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Vol. 126 (2017) No. 2 特集号:沈み込む海洋プレート科学の最前線-アウターライズ海洋掘削に向けて- p. 163-179




 The source mantle of the ocean crust on the Pacific Plate is examined using Pb–Nd–Hf isotopes and compared to a global isotope database of ocean basalts. The entire eastern half of the Pacific Plate, formed from an isotopically distinct Pacific mantle along the East Pacific Rise and the Juan de Fuca Ridge, largely remains on the seafloor. Conversely, the western half of the Pacific Plate becomes younger westward and is thought to have formed from the Izanagi–Pacific Ridge (IPR). The ridge subducted along the Kurile–Japan–Nankai–Ryukyu (KJNR) Trench at 70-65 Ma and currently forms the leading edge of the Pacific Plate stagnated in the mantle transition zone beneath China. The subducted IP formed from both Pacific and Indian mantles. Isotopic compositions of the basalts from borehole cores of 165-130 Ma in the western Pacific show that these are of Pacific mantle origin. However, the scraped-off ocean floor basalts (80-70 Ma) in the accretionary prism along the KJNR Trench have Indian mantle signatures. This indicates: (1) the younger western Pacific Plate of IPR origin formed from the Indian mantle, (2) the Indian–Pacific mantle boundary has been stationary in the western Pacific at least since the Cretaceous, and (3) the IPR moved over the boundary. The Indian mantle is thought to have formed from a depleted MORB source mantle (DMM) due to an ancient melt depletion event (2-3 Ga) and subsequent isotopic growth and mixing with a sub-continental lithospheric mantle. In contrast, the Pacific mantle originated from a primitive mantle at 3-1.5 Ga followed by isotopic growth alone. These different formation processes may relate to the formation of the supercontinent and superocean where the Indian mantle was formed in a sub-continental environment whereas the Pacific mantle formed in an oceanic ridge environment.

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