¹François Faure,¹Marion Auxerre,¹Valentin Casola
Earth and Planetary Science Letters 593, 117649 Link to Article [https://doi.org/10.1016/j.epsl.2022.117649]
¹Université de Lorraine, CNRS, CRPG, UMR 7358, 15 rue Notre Dame des Pauvres, F-54501 Vandoeuvre-lès-Nancy, France
Copyright Elsevier

Barred olivine (BO) chondrules are small ferromagnesian silicate igneous droplets with unique dendritic textures that are considered to have formed in the early solar system during one or more brief high-temperature episodes, followed by rapid cooling in a gas. Rapid cooling rates of 100–7200 °C/h during chondrule formation have been proposed based on experiments attempting to reproduce BO crystal textures. However, the BO texture has never truly been reproduced under such rapid cooling conditions. Here, we experimentally show that true BO textures can be produced either after rapid cooling (>50 °C/h) following by reheating step or by cooling rates slower than 10 °C/h. Regardless of the thermal history considered, the chemical compositions of glass inclusions trapped within olivines of BO chondrules imply a final slow cooling rate one to two orders of magnitude below previous estimates. Such slow cooling rates are consistent with those estimated for plagioclase-bearing porphyritic chondrules and magmatic type-B Ca-Al-rich inclusions, suggesting that slow cooling rates are common to all similar chondritic objects.

¹Wolf Uwe Reimold et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13833]
¹Laboratory of Geochronology and Isotope Geochemistry, Instituto de Geociências, Universidade de Brasília, Brasília, DF, CEP70910-900 Brazil
Published by arrangement with John Wiley & Sons

The Nova Colinas structure is an approximately 7 km wide, nearly circular feature centered at 07°09′33″S/46°06′30″W in Nova Colinas municipality in southwestern Maranhão State, Brazil. The area has been investigated for 40 yr and it has been suggested repeatedly that the structure could be of impact origin—without proof having been furnished. Magnetic anomaly maps depict the structure clearly with a strong, positive magnetic anomaly over the apparent rim zone. The central area is characterized by significant positive K and Th radiometric anomalies. Fieldwork showed that the structure has annular features along the outside and some prominent, structurally dissected hills in the interior. Thirty-three arenite samples were collected for petrographic analysis, mostly from within the structure. Microdeformation, in the form of cataclasis; concussion fractures related to compaction, and presence of planar fractures, feather features, and planar deformation features in quartz are reported. Three samples with a multitude of quartz grains with these microdeformations were analyzed by universal stage to determine the crystallographic orientations of planar fractures and planar deformation features. The results provide robust evidence that these microdeformation features represent shock metamorphism, with low (approximately 5–10 GPa) to moderate (10–16 and 10–20 GPa) shock levels. Thus, the Nova Colinas structure is now confirmed as a bona fide meteorite impact structure. The structure is moderately eroded, as shown by the absence of stronger shock deformation. The still limited available structural geological field evidence, paired with remote sensing and geophysical data, indicates that the innermost part of the structure may have a sizable remnant of a central uplift. The Nova Colinas impact age is only poorly constrained from stratigraphic inference to an upper limit of about 200–250 Ma.

^1,2Imene Kerraouch et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.07.010]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Str. 10, D-48149 Münster, Germany
²Astromaterials Research and Exploration Science, NASA Johnson Space Center, Houston TX, 77058, USA
Copyright Elsevier

On April 23rd, 2019, the Aguas Zarcas meteorite fall occurred in Costa Rica. Because the meteorite was quickly recovered, it contains valuable extraterrestrial materials that have not been contaminated by terrestrial processes. Our X-ray computed tomography (XCT) and scanning electron microscopy (SEM) results on various pre-rain fragments from earlier work (Kerraouch et al., 2020; 2021) revealed several distinct lithologies: Two distinct metal-rich lithologies (Met-1 and Met-2), a CM1/2 lithology, a C1 lithology, and a brecciated CM2 lithology consisting of different petrologic types. Here, we further examined these lithologies in the brecciated Aguas Zarcas meteorite and report new detailed mineralogical, chemical, isotopic, and organic matter characteristics. In addition to petrographic differences, the lithologies also display different chemical and isotopic compositions. The variations in their bulk oxygen isotopic compositions indicate that the various lithologies formed in different environments and/or under diverse conditions (e.g., water/rock ratios). Each lithology experienced a different hydration period during its evolution. Together, this suggests that multiple precursor parent bodies may have been involved in these processes of impact brecciation, mixing, and re-assembly. The Cr and Ti isotopic data for both the CM1/2 and Met-1 lithology are consistent with those of other CM chondrites, even though Met-1 displays a significantly lower ε50Ti isotopic composition that may be attributable to sample heterogeneities on the bulk meteorite scale and may reflect variable abundances of refractory phases in the different lithologies of Aguas Zarcas. Finally, examination of the organic matter of the various lithologies also suggests no strong evidence of thermal events, but a short-term heating cannot completely be excluded. Raman parameters indicate that the peak temperature has been lower than that for Yamato-793321 (CM2, ∼400°C). Considering the new information presented in this study, we now better understand the origin and formation history of the Aguas Zarcas daughter body.

^1,2,3David V. Bekaert,¹Joshua Curtice,⁴Matthias M. M. Meier,⁵David J. Byrne,⁵Michael W. Broadley,¹Alan Seltzer,¹Peter Barry,¹Mark D. Kurz,^2,3Sune G. Nielsen
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13887]
¹Marine Chemistry & Geochemistry Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, 02543 USA
²Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, 02543 USA
³NIRVANA Laboratories, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, 02543 USA
⁴Naturmuseum St. Gallen, Rorschacher Strasse 263, CH-9016 St. Gallen, Switzerland
⁵Centre de Recherches Pétrographiques et Géochimiques, Vandoeuvre-Lès-Nancy, France
Published by arrangement with John Wiley & Sons

We present He-Ne-Ar isotope data for 23 meteorite samples mainly recovered in Antarctica (six ordinary chondrites [OC], two CV chondrites, eight eucrites, one diogenite, and six ureilites), which are used to compute radiogenic gas retention ages and cosmic ray exposure (CRE) ages using both empirical and modeling approaches. For all samples where both 40K-40Ar and U,Th-4He retention ages could be derived, we find that U,Th-4He ages are systematically lower than 40K-40Ar ages, likely reflecting preferential diffusive loss of He relative to Ar. There is good agreement between empirically derived CRE ages calculated by (22Ne/21Ne)cos-3Hecos and (22Ne/21Ne)cos-21Necos approaches; where discrepancies occur, the (22Ne/21Ne)cos-3Hecos approach systematically yields lower CRE ages, also likely due to 3He loss. Overall, CRE ages derived from the empirical and modeling approaches show excellent agreement, within ∼10%. CRE ages derived for OC (4–24 Myr), CV chondrites (12–26 Myr), eucrites (4–45 Myr), the diogenite (30 Myr), and ureilites (<10 Myr) are in line with previous investigations of these meteorite groups. Some ureilites and one eucrite exhibit remarkably high cosmogenic 22Ne/21Ne > 1.24, as previously observed in various other rare achondrites. These samples likely contain solar cosmic ray-produced Ne (SCR-Ne) in addition to the commonly found galactic cosmic ray-produced Ne (GCR-Ne), implying low pre-atmospheric shielding and limited ablation upon atmospheric entry. The presence of SCR-Ne complicates the determination of the pure GCR-22Ne/21Ne, hampering its use as a shielding indicator. Nonetheless, we suggest that a first-order correction for SCR-Ne contribution can be used to derive a range of potential CRE ages for each sample.

¹Aaron J. Cavosie,²William D.A. Rickard,^1,2Noreen J. Evans,²Kai Rankenburg,
³Malcolm Roberts,⁴Catherine A. Macris,⁵Christian Koeberl
American Mineralogist 107, 1325-1340 Link to Article [http://www.minsocam.org/MSA/AmMin/TOC/2022/Abstracts/AM107P1325.pdf]
¹School of Earth and Planetary Sciences, Curtin University, Perth, Western Australia 6102, Australia
²John de Laeter Centre, Curtin University, Perth, Western Australia 6102, Australia
³Centre for Microscopy, Characterisation, and Analysis, University of Western Australia, Perth, Western Australia 6009, Australia
⁴School of Science, Indiana University–Purdue University, Indianapolis, Indiana 46202, U.S.A.
⁵Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria
Copyright: The Mineralogical Society of America

Identifying and determining the origin of β-cristobalite, a high-temperature silica polymorph, in
natural samples is challenging as it is rarely, if ever, preserved due to polymorphic transformation to
α-cristobalite at low temperature. Formation mechanisms for β-cristobalite in high-silica rocks are
difficult to discern, as superheating, supercooling, bulk composition, and trace element abundance all
influence whether cristobalite crystallizes from melt or by devitrification. Here we report a study of
α-cristobalite in Libyan Desert Glass (LDG), a nearly pure silica natural glass of impact origin found
in western Egypt, using electron microprobe analysis (EMPA), laser ablation inductively coupled mass
spectrometry (LA-ICP-MS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), scanning
electron microscopy (SEM), and electron backscatter diffraction (EBSD). The studied grains are
mostly 250 μm in diameter and consist of ~150 μm wide cores surrounded by ~50 μm wide dendritic
rims. Compositional layering in LDG continues across cristobalite grains and mostly corresponds to
variations in Al content. However, layering is disrupted in cores of cristobalite grains, where Al distribution records oscillatory growth zoning, whereas in rims the high Al occurs along grain boundaries.
Cristobalite cores thus nucleated within layered LDG at conditions that allowed mobility of Al into
crystallographically controlled growth zones, whereas rims grew when Al was less mobile. Analysis
of 37 elements indicates little evidence of preferential partitioning; both LDG and cristobalite are
variably depleted relative to the upper continental crust, and abundance variations correlate to layering in LDG. Orientation analysis of {112} twin systematics in cristobalite by EBSD confirms that
cores were formerly single β-cristobalite crystals. Combined with published experimental data, these
results provide evidence for high-temperature (>1350 °C) magmatic crystallization of oscillatory zoned
β-cristobalite in LDG. Dendritic rims suggest growth across the glass transition by devitrification, driven
by undercooling, with transformation to α-cristobalite at low temperature. This result represents the
highest formation temperature estimate for naturally occurring cristobalite, which is attributed to the
near pure silica composition of LDG and anomalously high temperatures generated during melting
by meteorite impact processes.

¹John P.Bradley,¹Hope A.Ishii,²Karen Bustillo,²James Ciston,³Ryan Ogliore,^4,5Thomas Stephan,⁶Donald E.Brownlee,⁶David J.Joswiak
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.06.036]
¹Hawai‘i Institute of Geophysics and Planetology, University of Hawai ‘i at Mānoa, 1680 East-West Road, Honolulu, HI 96822, USA
²National Center for Electron Microscopy, Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
³Laboratory for Space Sciences, Washington University, St. Louis, MO 63130, USA
⁴Department of Geophysical Sciences, The University of Chicago, Chicago, IL 60637, USA
⁵Chicago Center for Cosmochemistry, Chicago, IL, USA
⁶Department of Astronomy, University of Washington, Seattle WA 98195, USA
Copyright Elsevier

The provenance of GEMS (glass with embedded metal and sulfides) in cometary type interplanetary dust particles is investigated using analytical scanning transmission electron microscopy and secondary ion mass spectrometry. We review the current state of knowledge and closely examine the densities, elemental compositions and distributions, iron oxidation states, and isotopic compositions of a subset of GEMS in chondritic porous interplanetary dust. We find that GEMS are underdense with estimated densities that are 35–65% of compositionally equivalent crystalline aggregates. GEMS low densities result in a lower contribution to the bulk compositions of IDPs than has been assumed based on their volume fraction. We also find that element/Si ratios, assumed to be primary (indigenous), are instead perturbed by contamination and secondary alteration, including pulse heating during atmospheric entry. Fe in pyrrhotite inclusions was oxidized and Mg, S, Ca, and Fe were depleted relative to lithophile Al and Si, resulting in reduction in element/Si ratios. Because they trap outgassing elements, Fe-rich oxide rims that formed on the surface of GEMS are serendipitous “witness plates” to the changes in composition that accompany atmospheric entry. As a result of alteration, GEMS elemental compositions cannot reliably inform about their provenance. Except for highly anomalous oxygen isotope ratios measured in some large GEMS grains that indicate a contribution from circumstellar dust, oxygen isotope compositions are generally poor indicators of provenance. Prior work indicates that most GEMS fall close to the terrestrial oxygen isotope composition, which, however, does not exclude a presolar interstellar origin. Nitrogen isotopic compositions are more diagnostic. Elevated 15N/14N ratios indicate that GEMS accreted in conjunction with formation of organic matter by ion–molecule reactions in a cold (<50 K) presolar environment like the extreme outer nebula or interstellar medium. Considering all observations, we conclude that GEMS are most likely processed interstellar silicates.

^1,2L.Folco,³L.Carone,^1,2M.D’Orazio,⁴C.Cordier,^5,1M.D.Suttle,⁶M.van Ginneken,^1,2M.Masotta
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.06.035]
¹Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, Pisa, Italy
²CISUP, Centro per la Integrazione della Strumentazione dell’Università di Pisa, Lungarno Pacinotti, Pisa, Italy
³Università di Camerino, Scuola di Scienze e Tecnologie, Sezione di Geologia, Via Gentile III da Varano 7, 62032 Camerino, Italy
⁴Université Grenoble Alpes, Université Savoie Mont Blanc, CNRS, IRD, IFSTTAR, ISTerre, 38000 Grenoble, France
⁵School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
⁶Centre for Astrophysics and Planetary Science, School of Physical Sciences, Ingram Building, University of Kent, Canterbury CT2 7NH, UK
Copyright Elsevier

Kamil crater (Egypt) is a natural laboratory for the study of processes and products associated with the impacts of small iron projectiles on the Earth’s crust. In particular, because of the distinctive composition of the impactor (an ungrouped Ni-rich ataxite) and the target (Cretaceous sandstones and minor wackes) it offers a unique opportunity to study impactor–target physical–chemical interactions. Continuing the study of impact melt ejecta, we investigated the mineralogy and geochemistry of 25 Fe-Ni spherules – representative of a suite of 135 – recovered from the soil around the crater. Samples were collected during our 2010 geophysical expedition and investigated by combining scanning electron microscope imaging, electron probe microanalyzer and Raman spectroscopy analyses. Spherules range in size from 100 to 500 µm and show a variety of dendritic textures and mineral compositions dominated by Fe-Ni oxides of the wüstite – bunsenite and magnetite – trevorite series or Fe-Ni metal. All these features indicate quenching of high temperature (1600–1500 °C) oxide or metal liquid droplets under varying oxidizing conditions. A geochemical affinity with the iron impactor recorded by the Fe, Co, Ni ratios in the constituent phases (average Ni/Co element ratio of 25.1 ± 7.6; average Ni/(Ni + Fe) molar ratio of 0.21 ± 0.13), combined with target contamination (i.e., the ubiquitous occurrence of Si and Al from trace to minor amounts), document their origin as impact melt spherules formed through the physical and chemical interaction between metal projectile and silicate target melts and air. We propose a petrogenetic model that envisions formation as liquid droplet residues of immiscible projectile in a mixed silicate melt and their subsequent separation as individual spherules by stripping during hypervelocity ejection. We also argue that this model applies to all impact events produced by small iron projectiles and that such individual Fe-Ni oxide and metal spherules should be common impact products, despite little documentation in the literature. Our detailed mineralogical and geochemical characterization will facilitate their distinction from other, similar spherules of different origin (cosmic spherules, ablation spherules) often encountered in the geologic record.

¹Andrew K. Matzen,²Alan Woodland,³John R. Beckett,¹Bernard J. Wood
American Mineralogist 107, 1442-1452 Link to Article [http://www.minsocam.org/MSA/AmMin/TOC/2022/Abstracts/AM107P1442.pdf]
¹Department of Earth Sciences, University of Oxford, Oxford, OX1 3AN, U.K.
²Goethe-Universität Institut für Geowissenschaften, Frankfurt, D-60438, Germany
³California Institute of Technology, MC 170-25, Pasadena, California 91125, U.S.A.
Copyright: The Mineralogical Society of America

We performed a series of experiments at 1 atm pressure and temperatures of 1300–1500 °C to
determine the effect of oxygen fugacity on the oxidation state of Fe in a synthetic martian basalt. Ferricferrous ratios were determined on the quenched glasses using Mössbauer spectroscopy. Following the conventional doublet assignments in the spectrum, we obtain a Fe3+/ΣFe value of 0.19 at 1450 °C and
an oxygen fugacity corresponding to the QFM buffer. If we apply the Berry et al. (2018) assignments
the calculated Fe3+/ΣFe drops to 0.09, and the slope of log(XFe melt O1.5/XFe melt O) vs. log( fO2) changes from 0.18 to 0.26.
Combining oxidation state data together with results of one additional olivine-bearing experiment
to determine the appropriate value(s) for the olivine (Ol)-liquid (liq) exchange coefficient, KD,Fe2+-Mg
= (FeO/MgO)Ol/(FeO/MgO)liq (by weight), suggests a KD,Fe2+-Mg of 0.388 ± 0.006 (uncertainty is one
median absolute deviation) using the traditional interpretation of Mössbauer spectroscopy and a value
of 0.345 ± 0.005 following the Mössbauer spectra approach of Berry et al. (2018).
We used our value of KD,Fe2+-Mg to test whether any of the olivine-bearing shergottites represent liquids.
For each meteorite, we assumed a liquid composition equal to that of the bulk and then compared that
liquid to the most Mg-rich olivine reported. Applying a KD,Fe2+-Mg of ~0.36 leads to the possibility that
bulk Yamato 980459, NWA 5789, NWA 2990, Tissint, and EETA 79001 (lithology A) represent liquids.