^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.