Search

Fukutoku‐Oka‐no‐Ba is a submarine volcano located at 24°17.1′ N/141°28.9′ E in the Izu–Bonin arc, and is one of the most active volcanoes in Japan. This volcano produced an explosive eruption in August 2021 that generated a large amount of volcaniclastic material, some of which drifted westward to Japan and the coastal area of East Asia as a pumice raft. The pumice clasts that drifted for >1000 km were mostly homogeneous and identical to those produced by past historical eruptions. The clasts have trachytic compositions (SiO2 = 61–63 mass% and Na2O + K2O = 8.6–10.0 mass%) and contain augite, plagioclase, olivine (Mg# ~65), and magnetite, along with a small number of mafic enclaves containing diopside and high‐Mg olivine (Mg# ~ 92). We undertook a research cruise to investigate the proximal volcaniclastic materials by dredging. The proximal materials include pumice, weakly vesiculated lapilli, and volcanic blocks, which have trachytic composition (SiO2 contents up to 64.5 mass%). The main minerals in the proximal material are similar to those in the drift pumice, although remnants of mafic magma do not occur in the SiO2‐rich samples. The petrographic and geochemical characteristics of the proximal and drift ejecta from Fukutoku‐Oka‐no‐Ba suggest the magma reservoir was stratified into two parts. The major part experienced magma mixing with a limited volume of mafic magma, whereas the other part was more differentiated. The differentiated high‐SiO2 magma accumulated in the upper part of the magma reservoir and avoided the mixing with and feed of volatile from the mafic magma, then were pushed out from the volcanic vent without extensive bubbling to sunk in the proximal area. [ABSTRACT FROM AUTHOR]

We present the first report of geochemical data for submarine basalts collected by a manned submersible from Rurutu, Tubuai, and Raivavae in the Austral Islands in the South Pacific, where subaerial basalts exhibit HIMU isotopic signatures with highly radiogenic Pb isotopic compositions. With the exception of one sample from Tubuai, the [sup.40]Ar/[sup.39]Ar ages of the submarine basalts show no significant age gaps between the submarine and sub-aerial basalts, and the major element compositions are indistinguishable at each island. However, the variations in Pb, Sr, Nd, and Hf isotopic compositions in the submarine basalts are much larger than those previously reported in subaerial basalts. The submarine basalts with less-radiogenic Pb and radiogenic Nd and Hf isotopic compositions show systematically lower concentrations in highly incompatible elements than the typical HIMU basalts. These geochemical variations are best explained by a two-component mixing process in which the depleted asthenospheric mantle was entrained by the mantle plume from the HIMU reservoir during its upwelling, and the melts from the HIMU reservoir and depleted asthenospheric mantle were then mixed in various proportions. The present and compiled data demonstrate that the HIMU reservoir has a uniquely low [sup.176]Hf/[sup.177]Hf decoupled from [sup.143]Nd/[sup.144]Nd, suggesting that it was derived from an ancient subducted slab. Moreover, the Nd/Hf ratios of the HIMU basalts and curvilinear Nd-Hf isotopic mixing trend require higher Nd/Hf ratios for the melt from the HIMU reservoir than that from the depleted mantle component. Such elevated Nd/Hf ratios could reflect source enrichment by a subducted slab during reservoir formation. Keywords HIMU * Ocean islands * Submarine basalts * Isotopes * Recycled crust * Mantle reservoir, Introduction The South Pacific region is unique from a geodynamical perspective because it has been recognized as the site of a superplume, which is a large-scale mantle upwelling from the [...]

Author(s): Yaakov Weiss (corresponding author) [1]; Cornelia Class (corresponding author) [1]; Steven L. Goldstein (corresponding author) [1, 2]; Takeshi Hanyu (corresponding author) [3] Mantle melting, which leads to the formation [...]

Geological research using helium isotope analysis shows that high 238 uranium/204 lead samples from between ten and eighteen myr ago have a uniform and low ratio of 6.8 + or - 0.9 R. In comparison, enriched-mantle lavas from different regions have higher signatures of an variable nature. Evidence suggests that these characteristics are typical of the mantle source region and do not involve metasomatism.