Scott HASSLER¹, Sandra BILLER², and Bruce M. SIMONSON³
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13223]
¹The Wilderness Society, One Kaiser Plaza, Oakland, California 94612, USA
²SNAP-Ed Program Manager, University of Wyoming Extension, 1000 E University Ave Dept. 3354, Laramie,Wyoming 82071, USA
³Geology Department, Oberlin College, Oberlin, Ohio 44074, USA
Published by arrangement with John Wiley & Sons

The ~2490 Ma DS4 impact layer in the Dales Gorge Member is the only bed in the Brockman Iron Formation (Hamersley Group, Western Australia) known to contain “splash form” impact spherules. At a newly discovered site in Munjina Gorge (MG), the internal stratigraphy of the DS4 impact layer differs from previously known occurrences; it ranges from 36 to 57 cm in total thickness and consists of two distinct subunits. The lower subunit contains abundant cobble‐ to boulder‐scale intraclasts and spherules supported by a finer matrix. We interpret this subunit as the product of poorly cohesive debris flows. The upper subunit is 11–15 cm of low‐density turbidites. The DS4 layer also consists of two newly recognized subunits at Yampire Gorge (YG). The lower subunit is rich in well‐sorted spherules, 0–22 cm thick, and comprises an unstratified bedform with an irregular or swaley upper surface. This is overlain by 2 dm‐scale, fine‐grained, irregularly laminated beds that we interpret as low density turbidites laterally equivalent to the upper subunit at MG. The bedform at YG could be the lateral equivalent of the debrite at MG, genetically related to the overlying turbidites, or a product of impact tsunami‐induced bottom return flow. Other DS4 layer sites that have debrites similar to the one at MG are geographically separated from one another by sites that both lack debrite facies and feature well‐sorted spherules like YG. These characteristics suggest the DS4 layer had a complex depositional history that generated multiple debrites.

S. B. Simon^a, A. N. Krot^b,f, K. Nagashima^b, L. Kööp^c,d, A. M. Davis^c,d,e
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1021/j.gca.2018.11.029]
^aInstitute of Meteoritics, University of New Mexico, Albuquerque, NM 87131
^bHawai‘i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, HI 96822
^cDepartment of the Geophysical Sciences, The University of Chicago, 5734 S. Ellis Ave., Chicago, IL 60637
^dChicago Center for Cosmochemistry, The University of Chicago, 5734 S. Ellis Ave., Chicago, IL 60637
^eEnrico Fermi Institute, The University of Chicago, Chicago, IL 60637
^fGeoscience Institute / Mineralogy, Goethe University Frankfurt, Altenhoeferallee 1, 60438 Frankfurt am Main, Germany
Copyright Elsevier

We have found two refractory inclusions in the CO3.00 carbonaceous chondrite Dominion Range (DOM) 08006 that appear to be primary condensates from the early solar nebula. One, inclusion 56-1, contains the first four phases predicted to form by equilibrium gas-solid condensation: corundum; hibonite; grossite; and perovskite. The other, 31-2, contains nine predicted condensate phases: hibonite; grossite; perovskite; melilite; spinel; FeNi metal; diopside; forsterite; and enstatite. Except for melilite/spinel, the phases occur in the predicted sequence from core to rim of the inclusion, which has an irregular shape inconsistent with a molten stage. This inclusion preserves the most complete record of condensation in the early solar nebula that has yet been found. The physical evidence reported here supports equilibrium condensation calculations that predict the observed sequence as well as the assumptions upon which they are based, such as total pressure (∼10^–3 atm), bulk system composition (solar), and C-O-H proportions. All phases in both inclusions and the associated ferromagnesian silicates are ¹⁶O-rich, with Δ¹⁷O between –25 and –20‰, implying that this is the original composition of the vast majority of primary condensates and that ¹⁶O-poor compositions observed in many isotopically heterogeneous inclusions are largely due to subsequent isotopic exchange. While the nebula was well-mixed with respect to oxygen isotopic composition, clearly resolved anomalies in Ca and Ti isotopic compositions indicate that some isotopic heterogeneity existed early and was preserved during condensation. Inclusion 31-2 did not incorporate live ²⁶Al and and has nucleosynthetic anomalies in the heavy Ca and Ti isotopes (i.e., δ⁴⁸Ca=4.3±1.9‰; δ⁵⁰Ti=8.8±2.0‰). In contrast, inclusion 56-1 has radiogenic ²⁶Mg excesses yielding a (²⁶Al/²⁷Al)₀ ratio of (1.0±0.1) × 10^–5and negative nucleosynthetic isotopic anomalies in Ca (δ⁴⁸Ca=–10.3±4.2‰) and Ti (δ⁵⁰Ti=–4.3±2.9‰). Thus, it represents a deviation from the mutual exclusivity relationship between ²⁶Al incorporation and large nucleosynthetic anomalies. The reservoirs in which these inclusions formed had similar O-isotopic and different Al-, Ca– and Ti-isotopic compositions, suggesting that while the CAI-forming region was well-mixed with respect to oxygen isotopic composition, clearly resolved anomalies in Ca and Ti isotopic compositions indicate that some isotopic heterogeneity existed and was preserved during condensation.

¹Hans-Peter Gail, ^2,3Mario Trieloff
Astronomy & Astrophysics 615, A147 Link to Article [https://doi.org/10.1051/0004-6361/201732456]
¹Zentrum für Astronomie, Institut für Theoretische Astrophysik, Heidelberg University, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
e-mail: gail@uni-heidelberg.de
²Institut für Geowissenschaften, Universität Heidelberg, Im Neuenheimer Feld 236, 69120 Heidelberg, Germany
e-mail: Mario.Trieloff@geow.uni-heidelberg.de
³Klaus-Tschira-Labor für Kosmochemie, Universität Heidelberg, Im Neuenheimer Feld 236, 69120 Heidelberg, Germany

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