09:15 〜 09:30
[SCG58-02] 三波川変成帯樋口蛇紋岩体のバデレアイトが示唆する沈み込み帯低温熱水変質に伴う強配位子場元素の溶解と沈殿
キーワード：蛇紋岩、三波川変成帯、バデレアイト、ジルコン、沈み込み帯
High field strength elements (HFSEs) are usually insoluble at low temperatures, so minerals such as zircon and baddeleyite, consisting mainly of HFSE, hardly react with low-T hydrothermal fluid. However, it has been remarked that HFSEs can react with highly alkaline fluids associated with serpentinization at low T, based on observations of hydrothermal zircons from rodingite and jadeitite. Here we report baddeleyite found from the Higuchi serpentinite body (HSB) in the Sanbagawa belt of the Kanto Mountains and their U-Pb dating results by using LA-ICPMS. The HSB is composed of antigolite serpentinite with numbers of carbonate-talc veins, 15 × 8 m in size, and surrounded by pelitic schist. The protolith peridotite of the HSB is regarded to be originated in the mantle wedge. The carbonate veins are considered to have formed by oxidation of organic matter in the surrounding pelitic rocks. The baddeleyite is an aggregate of brown fibrous crystals, each aggregate reaching up to 2 mm or more in size. The periphery of the baddeleyite aggregate is surrounded by white porous zircons approximately ~tens of micrometres thick. The LA-ICPMS dating results indicate that both the baddeleyites and the porous zircons have ca. 95 Ma U-Pb ages. Based on detrital zircon U-Pb ages, the depositional age of the surrounding pelitic schist is younger than 95 Ma. Therefore, the crystallization of baddeleyite and silicification to zircon are not related with fluid supply from the surrounding pelitic schist and the carbonate vein intrusion. These results suggest that conditions exist for the dissolution, concentration, and precipitation of HFSEs during low-T hydrothermal activity associated with serpentinisation of the wedge mantle. The presence of such conditions may require a revision of the mechanisms of elemental cycling in plate subduction zones.