We report the occurrence of glimmerite and melteigite xenoliths from a hybrid Early Cretaceous camptonite - tinguaite dyke from the Nongchram fault zone, East Garo Hills, Shillong Plateau, North East India. Glimmerite xenolith contains predominantly biotite with subordinate to minor amounts of pyroxene, ferro- dolomite, rutile, and ilmenite. The melteigite xenolith is dominated by pyroxene and contain pseudo-nepheline (altered to analcime) and rutile. The host campto-tinguaite dyke features a distinctive porphyritic-panidiomorphic and a tinguaitic texture with clinopyroxenes as macrocrysts, microcrysts and clots and amphibole as phenocrysts with accessory minerals such as apatite, magnetite, rutile, and ilmenite. In-situ trace element geochemistry of pyroxenes from the xenoliths and the host rock reveals significant enrichment in LILEs such as Ba and Sr, as well as LREEs like La and Ce, suggesting an enriched mantle source. The geothermobarometric data for pyroxenes from the (i) glimmerite and melteigite xenoliths and macrocrysts and (ii) microcrysts and clots from the campto-tinguaite suggest varying crystallization pressures and temperatures, indicating different depths of origin ranging from 23 - 96 km. The mineral composition, in-situ trace element data of pyroxenes as well as the geothermobarometric study of clinopyroxenes from both the host and the xenoliths, suggest involvement of multiple shallower magma chambers composed of the camptonitic and tinguaitic magmas generated during distinct pulses, thereby forming a complex magmatic plumbing system. The presence of orogenic geochemical signatures in the minerals of xenoliths and anorogenic geochemical signatures in the host campto-tinguaite pyroxenes imply a complex tectono-magmatic setting, with contributions from both the plume and subduction-modified mantle sources.

An earthquake swarm occurred at Sofu Seamount near Izu-Torishima in the Izu-Ogasawara (Bonin) Arc, Japan, on 8 October 2023, followed by the arrival of unexpectedly-large tsunamis over a wide area of the Pacific coast of southwest Japan. On 20 October, aerial observation identified floating pumice rafts extending for ~80 km in the area near the seamount, and subsequently the pumice clasts that may have constituted the raft were sampled and found to comprise white-colored rhyolitic pumice containing dark-colored patches. Subsequently, in the summer and autumn of 2024, white pumice, which can be clearly distinguished from the pre-existing gray and black pumice from recent eruptions of oceanic volcanoes, was stranded on the coasts of the Nansei and Ogasawara Islands. This study presented petrographic and geochemical characteristics of the white drift pumice to investigate the dispersal of the small-scale pumice rafting.White pumice clasts were collected at Okinawa (Nansei Islands) and Hahajima (Ogasawara Islands), and found to have the same petrographic and geochemical characteristics. The clasts consist of microlite-free white pumice with fine vesicles and gray-colored frothy patches, similar to the clasts collected immediately after the earthquake swarm at Sofu Seamount in October 2023. In addition, the clasts contain black enclaves containing Mg-rich olivine and clinopyroxene, and Ca-rich plagioclase. Although the white pumice clasts collected at similar times included those with different characteristics, the finding of the same pumice type over a distance of >1000 km suggests that the pumice clasts drifted over a wide area despite the small size of the pumice rafts.

Hydrous alteration of ultramafic rocks produces unique reducing environments accompanied by hydrogen (H₂) generation. To understand the early stages of the reaction, batch experiments were conducted at 90°C for 2 weeks using a NaNO₃ solution, natural dunite and harzburgite samples with variable degrees of serpentinization. Our results indicate that the fresh ultramafic rocks generate more H₂ than serpentinized rocks, showing that the dissolution of major minerals in fresh ultramafic rocks (i.e., pyroxene and olivine) is the dominant factor in H₂ generation. Fresh harzburgite yielded higher amounts of H₂ of up to 322.1 μmol/kg than the other ultramafic rocks. Fresher samples had higher dissolved Si and Ca concentrations in the solutions with higher H₂ generation than the serpentinized samples, which can be explained by the dissolution of pyroxene, because the main host mineral for Ca is clinopyroxene. Magnesium-bearing silicates were observed in the experiments using a fresh harzburgite, probably due to the more effective Si supply from pyroxene dissolution than olivine. Another series of experiments using a fresh harzburgite with different chemical reagents showed that the addition of Si enhanced H₂ generation, suggesting that H₂ generation was regulated by the precipitation of magnesium silicate hydrate. Thermodynamic calculations indicated that the solution chemistry during the hydration of fresh ultramafic rocks was regulated by magnesium silicate hydrate, whereas the solution chemistry during the hydration of serpentinized rocks was buffered by brucite. Our study suggests that fresh harzburgite is more favorable for H₂ generation than dunite at 90°C because of its higher pyroxene content.