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TitleIridium anomaly, impact spherules, and Ni-rich spinels in an Upper Triassic (Norian) deep-sea deposit from Japan
AuthorSato, H; Onoue, T; Nakamura, T; Noguchi, T; Hatsukawa, Y; Osawa, T; Toh, Y; Koizumi, M; Orchard, M JORCID logo; Nedachi, M; Ebihara, M
SourceProceedings of the 21st Canadian Paleontology Conference; by Haggart, J W (ed.); Smith, P L (ed.); Canadian Paleontology Conference Proceedings no. 9, 2011 p. 42-43
Alt SeriesEarth Sciences Sector, Contribution Series 20120091
Meeting21st Canadian Paleontology Conference; Vancouver; CA; August 19-22, 2011
Lat/Long WENS135.0000 137.0000 35.5000 34.5000
Subjectsextraterrestrial geology; meteorite craters; spinel; Upper Triassic; claystones; Norian; nickel; iridium; Triassic
ProgramGEM: Geo-mapping for Energy and Minerals Yukon Sedimentary Basins
AbstractLate Triassic is marked by the formation of several large impact structures on Earth, including the 100-km-diameter Manicouagan crater in Canada. Although the age of the Late Triassic impact structures is well constrained by U-Pb dating, deposits containing their ejecta are known only from southwestern Britain. Here, we report that an Upper Triassic deep-sea deposit in Japan contains spherules, Ni-rich spinels, and iridium anomaly that are characteristic of impact ejecta.
Spherules and Ni-rich spinels were discovered from a claystone layer (~5 cm thick) in an Upper Triassic (middle to late Norian) radiolarian chert succession of the Mino terrane, central Japan. The base of the layer contains 10-15% (by rock volume) green spherules. Spherules range in size from 200 to 300 :m. Synchrotron X-ray diffraction analysis indicates that the spherules are composed mainly of clinoclore. Some spherules contain a high proportion of small, euhedral to subhedral crystals of oxidized Ni-rich spinels. These spinels are 5-20 :m in size and commonly have dendritic, skeletal, or octahedral morphologies. They are distinguished from typical igneous spinels by high contents of NiO (1.1-3.4 wt.%) and Fe3+. These characteristics of the spinel crystals are similar to those of Ni-rich spinels found at the Cretaceous-Paleogene boundary.
Examination of the clay layer using a multiple gamma-ray analysis system after neutron activation reveals that spherules and Ni-rich spinels occur associated with anomalously high contents of iridium, up to 40 ppb, comparable to the levels found at the Cretaceous-Paleogene boundary. The discovery of iridium anomaly, spherules, and Ni-rich spinels in Norian claystone from Japan suggests an important sedimentary record of an extraterrestrial impact in the Late Triassic. Additional research is needed to confirm the source for the Norian claystone layer in Japan, as well as to constrain the origin of the clinoclore spherules and Ni-rich spinels.

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