Paul L. G€OLLNER¹, Torben W€USTEMANN¹, Lisa BENDSCHNEIDER¹, Sebastian REIMERS¹, Martin D. CLARK^1,4, Lisa GIBSON², Peter C. LIGHTFOOT³, and Ulrich RILLER¹
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13268]
¹Institut für Geologie, Universität Hamburg, Bundesstraße 55, 20146 Hamburg, Germany
²Vale, North American Exploration, 337 Power St., Copper Cliff, Ontario, Canada
³Department of Earth Sciences, University of Western Ontario, 1151 Richmond Street N., London, Ontario N6A 5B7, Canada
⁴Present address: Department of Geology, University of the Free State, 205 Nelson Mandela Drive, Bloemfontein, South Africa
Published by arrangement with John Wiley & Sons

The 1.85 Ga Sudbury Igneous Complex (SIC) and its thermal aureole are unique on Earth with regard to unraveling the effects of a large impact melt sheet on adjacent target rocks. Notably, the formation of Footwall Breccia, lining the basal SIC, remains controversial and has been attributed to impact, cratering, and postcratering processes. Based on detailed field mapping and microstructural analysis of thermal aureole rocks, we identified three distinct zones characterized by static recrystallization, incipient melting, and crystallization textures. The temperature gradient in the thermal aureole increases toward the SIC and culminates in a zone of partial melting, which correlates spatially with the Footwall Breccia. We therefore conclude that assimilation of target rock into initially superheated impact melt and simultaneous deformation after cratering strongly contributed to breccia formation. Estimated melt fractions of the Footwall Breccia amount to 80 vol% and attest to an extreme loss in mechanical strength and, thus, high mobility of the Breccia during assimilation. Transport of highly mobile Footwall Breccia material into the overlying Sublayer Norite of the SIC and vice versa can be attributed to Raleigh–Taylor instability of both units, long‐term crater modification caused by viscous relaxation of crust underlying the Sudbury impact structure, or both.

Søren Grube PEDERSEN, Martin SCHILLER*, James N. CONNELLY, and Martin BIZZARRO
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13269]
Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7,DK-1350, Denmark
Published by arrangement with John Wiley & Sons

Planetary bodies a few hundred kilometers in radii are the precursors to larger planets but it is unclear whether these bodies themselves formed very rapidly or accreted slowly over several millions of years. Ordinary H chondrite meteorites provide an opportunity to investigate the accretion time scale of a small planetary body given that variable degrees of thermal metamorphism present in H chondrites provide a proxy for their stratigraphic depth and, therefore, relative accretion times. We exploit this feature to search for nucleosynthetic isotope variability of ⁵⁴Cr, which is a sensitive tracer of spatial and temporal variations in the protoplanetary disk’s solids, between 17 H chondrites covering all petrologic types to obtain clues about the parent body accretionary rate. We find no systematic variability in the mass‐biased corrected abundances of ⁵³Cr or ⁵⁴Cr outside of the analytical uncertainties, suggesting very rapid accretion of the H chondrite parent body consistent with turbulent accretion. By utilizing the μ⁵⁴Cr–planetary mass relationship observed between inner solar system planetary bodies, we calculate that the H chondrite accretion occurred at 1.1 ± 0.4 or 1.8 ± 0.2 Myr after the formation of calcium‐aluminum‐rich inclusions (CAIs), assuming either the initial ²⁶Al/²⁷Al abundance of inner solar system solids determined from angrite meteorites or CAIs from CV chondrites, respectively. Notably, these ages are in agreement with age estimates based on the parent bodies’ thermal evolution when correcting these calculations to the same initial ²⁶Al/²⁷Al abundance, reinforcing the idea of a secular evolution in the isotopic composition of inner disk solids.

Jan Render, Samuel Ebert, Christoph Burkhardt, Thorsten Kleine, Gregory A. Brennecka
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.03.011]
Institut für Planetologie, University of Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
Copyright Elsevier

Insights into the earliest stages of our Solar System can be derived from its oldest dated solids, calcium-aluminum-rich inclusions (CAIs). In particular, investigating isotopic anomalies of nucleosynthetic origin in CAIs offers potential clues to the genetic heritage of refractory inclusions and the reservoir(s) involved in their formation. To this point, however, nucleosynthetic anomalies in refractory inclusions have almost exclusively been recognized in (1) relatively large CAIs from CV3 chondrites, employing chemical purification and high-precision mass spectrometry, or (2) from sub-mm-sized hibonite-rich objects (e.g., PLACs, SHIBs) from the Murchison CM2 chondrite using much less precise in-situ techniques. Whereas the latter have been shown to be highly anomalous in their isotopic compositions, their genetic connection to ‘regular’ CAIs from carbonaceous chondrites remains poorly understood.

Here, we aim to address this issue by taking advantage of a new technique that allows for high-precision analysis of sub-mm-sized inclusions. Using this method, we report Ti isotope anomalies in a suite of twelve CAIs from five different CO carbonaceous chondrites, as well as ten refractory inclusions from the CM2 chondrite Jbilet Winselwan using MC-ICPMS. We find that these CO and CM CAIs exhibit Ti isotopic compositions very similar to those of previously investigated CV3 (and of two CK3) CAIs, suggesting a fundamental genetic relationship of CAIs found within these chondrite groups. As such, our data indicates that CAIs from various groups of carbonaceous chondrites formed from similar matter and in a single region of the solar nebula (i.e., derived from a single common CAI-formation region). Collectively, these data show evidence of large-scale transport of CAIs over a significant range of heliocentric distances, covering at least the accretion areas of the CV, CK, CO, and CM chondrites. In addition, we report two inclusions consisting of hibonite-rich crystal aggregates from Jbilet Winselwan that exhibit highly irregular nucleosynthetic Ti signatures, implying a distinct origin from the aforementioned CAIs. These inclusions may represent an earlier generation of refractory material, perhaps more akin to the previously mentioned PLACs and/or SHIBs.

V. Assis Fernandes^a,b,c,d, J. Hopp^c, W.H. Schwarz^c, J. P. Fritz^f,g, M. Trieloff^c and H. Povenmire^fGeochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.03.009]
^aMuseum für Naturkunde, Leibniz-Institute for Evolution and Biodiversity Research, Invalidenstraße 43, 10115 Berlin, Germany
^bSchool of Earth and Environmental Sciences, University of Manchester, Oxford Road, M13 9PL Manchester, United Kingdom
^cKlaus-Tschira-Labor für Kosmochemie, Institut für Geowissenschaften, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 234-236, 69120 Heidelberg, Germany
^dInstituto Dom Luiz, University of Lisbon, 1749-016 Lisbon, Portugal
^eSaalbau Weltraum Projekt, Liebigstrasse 6, 64646 Heppenheim, Germany
^fZentrum für Rieskrater und Impaktforschung (ZERIN), Nördlingen, Vordere Gerbergasse 3, 86720 Nördlingen, Germany
^gFlorida Institute of Technology (Retired), Melbourne, FL 32901, U.S.A
Copyright Elsevier

This study presents ⁴⁰Ar-³⁹Ar step heating ages of four North American tektites (three bediasites and one georgiaite) and two groundmass samples extracted at different depths from clast-rich impact melt rocks (CB-W61 and CB-W84) recovered by the USGS-ICDP Eyreville B drill-core about 9 km from the centre of the Chesapeake Bay impact structure. Radiometric age determination on both North American tektites and impact melt rocks from within Chesapeake Bay craters offers the first possibility to confirm the origin of these tektites. For this aim, argon data from 13 samples/aliquots of tektite rims, cores and bulk, and 4 samples/aliquots from two impact melt rocks were obtained over 15 to 26 step heating extractions. Age spectra of all tektite samples show plateaux comprising 62-98% of the ³⁹Ar release over consecutive intermediate and high temperature heating steps. Few low temperature extractions indicate excess ⁴⁰Ar. Inverse isochron ⁴⁰Ar/³⁶Ar intercepts of tektite samples are indistinguishable from air (295.5). However, impact melt rock spectra presented complex Ar-release affecting primarily the low temperature heating-steps. Inverse isochrones indicated excess argon from which the ⁴⁰Ar/³⁶Ar intercept was used to correct the age calculation. CB-W61 and CB-W61-2 ⁴⁰Ar/³⁶Ar intercepts are 354.5±2.5 and 327.2±6.3, respectively, and those for CB-W84 and CB-W84-2 are 332.0±7.3 and 329.6 ± 5.6, respectively. The inverse isochron weighted mean age (according to currently suggested K-decay constant revisions by Schwarz et al., 2011, Renne et al., 2011) for all four tektites is 34.86±0.25 Ma (MSWD=0.96, P=0.41; n=4) and for the two impact melt rocks is 37.16±3.65 Ma (MSWD=0.83, P=0.36). The combined tektite and impact melt rocks isochron mean age of 34.86±0.23 (0.32) Ma (MSWD=0.87, P=0.43) is slightly – though not significantly – higher than the plateau mean age of 34.55 ±0.27 (0.36) Ma (MSWD=0.66, P=0.62). Placing the age in the Global Stratotype Section and Point (GSSP) marine section exposed at Massignano, Italy, it falls below the Eocene/Oligocene (E/O) boundary overlapping with the 10.28 m Ir-anomaly. These results agree within errors with previously reported ages of 35.20±0.54 Ma mainly those derived from K-Ar and Ar-Ar total fusion analysis. An age of 34.86±0.32 Ma sets the Chesapeake Bay impact event close to the youngest of the three Ir anomalies at ∼35.0 Ma in the case the impactor was Ir-rich (e.g, a chondrite, primitive achondrite, stony-iron or iron meteorite). The concordance with the E/O boundary at ∼ 33.9 Ma seems only marginally possible, and only if the Ir contribution from the ejecta were, potentially, due either to its small amount becoming diluted in the geologic record or the impactor being Ir poor, e.g., of differentiated achondritic composition. This study also brings to front the need to re-establish the stratigraphic and palaeo-magnetic correlations across the globe for the Ir-anomalies and the magneto-stratigraphy during the mid- to late-Eocene and early-Oligocene, and the need to re-evaluate the markers for the Eocene-Oligocene boundary.