82. Umino, S., Kitamura, K., Kanayama, K., Tamura, A., Sakamoto, N., Ishizuka, O. and Arai, S., 2015. Thermal and chemical evolution of the subarc mantle revealed by spinel-hosted melt inclusions in boninite from the Ogasawara (Bonin) Archipelago, Japan. Geology, 43, 151-154, doi: 10.1130/G36191.1 .Primitive melt inclusions in chrome spinel from the Ogasawara Archipelago comprise two discrete groups of high-SiO2, MgO (high-Si) and low-SiO2, MgO (low-Si) boninitic suites with ultra-depleted dish- and V-shaped, and less depleted flat rare earth element (REE) patterns. The most magnesian melt inclusions of each geochemical type were used to estimate the T-P conditions for primary boninites, that range from 1345°C-0.56 GPa to 1421°C-0.85 GPa for the 48-46 Ma high-Si and low-Si boninites, and 1381°C-0.85 GPa for the 45 Ma low-Si boninite. The onset of the Pacific Slab subduction at 52 Ma forced upwelling of depleted mid-ocean ridge basalt mantle (DMM) to yield proto-arc basalt (PAB). With the rise of DMM, refractory harzburgite ascended without melting. At 48-46 Ma, introduction of slab fluids induced melting of the PAB residue and high-T harzburgite, resulted in the low-Si and high-Si boninites, respectively. Meanwhile, convection within the mantle wedge brought the less depleted residue of PAB and DMM into the region fluxed by slab fluids, which melted to yield the less depleted low-Si boninite at 45 Ma, and fertile arc basalts, respectively.
Keyword: Ogasawara Archipelago, boninite, Izu-Ogasawara (Bonin)-Mariana Arc, subduction initiation, mantle wedge
キーワード: 小笠原群島，無人岩，伊豆ー小笠原ーマリアナ弧，沈み込み帯形成，マントルウェッジ