ44/40Caoldhamite-silicate correlate well with previous theoretical calculation and suggests that the oldhamites in Norton County are in isotopic equilibrium with co-existing silicates, and therefore were formed during magmatic processes. However, in enstatite chondrites, the large variation on Δ44/40Caoldhamite-silicate and its negative correlation with metamorphic temperature reflects the redistribution and equilibration of Ca isotopes during metamorphism. The variable Δ44/40Caoldhamite-silicate found in unequilibrated chondrites reflect kinetic Ca isotope fractionation between oldhamite and nebular gas and therefore is evidence for the formation of oldhamite by condensation in the solar nebula

¹Daiki Yamamoto,²Noriyuki Kawasaki,³Shogo Tachibana,⁴Lily Ishizaki,⁴Ryosuke Sakurai,²Hisayoshi Yurimoto
Geochimica et Cosmochimica Acta (in Press) Open Access Link to Article [https://doi.org/10.1016/j.gca.2024.04.014]
¹Department of Earth and Planetary Sciences, Kyushu University, Motooka, Fukuoka 819-0395, Japan
²Department of Natural History Sciences, Hokkaido University, Sapporo, Hokkaido 060-0810, Japan
³UTokyo Organization for Planetary Space Science, The University of Tokyo, Hongo, Tokyo 113-0033, Japan
⁴Department of Earth and Planetary Science, The University of Tokyo, Hongo, Tokyo 113-0033, Japan
Copyright Elsevier

The reaction mechanism and kinetics of oxygen isotope exchange between tens of nanometer-sized amorphous silicate grains with forsterite composition (amorphous forsterite) and low-pressure carbon monoxide (CO) gas (PCO) of 0.05–1 Pa at 643–883 K were examined to investigate oxygen isotopic evolution in the protosolar disk that led to the mass-independent oxygen isotopic variation of planetary materials. Both CO gas supply- and diffusion-controlled isotope exchange reactions were observed. At 753–883K and PCO of 0.05–1 Pa, the supply of CO gas controls the isotope exchange reaction, and its rate is 2–3 orders of magnitude smaller than that of the H2O supply-controlled isotope exchange reaction. The diffusion-controlled isotope exchange occurred at 643–703 K and PCO of 0.3 Pa, and the reaction rate of D (m2/s) = (3.1 ± 2.3) × 10−23 exp[−41.7 ± 9.6 (kJ mol−1) R−1 (1/T − 1/1200)] was obtained.

We found that the oxygen isotope exchange rates of amorphous forsterite with CO and H2O gases are larger than those of gaseous isotope exchange between CO and H2O gases at a wide range of temperatures, wherein amorphous forsterite crystallization does not precede the isotope exchange reaction of amorphous forsterite with these gases. The most sluggish isotope exchange rate between H2O and CO in the gas phase suggests that amorphous forsterite would play a role in accelerating gaseous isotopic equilibrium through the isotope exchange of amorphous forsterite with both CO and H2O. We found that the oxygen isotopic equilibrium between 0.1 μm-sized amorphous forsterite, CO, and H2O would be accomplished through the isotope exchange of amorphous forsterite at temperatures as low as ∼600–700 K in the dynamically accreting protosolar disk, which is significantly lower than expected for the case of gaseous isotope exchange (>∼800 K).

^1,2Justin Y. Hu,³Ingo Leya,¹Nicolas Dauphas,²Auriol S.P. Rae,²Helen M. Williams
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2024.04.013]
¹Origins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA
²Department of Earth Sciences, University of Cambridge, Cambridge, CB2 3EQ, UK
³Space Sciences and Planetology, University of Bern, Bern 3012, Switzerland
Copyright Elsevier

The regolith evolution of airless bodies, like the Moon, is primarily controlled by impact cratering. Since the Apollo Era, measurements of cosmic ray exposure (CRE)-induced Sm and Gd isotopes in lunar drill cores have provided insights into the secondary neutron spectra in the lunar regolith. Since the production and transport of secondary neutrons vary with the regolith’s chemical composition and depth, the neutron fluence profile can be employed to track the evolution of lunar and asteroidal regolith. We developed a stochastic model that incorporates state-of-the-art cosmogenic production rate calculations for Sm and Gd isotopes in an effort to understand regolith evolution in the presence of meteoroid bombardments. By comparing the simulated depth profiles to those observed in the lunar drill cores from the Apollo 15, 16, and 17 missions, we find that the deviations from a static profile are due to continuous surface meteoroid bombardments. These bombardments result in the formation of nuclear-reworked zones near the lunar surface. Based on the surface neutron fluence of lunar rocks and regolith, our modeling shows that the regolith surface is reset by large impact-induced excavation and deposition of blanket ejecta every few hundred million years.