¹R.D.Hanna,²V.E.Hamilton,³C.W.Haberle,^4,5A.J.King,⁶N.M.Abreu,^7,8J.M.Friedrich
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113760]
¹Jackson School of Geosciences, University of Texas at Austin, Austin, TX 78712, USA
²Department of Space Studies, Southwest Research Institute, Boulder, CO 80302, USA
³School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA
⁴School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
⁵Planetary Materials Group, Department of Earth Sciences, Natural History Museum, Cromwell Road, London SW7 5BD, UK
⁶Earth Science Program, Pennsylvania State University, Du Bois, PA 15801, USA
⁷Department of Chemistry, Fordham University, Bronx, NY 10458, USA
⁸Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024, USA
Copyright Elsevier

Using infrared (IR) spectroscopy of thin sections, we characterize the relative degree of aqueous alteration and subsequent heating of a suite of CM chondrites to document spectral indicators of these processes that can contextualize observations of carbonaceous asteroids. We find that the progressive aqueous alteration of CMs manifests in two spectral regions. The low-wavenumber region (1200–400 cm⁻¹; 8–25 μm) records the increasing proportion of MgFe phyllosilicates relative to anhydrous silicates as aqueous alteration proceeds, with a highly correlated shift of the Christiansen feature (CF) to lower wavenumber and the SiO bending band minimum to higher wavenumber, and an increase in depth of the Mg-OH band (~625 cm⁻¹). The strongest correlation (R² = 0.90) with petrologic subtype is the distance between the CF and SiO stretching band minimum, which predicts the petrologic subtype of the sample to within 0.1. The high-wavenumber region (4000–2500 cm⁻¹, ≤3.33 μm) probes the variation in abundance and composition of MgFe serpentine and tochilinite among the altered CMs. All moderately to highly altered CMs (≤2.3) have an OH/H₂O (‘3 μm’) band emission maximum of 3690 cm⁻¹ (2.71 μm) indicative of Mg-bearing serpentine, and mildly aqueously altered CMs (≥ 2.5) have a wider band with a complex shape that results from contributions of Fe-bearing serpentine and tochilinite. Among weakly heated CMs (Stage II; 300–500 °C), the low-wavenumber region exhibits spectral features resulting from the dehydration and dehydroxylation of phyllosilicates that include broadening of the SiO stretching band and a shift of its minimum to lower wavenumber, and the disappearance of the Mg-OH band. The location of the SiO bending band minimum appears to be unaffected by mild heating. Extensively heated CMs (Stage III+; >500 °C) have a low-wavenumber region dominated by the spectral features of secondary, Fe-bearing olivine and low-Ca pyroxene and thus are readily distinguished from unheated and mildly heated CMs. The OH/H₂O band of all heated CMs is broad and rounded with an emission peak at higher wavenumbers (≤3636 cm⁻¹; ≥2.75 μm) than in unheated CMs. However, spectral and petrographic evidence suggests that our heated CMs have been compromised by terrestrial rehydration. Our study confirms that thermal metamorphism effects are concentrated within the matrix and suggests that the matrix of the CM WIS 91600 had a CI-like mineralogy prior to heating.

¹Jaques S. Schmidt,²Ruth Hinrichs
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13467]
¹Division of Geochemistry, PETROBRAS R&D Center, Rua Horácio Macedo 950, Rio de Janeiro, RJ, Brazil
²Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves 9500, Porto Alegre, RS, Brazil
Published by arrangement with John Wiley & Sons

A Raman method to estimate the aromatization degree of carbonaceous matter (CM) was established, and calibrated by literature data respecting differences in acquisition procedures. The generated equations allowed a comparison of the G‐band width to vitrinite reflectance, and to additional structural organization parameter extracted from X‐ray powder diffraction. The relations between aromatization degree and peak temperatures from literature were then propagated to G‐band width parameter. The applicability of these geothermometers is demonstrated by the consistency of data obtained for carbonaceous chondrites with previous estimations. Since the CM aromatization is controlled by peak temperature and time span, it is possible to estimate the duration of chondrite’s heating events. The G‐band width relation to elemental ratio trends of organic matter in chondrites, interplanetary dust particles, glucose, and saccharose‐based semi‐coke favors synthesis by formose route. These findings provide new approaches for the future development of chemical kinetic models for organic matter of chondrites. A reliable chemical model may allow the external calibration of numerical models for accurately evaluating the peak temperatures and cooling durations of chondrites.

^1,2Sheng Gou et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113776]
¹State Key Laboratory of Remote Sensing Science, Aerospace Information Research Institute, Chinese Academy of Sciences, Beijing 100101, China
²State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology, Macau, China
Copyright Elsevier

China’s Chang’e-4 probe achieved the first soft landing within the South Pole-Aitken (SPA) basin, which is the oldest, largest, and deepest basin on the lunar farside. The deployed Chang’e-4 rover made in situ spectral measurements along the rover traverse during a nominal three-month mission period. Spectral analyses imply that materials at the Chang’e-4 landing site have a forsteritic olivine (OL) and magnesium (Mg)-rich orthopyroxene (OPX) assemblage in almost equal fractions. The Chang’e-4 rover measured materials, which were essentially mixture of multiple sources, were primarily the weathering products of Finsen crater ejecta. Plagioclase (PLG), which has significant implication for the provenance, is often spectrally transparent or featureless in the wavelength range (450–2395 nm) of the in situ measured spectrum. Depending on the possible absence or presence of abundant PLG, the materials likely originated from a differentiated SPA impact melt pool or from an Mg-suite pluton in the lunar lower crust, and were less likely to originate from the lunar upper mantle.

^1,2Levke Kööp,³Kazuhide Nagashima,^1,2,4Andrew M. Davis,¹Alexander N. Krot
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13434]
¹Department of the Geophysical Sciences, The University of Chicago, Chicago, Illinois, 60637 USA
²Chicago Center for Cosmochemistry, The University of Chicago, Chicago, Illinois, 60637 USA
³Hawai’i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai’i at Mānoa, Honolulu, Hawai’i, 96822 USA
⁴Enrico Fermi Institute, The University of Chicago, Chicago, Illinois, 60637 USA
Published by arrangement with John Wiley & Sons

NASA’s Genesis mission revealed that the Sun is enriched in ¹⁶O compared to the Earth and Mars (the Sun’s Δ¹⁷O, defined as δ¹⁷O–0.52×δ¹⁸O, is –28.4 ± 3.6‰; McKeegan et al. 2011). Materials as ¹⁶O‐rich as the Sun are extremely rare in the meteorite record. Here, we describe a Ca‐Al‐rich inclusion (CAI) from a CM chondrite that is as ¹⁶O‐enriched as the Sun (Δ¹⁷O = –29.1 ± 0.7‰). This CAI also has large nucleosynthetic anomalies in ⁴⁸Ca and ⁵⁰Ti (δ‐values are –8.1 ± 3.3 and –11.7 ± 2.4‰, respectively) and shows no clear evidence for incorporation of live ²⁶Al; (²⁶Al/²⁷Al)₀ = (0.03 ± 0.11) × 10^–5. Due to their anomalous isotopic characteristics, the rare CAIs consistent with the Genesis value could be among the first materials that formed in the solar system. In contrast to the CAI studied here, the majority of CAIs formed in or interacted with a reservoir characterized by a Δ¹⁷O value near –23.5‰. Combined with ²⁶Al‐²⁶Mg systematics, the oxygen isotopic compositions of FUN (fractionation and unidentified nuclear effects), UN, and normal CAIs suggest that nebular conditions were favorable for solids to inherit this value for an extended period of time. Many later‐formed materials, such as chondrules, planetesimals, and terrestrial planets, formed in reservoirs with Δ¹⁷O near 0‰. The distribution could be easier to explain if the common CAI value of –23.5‰, which is consistent with the Genesis value within 3σ, represented the average composition of the protoplanetary disk.

^1,2Vladimir Svetsov,^1,2Valery Shuvalov,¹Igor Kosarev
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13470]
¹Institute for Dynamics of Geospheres, Russian Academy of Sciences, Moscow, 119334 Russia
²Moscow Institute of Physics and Technology, 9 Institutskiy per., Dolgoprudny, Moscow Region, 141701 Russia
Published by arrangement with John Wiley & Sons

Libyan Desert Glass contains meteoritic material and, therefore, its origin is most likely associated with an impact event. However, the impact crater has not been found. We performed numerical simulations of impacts of stony and cometary bodies in order to confirm the version that this glass was formed from silica heated by radiation from aerial bursts near the ground. Asteroids were treated as strengthless bodies from dunite with a density of 3.3 g cm−3, and comets as icy bodies with a density of 1 g cm−3. The simulations based on hydrodynamic equations included the equations of radiation transfer. Melting and vaporization of a silica target under action of radiation incident on a planar surface were modeled using a one‐dimensional hydrodynamic equation of energy and equations of radiation transfer in two‐flux approximation. We selected those variants of simulations in which a crater is not formed, a fireball touches the earth surface, and the area of a molten target corresponds to the area of the Libyan Desert Glass strewn field. Appropriate options include the impact of an asteroid with a diameter of 300 m, an entry speed of 35 km s−1, and an entry angle of 8°, and cometary bodies with diameters from 150 to 300 m, speeds of 50–70 km s−1, and entry angles from 15° to 45°. Impact options with crater formation are also discussed. The maximum depth of molten silica at ground zero reaches 10 cm with the cometary impacts and 3–4 cm with the asteroidal impact. Melting occurs during a period of time from 50 to 400 s.

¹C.K.Sio,¹L.E.Borg,¹W.S.Cassata
Earth and Planetary Science Letters 538, 116219 Link to Article [https://doi.org/10.1016/j.epsl.2020.116219]
¹Lawrence Livermore National Laboratory, 7000 East Ave. L-231, Livermore, CA 94550, USA
Copyright Elsevier

Ferroan anorthosite suite (FAS) rocks are widely interpreted to represent primordial lunar crust. Despite their importance in pinpointing the timing of lunar crust formation, robust chronological investigations for this rock type are scarce. Here, we report the Ar-Ar, Rb-Sr, and Sm-Nd isotopic systematics for the FAS troctolitic anorthosite 62237. The Ar-Ar isotopic system has been reset by a thermal event at 3710 ± 48 Ma, and the Rb-Sr isotopic systematics has been disturbed such that a Rb-Sr isochron age cannot be determined. However, an internal isochron for the Sm-Nd isotopic system has yielded an age of 4350 ± 73 Ma (MSWD = 2.0) with an initial Nd_CHUR of −0.53 ± 0.26. The mineral and whole-rock fractions of 62237 plot on the same internal isochron as FAS sample 60025. The combined datasets define an age of 4372 ± 35 Ma (MSWD = 4.0) with an initial Nd_CHUR of −0.17 ± 0.22. Literature Sm-Nd data for FAS and Mg-suite whole-rocks also plot on the 60025-62237 isochron. The coherence of data from both FAS and Mg-suite rocks examined thus far suggests that both rock suites formed contemporaneously from identical, or nearly identical, sources. In addition, the concordance of FAS and Mg-suite ages suggests that primordial crust solidification either involved both magmatic suites, or that Mg-suite magmatism was contemporaneous with FAS magmatism within resolution of the Sm-Nd chronometer. The ages for FAS and Mg-suite also coincide with the formation ages of the mare basalt source regions and urKREEP. Ferroan anorthosite suite rocks and urKREEP are thought to represent primordial LMO solidification products, whereas Mg-suite and the mare basalt source regions are argued to represent mixtures of various LMO crystallization products that were formed during density-driven overturn of the LMO. The concordance of ages implies that the 4372 ± 35 Ma Sm-Nd isochron records the age of mantle overturn, and that overturn occurred during, or shortly after, solidification of the LMO.

¹A.Morbidelli,¹G.Libourel,²H.Palme,³ S.A.Jacobson,⁴D.C.Rubie
Earth and Planetary Science Letters 538, 116220 Link to Article [https://doi.org/10.1016/j.epsl.2020.116220]
¹Laboratoire Lagrange, UMR7293, Université de Nice Sophia-Antipolis, CNRS, Observatoire de la Côte d’Azur, Boulevard de l’Observatoire, 06304 Nice Cedex 4, France
²Senckenberg, world of biodiversity, Sektion Meteoritenforschung, Senckenberganlage 25, 60325 Frankfurt am Main, Germany
³Northwestern University, Dept. of Earth and Planetary Sciences, Evanston, 60208 IL, United States of America
⁴Bayerisches Geoinstitut, University of Bayreuth, 95440, Bayreuth, Germany
Copyright Elsevier

The Al/Si and Mg/Si ratios in non-carbonaceous chondrites are lower than the solar (i.e., CI-chondritic) values, in sharp contrast to the non-CI carbonaceous meteorites and the Earth, which are enriched in refractory elements and have Mg/Si ratios that are solar or larger. We show that the formation of a first generation of planetesimals during the condensation of refractory elements implies the subsequent formation of residual condensates with strongly sub-solar Al/Si and Mg/Si ratios. The mixing of residual condensates with different amounts of material with solar refractory/Si element ratios explains the Al/Si and Mg/Si values of non-carbonaceous chondrites. To match quantitatively the observed ratios, we find that the first-planetesimals should have accreted when the disk temperature was ∼1,330–1,400 K depending on pressure and assuming a solar C/O ratio of the disk. We discuss how this model relates to our current understanding of disk evolution, grain dynamics, and planetesimal formation. We also extend the discussion to moderately volatile elements (e.g., Na), explaining how it may be possible that the depletion of these elements in non-carbonaceous chondrites is correlated with the depletion of refractory elements (e.g., Al). Extending the analysis to Cr, we find evidence for a higher than solar C/O ratio in the protosolar disk’s gas when/where condensation from a fractionated gas occurred. Finally, we discuss the possibility that the supra-solar Al/Si and Mg/Si ratios of the Earth are due to the accretion of ∼40% of the mass of our planet from the first-generation of refractory-rich planetesimals.

^1,2,3E.S.Steenstra,³J.Berndt,³S.Klemme,¹Y.Fei,²W.van Westrenen
Earth and Planetary Science Letters 538, 116222 Link to Article [https://doi.org/10.1016/j.epsl.2020.116222]
¹The Earth and Planets Laboratory, Carnegie Institution for Science, Washington, DC, USA
²Faculty of Science, VU Amsterdam, Amsterdam, the Netherlands
³Institute of Mineralogy, University of Münster, Münster, Germany
Copyright Elsevier

The formation of the Moon is thought to be the result of a giant impact between a Mercury-to-proto-Earth-sized body and the proto-Earth. However, the initial thermal state of the Moon following its accretion is not well constrained by geochemical data. Here, we provide geochemical evidence that supports a high-temperature origin of the Moon by performing high-temperature (1973–2873 K) metal-silicate partitioning experiments, simulating core formation in the newly-formed Moon. Results indicate that the observed lunar mantle depletions of Ni and Co record extreme temperatures (>2600–3700 K depending on assumptions about the composition of the lunar core) during lunar core formation. This temperature range is within range of the modeled silicate evaporation buffer in a synestia-type environment. Our results provide independent geochemical support for a giant-impact origin of the Moon and show that lunar thermal models should start with a fully molten Moon. Our results also provide quantitative constraints on the effects of high-temperature lunar differentiation on the lunar mantle geochemistry of volatile, and potentially siderophile elements Cu, Zn, Ga, Ge, Se, Sn, Cd, In, Te and Pb. At the extreme temperatures recorded by Ni and Co, many of these elements behave insufficiently siderophile to explain their depletions by core formation only, consistent with the inferred volatility-related loss of Cr, Cu, Zn, Ga and Sn during the Moon-forming event and/or subsequent magma-ocean degassing.

¹Hannah Bloom,¹Katharin Lodders,^1,2Heng Chen,^1,3Chen Zhao,¹Zhen Tian,¹Piers Koefoed, ⁴Mária K.Pető,^5,6Yun Jiang,¹Kun Wang (王昆)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.03.018]
¹Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University in St. Louis, One Brookings Drive, St. Louis, MO 63130, USA
²Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA
³Faculty of Earth Sciences, China University of Geosciences, Wuhan, Hubei 430074, China
⁴Konkoly Observatory, Research Center for Astronomy and Earth Sciences, H-1121 Budapest, Hungary
⁵CAS Key Laboratory of Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China
⁶CAS Center for Excellence in Comparative Planetology, China
Copyright Elsevier

Among solar system materials there are variable degrees of depletion in moderately volatile elements (MVEs, such as Na, K, Rb, Cu, and Zn) relative to the proto-solar composition. Whether these depletions are due to nebular and/or parent-body (asteroidal or planetary) processes is still under debate. In order to help decipher the MVE abundances in early solar system materials, we conducted a systematic study of high-precision K stable isotopic compositions of a suite of whole-rock samples of well-characterized carbonaceous and ordinary chondrites. We analyzed 16 carbonaceous chondrites (CM1-2, CO3, CV3, CR2, CK4-5 and CH3) and 28 ordinary chondrites covering petrological types 3 to 6 and chemical groups H, L, and LL. We observed significant K isotope (δ41K) variations (−1.54 to 0.70 ‰) among the carbonaceous and ordinary chondrites. In general, the two major chondrite groups are distinct: The K isotope compositions of carbonaceous chondrites are largely higher than the Bulk Silicate Earth (BSE) value, whereas ordinary chondrites show K isotope compositions that are typically lower than the BSE value. Neither carbonaceous nor ordinary chondrites show clear/resolvable correlations between K isotopes and chemical groups, petrological types, shock levels, cosmic-ray exposure ages, fall/find occurrence, or terrestrial weathering. Importantly, the lack of a clear trend between K isotopes and K content among chondrites indicates that the K isotope fractionations were decoupled from the relative elemental K depletions, which is inconsistent with a single-stage partial vaporization or condensation process to account for these MVE depletion patterns among chondrites. The range of K isotope variations in the carbonaceous chondrites in this study is consistent with a four-component (chondrule, refractory inclusion, matrix and water) mixing model that is able to explain the bulk elemental and isotopic compositions of the main carbonaceous chondrite groups, but requires a fractionation in K isotopic compositions in chondrules. We propose that the major control of the isotopic compositions of group averages is condensation and/or vaporization in pre-accretional (nebular) environments that is preserved in the compositional variation of chondrules. Parent-body processes, such as aqueous alteration, thermal metamorphism, and metasomatism, can mobilize K and affect the K isotopes in individual samples. In the case of the ordinary chondrites, the full range of K isotopic variations can only be explained by the combined effects of the size and relative abundances of chondrules, parent-body aqueous and thermal alteration, and possible sampling bias.

¹Renaud Merle,¹Yuri Amelin,²Qing-Zhu Yin,²Magdalena H.Huyskens,²Matthew E.Sanborn,³Kazuhide Nagashima,⁴Katsuyuki Yamashita,¹Trevor R.Ireland,³Alexander N.Krot,^1,5Melanie J.Siebera
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.03.010]
¹Research School of Earth Sciences, The Australian National University, Canberra, 2601 Australia
²Department of Earth and Planetary Sciences, University of California-Davis, One Shields Avenue, Davis, CA, USA
³Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
⁴Graduate School of Natural Science and Technology, Okayama University, Japan
⁵GFZ, German Research Centre for Geoscience, Telegrafenberg, D.-14473 Potsdam,Germany
Copyright Elsevier

Chondrules in chondritic meteorites are unique witnesses of nebular and asteroidal processes that preceded large-scale planetary accretion. Together with refractory calcium-aluminium-rich inclusions (CAIs), they are the sources of our knowledge of the initial evolution of the early Solar System. We have investigated a single very large (>10 mm in longer dimension) chondrule, hereafter, the mega-chondrule A25-2, extracted from the Allende CV3 chondrite. We characterised texture, mineralogy and mineral chemistry of this chondrule, and studied its Al-Mg, U-Pb and U-isotope systematics. We also studied the distribution of U, Th and Pb, and measured Pb isotopic composition in individual minerals of A25-2 by secondary ion mass-spectrometry (SIMS). The main difficulty in absolute age determination was the presence of pervasive and resilient non-radiogenic Pb. In the search for the best way to separate radiogenic Pb from non-radiogenic Pb components of terrestrial and asteroidal origins, we used various protocols of multi-step leaching and assessed their efficiency in generating data suitable for the construction of an isochron. Testing the data filtering procedure led us to explore the behaviour of the stepwise leaching method in the presence of pervasive and resilient non-radiogenic Pb. The model age patterns observed in the final HF partial dissolution steps were probably induced by isotopic fractionation. Although step leaching did not yield fractions with highly radiogenic Pb, a Pb-Pb isochron age corrected for measured ²³⁸U/²³⁵U was obtained by: (1) data filtering process based on strict analytical and geochemical criteria to include in the Pb-Pb isochron only leaching steps free from terrestrial contamination and (2) arithmetically recombined analyses to cancel the effects of leaching-induced isotopic fractionation.

This extensive data processing yielded the age of 4568.5±3.0 Ma, which we consider reliable within its uncertainty limits, although it is not as precise as, and more model dependent than, the age that could have been obtained if Pb isotopic compositions were more radiogenic. The ²³⁸U/²³⁵U ratio of the mega-chondrule is 137.764±0.016, which is similar to the ratios obtained from single chondrules yet slightly different from small pooled Allende chondrules. The initial ²⁷Al/²⁶Al ratio inferred from internal isochron obtained from SIMS Al-Mg isotope measurements is (5.4±6.5)×10^–6, which corresponds to 4565.0 +0.8/-∞ Ma, assuming homogeneous distribution of ²⁶Al throughout the protoplanetary disk at the canonical level (∼5.2×10⁻⁵). This age is 3.5±3.1 Ma younger than the Pb-isotopic age. Calculation of ²⁶Al-²⁶Mg age assuming initial (²⁷Al/²⁶Al)₀ of (1.36±0.72)×10^–5 in the chondrule-forming region yields the age of 4566.4+0.8/-∞, which is consistent with the Pb-isotopic age.