研究業績

Researchers have gained insights into the lithology and geochemistry of oceanic crust through the investigation of mid-ocean ridge basalts (MORBs), rocks exposed along the spreading ridges (e.g., East Pacific Rise), ophiolites, and samples retrieval through seafloor drilling. However, obtaining samples of old and deep oceanic crust, particularly formed at fast spreading ridges, remains challenging in ocean basins. Much of what we know about the matured oceanic crust comes from studies of ophiolite sequences, whose origins are often uncertain. This study investigates the whole-rock and mineral chemistry, Sr–Nd–Pb isotopic ratios, and petrography of basaltic, doleritic, and gabbroic xenoliths in a petit-spot volcano on the northwestern Pacific Plate in order to characterize the deeper lithologies of in situ old oceanic crust which have not been confirmed. The major-element compositions of mafic xenoliths were non alkaline and generally align with the global MORBs. Their entirely N-MORB-like trace-element-patterns and the radiogenic isotopic ratios, aligning with the Pacific MORB and drilled uppermost basaltic crusts in the vicinity of the study area, indicate that these mafic xenoliths are fragments of oceanic crust formed at the fast-spreading Izanagi–Pacific Ridge at 130–140 Ma unrelated to petit-spot volcanism. The apparent correlation between grain size and chemical composition may reflect the lithological variation within oceanic crust owing to crystal fractionation and replenishment at the MOR. The presence of “granoblastic” dolerite supports that the contact metamorphism of lower sheeted dikes due to melt lenses is a ubiquitous phenomenon beneath the fast-spreading ridge axis. While quantitative extraction of the original depth information for these mafic xenoliths is challenging, these results substantiate the hypothesis on the Penrose-type lithostratigraphy of the oceanic crust formed at fast-spreading ridge and its relationship with chemical composition, established through studies in limited areas, even in old oceanic crust that had never been examined before.