多段階の堆積サイクルを経た堆積物・堆積岩における源岩の初期Sr同位体比の保持―淡路島の細粒河川堆積物を例に―

抄録

We have elucidated retention of primary Sr isotopic ratios in source rocks to sediments and sedimentary rocks during repeated sedimentation recycling. Twenty-three fine stream sediments (<180 μm) in Awajishima Island were used for the study. The ⁸⁷Rb/⁸⁶Sr and ⁸⁷Sr/⁸⁶Sr of stream sediments originated from Cretaceous Ryoke granitic rocks and Cretaceous sedimentary rocks (Izumi Group) were similar to those of the respective source rocks. Incidentally, the ⁸⁷Rb/⁸⁶Sr-⁸⁷Sr/⁸⁶Sr of stream sediments in Awajishima Island suggests that Cretaceous felsic igneous rocks are the dominant source of clastics in Izumi Group. Stream sediments derived from Neogene and Quaternary sediments, which are originated from granitic rocks and Izumi Group, had comparable isotopic ratios to those of the respective primitive source rocks. Accordingly, ⁸⁷Rb/⁸⁶Sr and ⁸⁷Sr/⁸⁶Sr can be retained during the repeated recycling process of sediments and sedimentary rocks. Stream sediment is presumed to consist of clastics supplied from respective lithologies according to their exposed areas. However, ⁸⁷Sr/⁸⁶Sr of stream sediment is not comparable to the values calculated from exposed areas of lithologies and isotopic data of parent rocks. The result suggests that amount of sedimentation denudation and production differs among different lithologies and is not simply proportional to their exposed areas.