[B-PT23_30AM2] 地球史解読：冥王代から現代まで

[BPT23-08] 日本の三畳紀後期イジェクタ堆積物の地球化学的検討：とくに衝突隕石の起源について

*佐藤 峰南¹、山下 勝行²、米田 成一³、白井 直樹⁴、海老原 充⁴、野崎 達生⁵、海老原 充⁵、尾上 哲治⁶ (1.九州大学、2.岡山大学、3.国立科学博物館、4.首都大学東京、5.海洋研究開発機構、6.熊本大学)

キーワード：隕石衝突, 白金族元素, オスミウム同位体, クロム同位体

Our previous studies have revealed that the Sakahogi section in central Japan contains an impact ejecta layer in the Late Triassic, which was derived from an extraterrestrial impact event. This ejecta layer is characterized by platinum group element (PGE) positive anomalies and Os isotope negative excursion together with enrichments in Ni and Cr, and abundant occurrences of Ni-rich magnetite grains and microspherules. PGE anomalies in the Late Triassic sediments were also discovered from deep-sea claystone layers at three bedded chert sections in southwest Japan as follows: (i) Unuma section in the Inuyama area, Mino Belt, (ii) Hisuikyo section in the Kamiaso area, Mino Belt, and (iii) Enoura section in the Tsukumi area, Chichibu Belt. Combined PGE and various isotope data from these ejecta layers are insightful so as to identify the meteoritic material which has caused the Late Triassic impact event. Here we report the PGE element ratios, and Cr and Os isotope compositions of these ejecta layers to understand the projectile component.The Ru/Ir and Pt/Ir ratios of all the claystone samples from the study sites are plotted along the mixing line between chondrites and upper continental crust. Although a chondrite cannot be distinguished from iron meteorites by using PGE/Ir ratios, the claystone layers show Cr/Ir ratios between 10⁴ to 10⁵, indicating that the claystone layers are clearly contaminated by chondritic material. The Os isotope compositions (¹⁸⁷Os/¹⁸⁸Os ratios) in the claystone have a narrow range from 0.126 to 0.128 and these values are well similar to those of chondrites. The Cr isotope data are useful to identify the extraterrestrial components in the ejecta deposits because meteorites of different classes have a distinct ⁵⁴Cr isotope anomaly. The presence of positive ε⁵⁴Cr anomaly in all claystone samples strongly suggests that a carbonaceous chondrite-like material was involved in the studied ejecta layers. Consequently, these geochemical lines of evidence indicate that the Upper Triassic ejecta layers in the Japanese accretionary complexes have been most likely derived from a carbonaceous chondrite.