^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.

^1,2CHRYSA AVDELLIDOU,^1,2MARCO DELBO,³DAVID NESVORNÝ,³KEVIN J. WALSH,^1,4ALESSANDRO MORBIDELLI
Science 384, 348-352 Link to Article [DOI: 10.1126/science.adg8]
¹Laboratoire Lagrange, Centre National de la Recherche Scientifique, Observatoire de la Côte d’Azur, Université Côte d’Azur, 06304 Nice, France.
²School of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK.
³Southwest Research Institute, Boulder, CO 80302, USA.
⁴Collège de France, Centre National de la Recherche Scientifique, Université Paris Sciences et Lettres, Sorbonne Université, 75014 Paris, France.
Reprinted with permission from AAAS

The giant planets of the Solar System formed on initially compact orbits, which transitioned to the current wider configuration by means of an orbital instability. The timing of that instability is poorly constrained. In this work, we use dynamical simulations to demonstrate that the instability implanted planetesimal fragments from the terrestrial planet region into the asteroid main belt. We use meteorite data to show that the implantation occurred >60 million years (Myr) after the Solar System began to form. Combining this constraint with a previous upper limit derived from Jupiter’s trojan asteroids, we conclude that the orbital instability occurred 60 to 100 Myr after the beginning of Solar System formation. The giant impact that formed the Moon occurred within this range, so it might be related to the giant planet instability.

^1,2Xinyi Xiang,^1,2Peixin Du,³Binlong Ye,⁴Hongling Bu,⁵Dong Liu,⁵Jiacheng Liu,⁴Jian Hua,^1,2Xiaolong Guo
Journal of Geophysical Research (Planets)(in Press) Link to Article [https://doi.org/10.1029/2023JE008031]
¹State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology, Macau, China
²CNSA Macau Center for Space Exploration and Science, Macau, China
³Department of Earth Sciences and Laboratory for Space Research, The University of Hong Kong, Hong Kong, China
⁴Guangdong Key Laboratory of Integrated Agro-environmental Pollution Control and Management, Guangdong Institute of Eco-environmental Science & Technology, Guangdong Academy of Sciences, Guangzhou, China
⁵CAS Key Laboratory of Mineralogy and Metallogeny / Guangdong Provincial Key Laboratory of Mineral Physics and Materials, CAS Center for Excellence in Deep Earth Science, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China
Published by arrangement with John Wiley & Sons

Ferrihydrite, a nanocrystalline iron (oxyhydr)oxide mineral, is widely distributed in soils and sediments on Earth and is probably an important component and/or precursor of widespread nanophase iron minerals on Mars. Terrestrial ferrihydrite often co-occurs with amorphous silica and/or contains a certain amount of Si in its structure. However, it remains ambiguous how environmental Si concentration affects the formation-evolution and structure-spectral features of ferrihydrite in the Fe(III)-Si systems. To this end, hydrolysis experiments were carried out for Fe-Si systems at an unprecedentedly wide range of initial Si/(Fe + Si) molar ratios (0–0.80), followed by characterizing the products detailly. X-ray diffraction, Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, Mössbauer spectroscopy, and transmission electron microscopy results showed that at Si/(Fe + Si) molar ratios ≤0.30, the main phase of the products was ferrihydrite, of which the unit cells enlarged, the crystallinity decreased, and the existing state of Fe changed with increased Si contents; at Si/(Fe + Si) molar ratios ≥0.40, ferrihydrite was no longer formed and a novel amorphous Fe-O-Si phase was instead obtained, with the excess Si forming amorphous silica. The visible and near-infrared spectroscopy, the most powerful tool to detect hydrous minerals on the surface of Mars at global or regional scales, showed weakness in identifying ferrihydrite-like materials obtained in the Fe-Si systems. Raman spectroscopy can identify ferrihydrite and Si-containing ferrihydrite but cannot differentiate between them. Mössbauer spectroscopy showed great potential in both identifying and differentiating between ferrihydrite and Si-containing ferrihydrite, and thus can be used to characterize the poorly ordered iron (oxyhydr)oxides on Mars.

^1,2A. Sehlke,^1,2D. W. G. Sears, the ANGSA Science Team
Journal of Geophysical Research (Planets)(in Press) Link to Article [https://doi.org/10.1029/2023JE008083]
¹NASA Ames Research Center, Moffett Field, CA, USA
²Bay Area Environmental Research Institute, Moffett Field, CA, USA
Published by arrangement with John Wiley & Sons

We explored the geological history of the Taurus-Littrow Valley at the Apollo 17 landing site through the induced thermoluminescence (TL) properties of regolith samples collected from the foothills of the Northern and Southern Massifs, from near the landing site, and from the deep drill core taken in proximity to the landing site. The samples were recently made available by NASA through the Apollo Next Generation Sample Analysis program in anticipation of the forthcoming Artemis missions. We found that the two samples from the foothills of the massifs exhibit induced TL values approximately four times higher than those of the valley samples. This observation is consistent with their elevated plagioclase content, indicating their predominantly highland material composition. Conversely, the valley samples display induced TL values characteristic of lunar mare material. The samples from the deep drill core demonstrate uniformly induced TL properties, despite originating from depths of up to 3 m. Notably, one of the samples from the lower section of the deep drill core presents anomalous-induced TL readings. This anomaly coincides with elevated levels of low-potassium KREEP along with reduced quantities of anorthositic gabbro and orange glass, and could be due to the traces of phosphate minerals. Alternatively, this observation raises the possibility that this sample contains Tycho impact material. The induced TL data is consistent with the regolith, extending to a depth of at least 3 m, having been deposited by a singular event approximately 100 million years ago. This timing aligns with the hypothesized formation of the Tycho crater.

¹R. J. Macke,¹C. P. Opeil,¹D. T. Britt,¹G. J. Consolmagno,¹A. Irving
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14171]
¹Vatican Observatory, Vatican City-State, Vatican
²Department of Physics, Boston College, Chestnut Hill, Massachusetts, USA
³Department of Physics, University of Central Florida, Orlando, Florida, USA
⁴Center for Lunar and Asteroid Surface Science, Orlando, Florida, USA
⁵University of Washington Earth & Space Sciences, Seattle, Washington, USA
Published by arrangement with John Wiley & Sons

Lunar meteorites are the most diverse and readily available specimens for the direct laboratory study of lunar surface materials. In addition to informing us about the composition and heterogeneity of lunar material, measurements of their thermo-physical properties provide data necessary to inform the models of the thermal evolution of the lunar surface and provide data on fundamental physical properties of the surface material for the design of exploration and resource extraction hardware. Low-temperature data are particularly important for the exploration of low-temperature environments of the lunar poles and permanently shadowed regions. We report low-temperature-specific heat capacity, thermal conductivity, and linear thermal expansion for six lunar meteorites: Northwest Africa [NWA] 5000, NWA 6950, NWA 8687, NWA 10678, NWA 11421, and NWA 11474, over the range 5 ≤ T ≤ 300 K. From these, we calculate thermal inertia and thermal diffusivity as functions of temperature. Additionally, heat capacities were measured for 15 other lunar meteorites, from which we calculate their Debye temperature and effective molar mass.

^1,2,3,4L.F. White,⁵D.E. Moser,³J.R. Darling,⁴B.G. Rider-Stokes,^1,2,5B. Hyde,^1,2K.T. Tait,⁶K. Chamberlain,^7,8A.K. Schmitt,³J. Dunlop,⁴M.Anand
Earth and Planetary Science Letters 636, 118694 Open Access Link to Article [https://doi.org/10.1016/j.epsl.2024.118694]
¹Department of Natural History, Royal Ontario Museum, Toronto, Ontario, M5S 2C6, Canada
²Department of Earth Sciences, University of Toronto, Toronto, Ontario, M5S 3B1, Canada
³School of Earth and Environmental Science, University of Portsmouth, Portsmouth, PO1 3QL, UK
⁴School of Physical Sciences, The Open University, Milton Keynes MK7 6AA, UK
⁵Western University, London, Ontario, Canada N6A 3KL
⁶Department of Geology and Geophysics, University of Wyoming, 1000 E. University Ave, Laramie, Wyoming 82071-3006, USA
⁷Institute of Earth Sciences, Ruprecht-Karls-Universitat Heidelberg, Im Neuenheimer Feld 236, D-69120 Heidelberg, Germany
⁸John de Laeter Centre, Curtin University, Bentley. WA 6102, Australia
Copyright Elsevier

A long-standing paradigm in planetary science is that the inner Solar System experienced a period of intense and sustained bombardment between 4.2 and 3.9 Ga. Evidence of this period, termed the Late Heavy Bombardment is provided by the 40Ar/39Ar isotope systematics of returned Apollo samples, lunar meteorites, and asteroidal meteorites. However, it has been largely unsupported by more recent and robust isotopic age data, such as isotopic age data obtained using the U-Pb system. Here we conduct careful microstructural characterisation of baddeleyite, zircon, and apatite in six different eucrites prior to conducting SIMS and LA-ICP-MS measurement of U, Th, and Pb isotopic ratios and radiometric dating. Baddeleyite, displaying complex internal twinning linked to reversion from a high symmetry polymorph in two samples, records the formation of the parent body (4554 ± 3 Ma 2σ; n = 8), while structurally simple zircon records a tight spread of ages representing metamorphism between 4574 ± 14 Ma and 4487 ± 31 Ma (n = 6). Apatite, a more readily reset shock chronometer, records crystallisation ages of ∼4509 Ma (n = 6), with structurally deformed grains (attributed to impact events) yielding U-Pb ages of 4228 Ma (n = 12). In concert, there is no evidence within the measured U-Pb systematics or microstructural record of the eucrites examined in this study to support a period of late heavy bombardment between 4.2 and 3.9 Ga.

¹Aimee Smith,¹Rhian.H. Jones
Geochimica et Cosmochimica Acta (in Press) Open Access Link to Article [https://doi.org/10.1016/j.gca.2024.04.016]
¹Department of Earth and Environmental Sciences, The University of Manchester, M13 9PL, UK
Copyright Elsevier

Silica-rich igneous rims (SIRs) occur commonly as an outer rim layer on porphyritic chondrules in CR (Renazzo-like) chondrites, as well as more rarely in other chondrite groups. Formation conditions for SIRs can provide insight into chemical and physical conditions in the chondrule-forming region of the protoplanetary disk, as well as helping to understand temporal changes to successive thermal events within the CR and other chondrule formation regions. Both accretion and condensation models have been proposed as formation mechanism of SIRs, but a lack of robust thermal constraints prevents a detailed interpretation. We have carried out an experimental study aimed to define the conditions of SIR formation by reproducing the texture, mineralogy, mineral chemistry, and silica polymorphs present in natural SIRs. Experiments conducted on an SIR bulk composition, at peak temperatures of 1310–1507 °C with linear cooling rates between 30 and 90 °C/hr successfully reproduce natural SIRs that contain cristobalite, low-Ca pyroxene, Ca-rich pyroxene, and glass. Microcrystalline mesostasis was formed in an experiment with lower cooling rates at low temperatures (6 °C/hr from <1200 °C). All silica in the experiments was cristobalite, including in experiments with maximum temperatures as low as 1310 °C. Since the cristobalite stability field in the silica phase diagram is >1470 °C, it is clear that silica polymorphs are not robust temperature indicators and that the presence of cristobalite in natural SIRs does not necessarily indicate high peak temperatures. We suggest that accretion of SIR precursors onto solidified chondrules, followed by melting, is the most likely scenario for their formation. Our constraints on SIR formation are similar to those that are usually discussed for chondrules. Therefore, in the CR chondrule-forming region, a repeatable heating mechanism is required that does not change significantly during the time in which condensing material evolved to highly silica-rich compositions. The need for recurring heating events is a general constraint for modeling chondrule formation.