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美濃帯南部、上部トリアス系層状チャートから見つかったイリジウム異常,Niに富むスピネル粒子,マイクロスフェルール

Iridium anomaly, Ni-rich spinels, and microspherules in upper Triassic chert of the Mino Terrane, Central Japan

佐藤 峰南*; 尾上 哲治*; 中村 智樹*; 野口 高明*; 初川 雄一; 大澤 崇人 ; 藤 暢輔  ; 小泉 光生 

Sato, Honami*; Onoue, Tetsuji*; Nakamura, Tomoki*; Noguchi, Takaaki*; Hatsukawa, Yuichi; Osawa, Takahito; Toh, Yosuke; Koizumi, Mitsuo

美濃帯南部にある上部トリアス系層状チャート中で発見された高ニッケル含有スピネル試料中にイリジウムの異常濃度を発見した。高濃度のイリジウムの起源として地球外物質衝突が最も有力な候補であり、この地層の年代(2億1550万年)よりカナダにある直径100キロメートルのマニコーガンクレータによる衝突事象において放出されたイジェクターと考えられる。イリジウム分析のみならず、シンクロトロンX線回折分析などを通して得られた知見の発表を行った。

Microspherules were discovered from a claystone layer (5 cm thick) in an Upper Triassic bedded chert succession of the Sakahogi section, Mino Terrane, Japan. An analysis of radiolarian fossils reveals that the claystone layer is correlated with the early to middle Norian stage of the Upper Triassic. The base of the layer contains 10-15% (by rock volume) green microspherules. Microspherules range in size from 200 to 300 microns. Synchrotron X-ray diffraction analysis indicates that the microspherules are composed mainly of clinoclore. Some microspherules contain a high proportion of small, euhedral to subhedral crystals of oxidized Ni-rich spinels. They are distinguished from typical igneous spinels by high contents of NiO and Fe$$^{3+}$$. The geochemical signals of extraterrestrial impact are recorded in the claystone layer. Examination of the clay layer using a multiple $$gamma$$-ray analysis system after neutron activation reveals that microspherules and Ni-rich spinels occur associated with an iridium anomaly, which levels comparable with those at the Cretaceous-Paleogene boudary. A scanning X-ray analytical microscope analysis shows high concentrations of siderophile elements such as nickel and cobalt, which would be expected from an extraterrestrial source.

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