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We report the whole-rock major and trace element and Sr – Nd – Pb isotopic compositions of volcanic rocks from the Nansatsu area in South Kyushu. We analysed samples from the Oldest stage (middle Miocene), Older stage (late Miocene), and Middle stage (early Pliocene) Nansatsu volcanic rocks, which are andesite to dacite in composition, with an island-arc trace element signature. The compositional variabilities within the Older and the Middle stage rocks are relatively small compared to those in the Oldest stage and Quaternary volcanic rocks. Isotopic characteristics suggested that the Oldest stage volcanic rocks had magma sources similar to regional ilmenite-series granitoids of later ages (Outer Zone granitoids), which incorporated a substantial amount of graphite-bearing accretionary sediments (Shimanto Supergroup). The sources for the Older and Middle stage magmas were similar to those of primary Quaternary basalts in South Kyushu, indicating that Pliocene and Quaternary volcanic rocks in South Kyushu share a common mantle source, despite the change in the nature of volcanic activity from andesite-dominated to more variable volcanism of basalt to rhyolite. [ABSTRACT FROM AUTHOR]

Petit-spot volcanoes, occurring due to plate flexure, have been reported globally. As the petit-spot melts ascend from the asthenosphere, they provide crucial information of the lithosphere–asthenosphere boundary. Herein, we examined the lava outcrops of six monogenetic volcanoes formed by petit-spot volcanism in the western Pacific. We then analyzed the 40Ar/39Ar ages, major and trace element compositions, and Sr , Nd , and Pb isotopic ratios of the petit-spot basalts. The 40Ar/39Ar ages of two monogenetic volcanoes were ca. 2.6 Ma (million years ago) and ca. 0 Ma. The isotopic compositions of the western Pacific petit-spot basalts suggest geochemically similar melting sources. They were likely derived from a mixture of high- μ (HIMU) mantle-like and enriched mantle (EM)-1-like components related to carbonatitic/carbonated materials and recycled crustal components. The characteristic trace element composition (i.e., Zr , Hf , and Ti depletions) of the western Pacific petit-spot magmas could be explained by the partial melting of ∼ 5 % crust bearing garnet lherzolite, with 10 % carbonatite flux to a given mass of the source, as implied by a mass-balance-based melting model. This result confirms the involvement of carbonatite melt and recycled crust in the source of petit-spot melts. It provides insights into the genesis of tectonic-induced volcanoes, including the Hawaiian North Arch and Samoan petit-spot-like rejuvenated volcanoes that have a similar trace element composition to petit-spot basalts. [ABSTRACT FROM AUTHOR]

The lithosphere–asthenosphere boundary (LAB), which can be seismically detected, stabilizes plate tectonics. Several conflicting hypotheses have been proposed as the causes of LAB discontinuity, such as the contribution of hydrated minerals, mineral anisotropy, and partial melts. The petit-spot melts ascending from the asthenosphere, owing to subducting plate flexures, support the partial melting at the LAB. Here, we observed the lava outcrops of six monogenetic volcanoes formed by petit-spot volcanism in the western Pacific. Thereafter, we determined the 40Ar/39Ar ages, major and trace element compositions, and Sr, Nd, and Pb isotopic ratios of the petit-spot basalts. The 40Ar/39Ar ages of two monogenetic volcanoes were ca. 2.6 Ma (million years ago) and ca. 0 Ma, respectively. The isotopic compositions of the western Pacific petit-spot basalts suggest their geochemically similar melting sources. They were likely derived from a mixture of high-μ (HIMU) mantle-like and enriched mantle (EM) -1-like components related to carbonatitic/carbonated materials and recycled crustal components. A mass balance-based melting model implied that the characteristic trace element composition (i.e., Zr, Hf, and Ti depletions) of the western Pacific petit-spot magmas could be explained by the partial melting of garnet lherzolite with a small degree of carbonatite melt flux with crustal components. This result confirms the involvement of carbonatite melt and recycled crust in the source of petit-spot melts and provides an implication for the genesis of tectonic-induced volcanism with similar geochemical signatures to those of petit-spots. [ABSTRACT FROM AUTHOR]

In this study, the Sr isotope ratios (IRs; 87Sr/86Sr) of ferromanganese (Fe–Mn) crusts are analyzed through laser ablation inductively coupled plasma multiple-collector mass spectrometry. A sample collected from off Minamitorishima Island showed uniform Sr IRs (0.70906–0.70927) similar to that of present-day seawater with more than 36 mm thickness. Meanwhile, a detritus-rich sample collected from off northeast (NE) Japan showed a wide variation in Sr IRs (0.707761–0.709963). The Sr IR variation in the Fe–Mn crust from off NE Japan suggests detrital influx contributions from both the NE Japan arc (<0.708) and aeolian dust from China (>0.718). Detrital flux from the NE Japan arc increases from the bottom to middle layers, possibly due to the uplift of the Ou backbone range that occurred after ~2 Ma. The increased influx of the aeolian dust in the outer layer is attributable to global cooling in the Quaternary that increased the loess dust transportation from China to the western North Pacific Ocean. Meanwhile, the influence of the detrital influx on the sample from off Minamitorishima Island appeared to be negligible. The Sr IR analysis with high spatial resolution proposed in this study possibly improves the burial history of Fe–Mn nodules. [ABSTRACT FROM AUTHOR]