09:15 〜 09:30
[MIS13-02] ペルム紀・三畳紀における地球外3He流入量の変動
キーワード：ペルム紀、三畳紀、層状チャート、ヘリウム同位体
Permian and Triassic bedded chert sequences in Japanese accretionary complexes are recognized as pelagic deep-sea deposits characterized by very low sedimentation rates (less than a few millimeters per thousand years) and the absence of coarse-grained terrigenous clastics. Owing to their low sedimentation rates and long-term continuous deposition, these bedded cherts may record the accretionary history of extraterrestrial material over geologic time. Helium is suitable for detecting extraterrestrial material in deep-sea sediments, because submillimeter interplanetary dust particles (IDPs) are enriched in 3He. 3He concentrations in ancient deep-sea sediments have been used to constrain the flux of IDPs onto the Earth for at least the last 100 Myr. However, the use of 3He in detecting IDP flux is often compromised by the diffusional loss of 3He in sedimentary rocks, with the exception of some Ordovician samples that record a period of unusually high extraterrestrial 3He (3HeET) flux. In this study, we report the preservation of 3HeET in deep-sea bedded chert from Permian and Middle Triassic sections in the Mino Belt, central Japan, which was deposited in the Panthalassa Ocean.
Analyzed samples yielded bulk 3He concentrations between 0.2 and 0.4 pcc/g in the late Guadalupian to Lopingian (middle-late Permian). 3He concentrations are relatively constant in the early to middle Anisian (Middle Triassic), but increase from 0.2 to 1.2 in the late Anisian to Ladinian. 3He/4He ratios follow a similar trend to 3He concentrations in the Middle Triassic, with the maximum 3He/4He ratio of 0.43 Ra being measured in the middle Ladinian bed. The 3He/4He ratios in the analyzed samples are significantly higher than the crustal ratios (0.02 Ra), indicating that the 3He in the investigated samples is mainly of extraterrestrial origin. The estimated variation of 3HeET concentrations shows an increasing trend in the Middle Triassic. The increase in 3HeET can be explained by a decrease in sedimentation rate as well as an increase in cosmic dust influx to the earth. Future studies are needed to compare the sedimentation rate and the 3HeET concentrations through the Middle Triassic bedded chert in the Mino Belt.
Analyzed samples yielded bulk 3He concentrations between 0.2 and 0.4 pcc/g in the late Guadalupian to Lopingian (middle-late Permian). 3He concentrations are relatively constant in the early to middle Anisian (Middle Triassic), but increase from 0.2 to 1.2 in the late Anisian to Ladinian. 3He/4He ratios follow a similar trend to 3He concentrations in the Middle Triassic, with the maximum 3He/4He ratio of 0.43 Ra being measured in the middle Ladinian bed. The 3He/4He ratios in the analyzed samples are significantly higher than the crustal ratios (0.02 Ra), indicating that the 3He in the investigated samples is mainly of extraterrestrial origin. The estimated variation of 3HeET concentrations shows an increasing trend in the Middle Triassic. The increase in 3HeET can be explained by a decrease in sedimentation rate as well as an increase in cosmic dust influx to the earth. Future studies are needed to compare the sedimentation rate and the 3HeET concentrations through the Middle Triassic bedded chert in the Mino Belt.