¹Michael E. Zolenksy et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13561]
¹ARES, NASA Johnson Space Center, Houston, Texas, 77058 USA
Published by arrangement with John Wiley and Sons

Cosmic ray exposure (CRE) ages of CM chondrites have been found to have multiple peaks (as many as four), in stark contrast to other groups of chondrites (Nishiizumi and Caffee 2012; Herzog and Caffee 2014). In this study, we sought correlations between the CRE ages and petrography of CM chondrites, and we conclude that the degree of aqueous alteration does appear to vary with the CRE ages—the CMs displaying the most aqueous alteration all have relatively short exposure ages. However, some CMs with low degrees of alteration also have short exposure ages—thus, this apparent correlation is not exclusive. We also found a definite inverse relation between the number of distinctive lithologies in a CM and its exposure age, which could indicate different responses of homogeneous and heterogeneous meteoroids to the space environment between their onset of exposure (exhumation and ejection from the parent body) and arrival at Earth. Breccias have more internal surfaces of lithologic discontinuity, possibly resulting in weaker meteoroids that disintegrate more readily than their more homogeneous counterparts. Our results suggest that CM chondrite regoliths consist of numerous genomict lithologies in a breccia with millimeter‐ to decimeter‐scale clasts, with varying degree of heating/metamorphism.

¹Jan L. Hellmann,^1,2Thomas S. Kruijer,¹Knut Metzler,¹Markus Patzek,³Andreas Pack,⁴Jasper Berndt,¹Thorsten Kleine
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13571]
¹Institut für Planetologie, University of Münster, Wilhelm‐Klemm‐Strasse 10, 48149 Münster, Germany
²Nuclear & Chemical Sciences Division, Lawrence Livermore National Laboratory, 7000 East Avenue L‐231, Livermore, California, 94550 USA
³Geowissenschaftliches Zentrum, University of Göttingen, Goldschmidtstr. 1‐3, 37077 Göttingen, Germany
⁴Institut für Mineralogie, University of Münster, Corrensstrasse 24, 48149 Münster, Germany
Published by arrangement with John Wiley & Sons

A large, igneous‐textured, and 2 cm‐sized spherical object from the L5/6 chondrite NWA 8192 was investigated for its chemical composition, petrography, O isotopic composition, and Hf‐W chronology. The petrography and chemical data indicate that this object closely resembles commonly found chondrules in ordinary chondrites and is therefore classified as a “macrochondrule.* As a result of metal loss during its formation, the macrochondrule exhibits elevated Hf/W, which makes it possible to date this object using the short‐lived 182Hf‐182W system. The Hf‐W data provide a two‐stage model age for metal–silicate fractionation of 1.4 ± 0.6 Ma after Ca‐Al‐rich inclusion (CAI) formation, indicating that the macrochondrule formed coevally to normal‐sized chondrules from ordinary chondrites. By contrast, Hf‐W data for metal from the host chondrite yield a younger model age of ~11 Ma after CAIs. This younger age agrees with Hf‐W ages of other type 5–6 ordinary chondrites, and corresponds to the time of cooling below the Hf‐W closure temperature during thermal metamorphism on the parent body. The Hf‐W model age difference between the macrochondrule and the host metal demonstrates that the Hf‐W systematics of the bulk macrochondrule were not disturbed during thermal metamorphism, and therefore, that the formation age of such objects can still be determined even in strongly metamorphosed samples. Collectively, this study illustrates that chondrule formation was not limited to mm‐size objects, implying that the rarity of macrochondrules reflects either that this process was very inefficient, that subsequent nebular size‐sorting decimated large chondrules, or that large precursors were rare.

¹L.Chimiak,²J.E.Elsila,¹B.Dallas,²J.P.Dworkin,^2,3J.C.Aponte,¹A.L.Sessions,¹J.M.Eiler
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.09.026]
¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, 91125, USA
²Solar System Exploration Division, Code 691, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
¹Department of Chemistry, Catholic University of America, Washington, D.C., 20064
Copyright Elsevier

Meteorites contain prebiotic, bio-relevant organic compounds including amino acids. Their syntheses could result from diverse sources and mechanisms and provide a window on the conditions and materials present in the early solar system. Here we constrain alanine’s synthetic history in the Murchison meteorite using site-specific 13C/12C measurements, reported relative to the VPDB standard. The δ13CVPDB values of –29 ± 10 ‰, 142 ± 20 ‰, and –36 ± 20 ‰ for the carboxyl, amine-bound, and methyl carbons, respectively, are consistent with Strecker synthesis of interstellar-medium-derived aldehydes, ammonia, and low-δ13C nebular or interstellar-medium-derived CN. We report experimentally measured isotope effects associated with Strecker synthesis, and use them to constrain the δ13C values of the alanine precursors, which we then use to construct a model that predicts the molecular-average δ13C values of 19 other organic compounds of prebiotic significance found in Murchison if they were made by our proposed synthetic network. Most of these predictions agree with previous measurements, suggesting that interstellar-medium-derived aldehydes and nebular and/or pre-solar CN could have served as substrates for synthesis of a wide range of prebiotic compounds in the early solar system.