^1,2L.Krämer Ruggiu et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.09.020]
¹Aix Marseille Univ, CNRS, IRD, INRAE, CEREGE, Aix-en-Provence, France
²Analytical-, Environmental- and Geo-Chemistry, Vrije Universiteit Brussel, Brussels, Belgium
Copyright Elsevier

We discuss if the detection of aqueous alteration depends on the techniques that are used. We apply different methods to estimate the extent of aqueous alteration on four ungrouped carbonaceous chondrites showing limited aqueous alteration and thermal metamorphism: Chwichiya 002, El Médano (EM) 200, Northwest Africa (NWA) 12957 and NWA 11750, classified as C3 or C3.00-ung. The aim is to propose a reliable methodology to identify the most primitive chondrites. Chwichiya 002, NWA 11750 and NWA 12957 display very primitive matrices and could be amongst the most primitive chondrites currently known, similar to the least altered lithologies of the CM chondrites Paris (CM2.9) and Asuka (A) 12085 (CM2.8), A 12236 (CM2.9) and A 12169 (CM3.0). The structure of organic matter and Cr2O3 in ferroan olivines show that the four meteorites have been less heated than the least metamorphosed standard/reference type 3 chondrite, Semarkona (LL3.00), with Chwichiya 002, NWA 12957 and NWA 11750 similar to the CO3.0s, Acfer 094 (C2-ung) and Paris meteorites. Chwichiya 002 and NWA 12957 show similar alteration phases and degree of alteration, with high abundances of amorphous material with embedded metal and sulfide, resembling Glass with Embedded Metal and Sulfide (GEMS)-like materials, and tochilinite-cronstedtite intergrowths (TCIs) as the major alteration phases. The matrix in NWA 11750 contains aggregates of nanoscale olivine crystals and abundant carbonates, observed as micrometer-sized carbonate veins surrounding chondrules, and as nanoscale carbonates mixed with the fine-grained materials. It also contains abundant grains of metal and a low abundance of phyllosilicates. El Medano 200 shows a high abundance of magnetite (∼ 10 vol%), nanoscale phyllosilicates, troilite, and organic matter. The variability of the secondary alteration phases in the meteorites suggests different alteration mechanisms, likely depending on both the starting composition of the meteorites and the composition of the fluids of alteration.

Scanning and transmission electron microscopy (SEM and TEM) allow the identification of primitive phases and the composition and spatial distribution of the secondary phases. X-ray diffraction (XRD) can detect alteration products, including some amorphous phases, although this is limited by the small coherence domains of small TCIs and other phyllosilicates. Transmission infrared (IR) spectroscopy can detect phyllosilicate and carbonate, but is ineffective for the detection of amorphous phases, metal, or sulfide. Both matrix defocused electron microprobe analyses (EMPA) and thermogravimetric analysis (TGA) allow detection of hydrated minerals, such as phyllosilicates and carbonates, but are strongly influenced by the presence of organic matter and do not reflect the overall alteration state of a meteorite. We conclude that the assessment of the primitivity of a chondrite is highly technique dependent. We propose a combination of XRD and the Cr2O3 in ferroan olivines or Raman spectroscopy for a rapid characterization of the alteration state of a chondrite and the detection of the most primitive meteorites. Finally, the combination of XRD and TEM allows for the detection of all primary and secondary phases and represents an ideal methodology for the characterization and detailed study of primitive chondrites and the different types of incipient aqueous alteration.

¹Rajdeep Dasgupta,¹Emily Falksen,¹Aindrila Pal,^1,2Chenguang Sun
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.09.012]
¹Department of Earth, Environmental and Planetary Sciences, Rice University, Houston, TX 77005, USA
²Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX 78712
Copyright Elsevier

Evolution of nitrogen (N), a life-essential volatile element, in highly reduced magmatic systems is a key for the origin of N on rocky planets formed via accretion of reduced chondritic parent body materials, planetesimals, and embryos that underwent partial or complete differentiation. However, the storage capacity of N in phases relevant for reduced silicate systems undergoing thermal processing is poorly known. To investigate the stability of N-bearing phases in partially molten silicate-rich systems as well as solubility of nitrogen in silicate melts and minerals, we performed laboratory experiments on a 80:20 synthetic basalt-Si3N4 mixture at 1.5-3.0 GPa and 1300-1600 °C in graphite capsules, yielding oxygen fugacity ranging from ∼IW– 3.0 to ∼IW – 4.0. All experiments produced silicate melt + nierite + Fe-rich alloy melt + N-rich vapor ± sinoite ± cpx. Sinoite was restricted to above while cpx was restricted below 1400-1500 °C. Nitrogen solubility and Nitrogen Concentration at Silicon-Nitride Saturation (NCNS) in silicate melts increases with increasing pressure and temperature and ranges between 3.6 and 9.5 wt %. Using our high pressure N solubility data and similar data at ambient and lower pressures, we derived a new N solubility model in silicate melts. Solubility of nitrogen in cpx was between 1.51 and 2.05 wt% and resulted in cpx/silicate melt partition coefficients for nitrogen, of ∼0.4 to ∼0.2. These are distinctly higher than those previously estimated at more oxidizing conditions, suggesting N maybe much less incompatible during thermal processing of rocky reservoirs at highly reducing conditions. Partition coefficient of N between Fe-rich alloy melt and cpx, was found to be between 1.6 and 2.1. The application of our N solubility data and model suggests that mobilization of N from the deeper, partially molten reservoirs to shallower reservoirs is possible in reduced planetesimals and internally differentiated meteorite parent bodies – leading to net loss of N via melt degassing or reprecipitation of N-bearing solid phases, depending on whether the surficial shell is oxidized or reduced, respectively. Similarly, comparison of the first measured values from our highly reducing experiments with those estimated at more oxidizing conditions suggest that N would be much less incompatible during internal and external magma ocean processing of rocky bodies under highly reducing conditions. Therefore, enrichment of N in the atmospheres of Earth and Venus is likely a result of more oxidizing penultimate phase of accretion, which would lead to N being more readily partitioned to residual liquid, which would also more readily degas at oxidizing conditions.

Michael Zolensky¹, Takashi Mikouchi², Kenji Hagiya³, Kazumasa Ohsumi^4,5, Mutsumi Komatsu⁶, Andrew Cheng⁷, Loan Le⁸
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13909]
¹ARES, NASA Johnson Space Center, Houston, Texas 77058, USA
²University Museum, University of Tokyo, Tokyo 113-0033, Japan
³Graduate School of Life Science, Universtiy of Hyogo, Hyogo 678-1297, Japan
⁴Japan Synchrotron Radiation Research Institute (JASRI), Hyogo 679-5198, Japan
⁵Japan and High Energy Accelerator Research Organization (KEK), Ibaraki 305-0801, Japan
⁶The Graduate University for Advanced Studies, SOKENDAI, Kanagawa 240-0193, Japan
⁷The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland 20723, USA
⁸Jacobs JETS, Johnson Space Center, Houston, Texas 77058, USA
Published by arrangement with John Wiley & Sons

We explore impact shock processing of the regolith of parent asteroids of carbonaceous chondrites, which has not been considered a major process for hydrous carbonaceous chondrites. We describe shock-produced minerals and features found in brecciated CI, CM, and CV chondrites, including agglutinates, a glassy melt pod, a shock melt vein, and melted sulfides. We also reexamine cognate clasts present in the Vigarano CV3 chondrites which appear to derive from asteroid ponds and exhibit cross-bedded dish structures.

Tarunika Ramprasad¹, Pierre Haenecour², Kenneth Dominik², Thomas J. Zega^1,2
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13910]
¹Department of Materials Science and Engineering, University of Arizona, 1235 E. James E. Rogers Way, Tucson, Arizona
²Lunar and Planetary Laboratory, University of Arizona, 1629 E. University Blvd, Tucson, Arizona 85721, USA
85721, USA
Published by arrangement with John Wiley & Sons

Compact type A calcium-aluminum-rich inclusions (CTA CAIs) are believed to have experienced partial melting that erased all information on their original nebular condensation. To investigate this question, we report new microstructural data on a CTA CAI, composed primarily of melilite, spinel, and perovskite, in the Northwest Africa 5028 CR2 chondrite. The melilite grains contain low (5–10 mole%) åkermanite contents and are not compositionally zoned. Spinel and perovskite each occur as near endmember compositions MgAl₂O₄ and CaTiO₃ and contain minor V and Al, respectively. A continuous rim composed of melilite, spinel, and perovskite, with minor hibonite grains occur around the CAI. We extracted two regions of interest from the interior CAI and two from the rim using focused ion beam techniques for detailed analysis using transmission electron microscopy. Evidence for thermal processing occurs as a perovskite–spinel–spinel triple junction in an interior section and a spinel inclusion within perovskite in a rim section. Evidence for parent-body alteration occurs in the form of Fe-rich sheet silicates in the rim, and localized amstallite in the interior of the CAI. While previous work suggested that many CTA CAIs experienced thermal processing in the solar nebula, including partial melting, our data show that signatures of primary condensation can be preserved in the form of more refractory phases contained within less refractory minerals, namely melilite and perovskite grains within spinel, and hibonite grains within perovskite, respectively. The inclusion we report on here has a complex history involving gas-phase condensation, nebular thermal processing, and parent-body alteration.

Dilyara M. Kuzina¹, Jérôme Gattacceca², Natalia S. Bezaeva³, Dmitry D. Badyukov³, Pierre Rochette², Yoann Quesnel², François Demory², Daniel Borschneck²
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.13906]
¹Institute of Geology and Petroleum Technologies, Kazan Federal University, 4/5 Kremlyovskaya Str, 420008 Kazan, Russia ²CNRS, Aix Marseille Univ, IRD, INRAE, Aix-en-Provence, France
³Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, 19 Kosygin Str, 119991 Moscow, Russia
Published by arrangement with John Wiley & Sons

We present a paleomagnetic study of the ~10 km diameter Karla impact structure in Russia. We sampled the target carbonate rocks, and a yet undocumented fragmental melt-bearing lithic breccia layer. This impact breccia, which contains carbonate melt, is enriched in stoichiometric magnetite by a factor of ~15 compared to the target lithologies, and carries a stable natural remanent magnetization. The weak remanent magnetization and the presence of both normal and reverse polarities down to the centimeter scale indicate that the breccia does not carry a thermoremanent magnetization (TRM), but rather a chemical remanent magnetization (CRM). The presence of stoichiometric magnetite and the absence of TRM suggest that the magnetite was formed during relatively low-temperature postimpact hydrothermalism that affected the porous impact breccia layer. During this process, the breccia acquired a CRM. The paleomagnetic direction is compatible with a Cenozoic age for the impact event, but cannot bring more precise constraint on the age because of the stable position of the Eurasian plate over the last 60 Myr. However, the presence of both polarities indicates that mild hydrothermalism took place over a period of time long enough to span at least one reversal of the geomagnetic field, that is, over a time scale of the order of 100 kyr. This confirms that protracted hydrothermal systems associated with impact craters are long lived, even in relatively small craters such as Karla, and are key features of the geologic and environmental effects of impacts on Earth.

Bidong Zhang¹, Nancz L. Chabot¹ and Alan E. Rubin¹
Science Advances – Link to Article [https://www.science.org/doi/full/10.1126/sciadv.abo5781]
¹Department of Earth, Planetary and Space Sciences, University California, Los Angeles, CA 90095-1567, USA.
²Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA.
Copyright Elsevier

The parent cores of iron meteorites belong to the earliest accreted bodies in the solar system. These cores formed in two isotopically distinct reservoirs: noncarbonaceous (NC) type and carbonaceous (CC) type in the inner and outer solar system, respectively. We measured elemental compositions of CC-iron groups and used fractional crystallization modeling to reconstruct the bulk compositions and crystallization processes of their parent asteroidal cores. We found generally lower S and higher P in CC-iron cores than in NC-iron cores and higher HSE (highly siderophile element) abundances in some CC-iron cores than in NC-iron cores. We suggest that the different HSE abundances among the CC-iron cores are related to the spatial distribution of refractory metal nugget–bearing calcium aluminum–rich inclusions (CAIs) in the protoplanetary disk. CAIs may have been transported to the outer solar system and distributed heterogeneously within the first million years of solar system history.

Marceau Lecasble, Laurent Remusat, Jean-Christophe Viennet, Boris Laurent, Sylvain Bernard
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.08.039]
Muséum National d’Histoire Naturelle, Sorbonne Université, CNRS UMR 7590, IMPMC, Paris, France
Copyright Elsevier

Carbonaceous chondrites contain a diverse suite of more or less soluble compounds, including polycyclic aromatic hydrocarbons (PAHs). These compounds are structured around two or more fused benzene rings, and have shown to be detected in various astrophysical environments. The origin of PAHs in carbonaceous chondrites is debated: they may originate from the interstellar medium (ISM) and thus potentially carry information on accretion processes. Alternatively, they may have formed or transformed during secondary processes on parent bodies and thus carry information about aqueous alteration conditions. Here, we investigate the nature, quantity, and isotopic composition of free PAHs in three recently recovered CM chondrites having experienced substantial and distinct degrees of alteration: the CM2.2 Aguas Zarcas, the CM2.0 Mukundpura and the CM1/2 Kolang. All the CMs investigated contain PAHs, with sizes ranging from 2 (naphthalene) to 5 cycles (benzopyrene). The concentration of PAHs is not correlated to the degree of alteration and larger PAHs are also the most depleted in ¹³C, suggesting an interstellar origin. Yet, the abundance of alkylated PAHs appears correlated to the degree of alteration and all the extracted PAHs are D-depleted, pointing towards hydrogen exchange with water having occurred during aqueous alteration. These combined results suggest that PAHs in CCs likely carry information on both accretion and alteration processes.

David W. Graham, Kevin Konrad¹
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.09.016]
College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331 USA
Copyright Elsevier

We report ³He and ⁴He concentrations in 57 sediment samples from the southeast Indian Ocean where ⁶⁰Fe excesses were previously identified in a subset of the same samples (Wallner et al., 2016). The coupled ⁶⁰Fe-³He data allow further evaluation of two competing hypotheses: 1) a nearby supernova (SN) showered Earth with exotic radionuclides such as ⁶⁰Fe during the last 3 million years, or 2) ⁶⁰Fe in terrestrial archives was generated by reactions of galactic cosmic rays (GCRs) on micrometeorite grains that were irradiated for hundreds of millions of years in the interstellar medium, where ³He production by GCRs is larger than the solar wind ³He flux.

^aKeqing Zong et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.06.037]
¹State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
Copyright Elsevier

Lunar soil is a fine mixture of local rocks and exotic components. The bulk-rock chemical composition of the newly returned Chang’E-5 (CE-5) lunar soil was studied to understand its chemical homogeneity, exotic additions, and origin of landing site basalts. Concentrations of 48 major and trace elements, including many low-concentration volatile and siderophile elements, of two batches of the scooped CE-5 soil samples were simultaneously obtained by inductively coupled plasma mass spectrometry (ICP-MS) with minimal sample consumption. Their major and trace elemental compositions (except for Ni) are uniform at milligram levels (2–4 mg), matching measured compositions of basaltic glasses and estimates based on mineral modal abundances of basaltic fragments. This result indicates that the exotic highland and KREEP (K, rare earth elements, and P-rich) materials are very low (<5%) and the bulk chemical composition (except for Ni) of the CE-5 soil can be used to represent the underlying mare basalt. The elevated Ni concentrations reflect the addition of about 1 wt% meteoritic materials, which would not influence the other bulk composition except for some highly siderophile trace elements such as Ir. The CE-5 soil, which is overall the same as the underlying basalt in composition, displays low Mg# (34), high FeO (22.7 wt%), intermediate TiO₂ (5.12 wt%), and high Th (5.14 µg/g) concentrations. The composition is distinct from basalts and soils returned by the Apollo and Luna missions, however, the depletion of volatile or siderophile elements such as K, Rb, Mo, and W in their mantle sources is comparable. The incompatible lithophile trace element concentrations (e.g., Ba, Rb, Th, U, Nb, Ta, Zr, Hf, and REE) of the CE-5 basalts are moderately high and their pattern mimics high-K KREEP. The pattern of these trace elements with K, Th, U, Nb, and Ta anomalies of the CE-5 basalts cannot be explained by the partial melting and crystallization of olivine, pyroxene, and plagioclase. Thus, the mantle source of the CE-5 landing site mare basalt could have contained KREEP components, likely as trapped interstitial melts. To reconcile these observations with the initial unradiogenic Sr and radiogenic Nd isotopic compositions of the CE-5 basalts, clinopyroxene characterized by low Rb/Sr and high Sm/Nd ratios could be one of the main minerals in the KREEP-bearing mantle source. Consequently, we propose that the CE-5 landing site mare basalts very likely originated from partial melting of a shallow and clinopyroxene-rich (relative to olivine and orthopyroxene) upper mantle cumulate with a small fraction (about 1–1.5 %) of KREEP-like materials.

Tiantian Liu^a Kai Wünnemann^a,b GregMichael^a
Earth and Planetary Science Letters (in Press) Link to Article [https://doi.org/10.1016/j.epsl.2022.117817]
^aMuseum für Naturkunde, Leibniz Institute for Evolution and Biodiversity Science, 10115 Berlin, Germany
^bFreie Universität Berlin, Malteserstr., 74-100, 12249 Berlin, Germany
Copyright: Elsevier

The early impact bombardment extensively fractured the lunar crust resulting in the formation of the so-called megaregolith. Previous estimates of megaregolith distribution vary significantly with respect to the vertical extent and the size-frequency distribution of fragments was rarely studied. We built a spatially resolved numerical model to simulate the process of cumulative impact fragmentation, aiming to backtrack the megaregolith evolution history and to constrain its fragment distribution. The results highlight the pivotal role of basin-forming events on the megaregolith formation. Especially the South-Pole Aitken (SPA) impact established the initial megaregolith structure which remained distinct after 0.5 Ga subsequent fragmentation. At 3.8 Ga, the megaregolith displays substantial lateral variation and layering: the highly fractured upper layer of ∼2.5 km is dominated by meter-scale fragments; the disturbed lower layer deeper than tens of kilometers is mainly consisting of kilometer-scale fragments; the transition zone >5 km contains fragments of various size scales.