The depth of crystallization differentiation was investigated for arc tholeiite basalts from the Takada Ohdake and Nishimori cones, within the volcanic front of the northern Honshu volcanic arc, Japan, based on the petrography of crystal-bearing melt inclusions, whole-rock trace element chemistry, and the simulation of fractional crystallization with MELTS. Olivine-hosted melt inclusions contain aluminous clinopyroxene, spinel, plagioclase (An_60–65), and rare garnet, suggesting melt-entrapment under garnet-granulite facies conditions. Removal of the observed phenocrysts (olivine, orthopyroxene, and anorthitic plagioclase) cannot account for either the major or trace element variations within the basalts. However, the major element variations are typical of a tholeiitic differentiation trend, showing an increase in Fe/Mg ratio with increasing SiO₂. The results of trace element modelling, assuming Rayleigh fractionation, indicate fractionation of clinopyroxene, plagioclase, and spinel. The estimated fractionated mineral assemblage is consistent with that in the melt inclusions except for the presence of garnet. The occurrence of garnet in some of the more differentiated andesitic melt inclusions implies its crystallization within the inclusions during the later stages of cooling. Fractional crystallization modelling using MELTS closely approximates the major element variations at 10 kbar under anhydrous conditions. The MELTS modelling is consistent both with the melt-inclusion mineralogy and with the trace element modelling. An estimated depth of crystallization of 34 km (∼10 kbar) corresponds to the seismic Moho in the region (35–36 km). Our data also suggest that the relatively anhydrous magmas that were emplaced at Moho depths became hydrous concurrent with differentiation, implying incorporation of water from the surrounding crustal rocks.

The Lamongan Volcanic Field (LVF), East Java, Indonesia, has experienced numerous maar eruptions, producing varied properties and morphologies of ash particles. This study conducted textural, morphometric, and geochemical analyses of the juvenile particles to elucidate the factors governing their heterogeneous characteristics. Two distinct types of juvenile ash were identified: A (black and brown ash) and B (orange-brown ash), reflecting different fragmentation processes. The blocky to slightly elongate shapes of juvenile A across heterogenous basaltic compositions (resulting in variable textures, rheological properties, and/or cooling histories) highlight the phreatomagmatic process as the primary control of their shape. In contrast, the irregular-fluidal shapes of juvenile B particles indicate magmatic fragmentation of basaltic andesite magma. This study reveals that variable magma properties yield diverse ash components, yet fragmentation dynamics govern pyroclast shapes in the LVF maar complex. Our integrated approach emphasizes the importance of considering multiple variables when interpreting heterogeneous volcanic ash deposits.

Aluminum-phosphate-sulfates (APS) of the alunite supergroup occur in igneous rocks within zones of advanced argillic and silicic alteration in porphyry and epithermal ore environments. In this study we report on the presence of woodhouseite-rich APS in ash from the 27 September 2014 hydrothermal eruption of Ontake volcano. Scanning electron microscope coupled with energy dispersive X-ray spectrometer (SEM-EDS) and field emission (FE)-SEM-EDS observations show two types of occurrence of woodhouseite: (a) as cores within chemically zoned alunite-APS crystals (Zoned-alunite-woodhouseite-APS), and (b) as a coherent single-phase mineral in micro-veinlets intergrown with similar micro-veinlets of silica minerals (Micro-wormy-vein woodhouseite-APS). The genetic environment of APS minerals at Ontake volcano is that of a highly acidic hydrothermal system existing beneath the volcano summit, formed by condensation in magmatic steam and/or ground waters of sulfur-rich magmatic volatiles exsolved from the magma chamber beneath Mt. Ontake. Under these conditions, an advanced argillic alteration assemblage forms, which is composed of silica, pyrophyllite, alunite and kaolinite/dickite, plus APS, among other minerals. The discovery of woodhouseite in the volcanic ash of the Ontake 2014 hydrothermal eruption represents the first reported presence of APS within an active volcano. Other volcanoes in Japan and elsewhere with similar phreatic eruptions ejecting altered ash fragments will likely contain APS minerals derived from magmatic-hydrothermal systems within the subvolcanic environment. The presence of APS minerals within the advanced argillic zone below the summit vent of Ontake volcano, together with the prior documentation of phyllic and potassically altered ash fragments, provides evidence for the existence within an active volcano in Japan of an alteration column comparable to that of porphyry copper systems globally.

The origin of extremely diverse arc magmas erupted along the 600‐km‐long Quaternary frontal arc of NE Japan was examined using Sr–Nd–Hf–Pb isotopic and major/trace elemental compositions. The major/trace element chemistry of the magmas varied with the various combinations of low‐K to medium‐K and calc‐alkaline to tholeiitic rock suites, even within a single volcano. Nevertheless, magmas from individual volcanoes showed quasi‐linear isotopic variations in the Pb–Pb and Nd–Pb isotope spaces irrespective of their complex rock suites. The isotope data indicate a binary mixing origin of the magmas derived from a mostly homogeneous mantle source and the others from an isotopically diverse crust represented by Cretaceous to Paleogene basement granitoids. Assimilation‐fractional‐crystallization (AFC) and melting‐assimilation‐storage‐hybridization (MASH) processes formed the tholeiitic and calc‐alkaline series, respectively. These would have occurred in the amphibolitic lower crust that constitutes the roots of the basement granitoids. This study demonstrates that the mafic lower crust extensively contributes to the chemical diversity of arc magmas, even for relatively undifferentiated basaltic andesites.

History and pattern of explosive eruptions at Chokai volcano, Japan, in the last 2500 years were investigated from tephra survey and accelerator mass spectrometry (AMS) radiocarbon dating. The tephrostratigraphy was established based on observations at eight hand-dug trenches and three outcrops. The well-correlated tephra layers were dated at c. 2.5 ka, 2.1–1.9 ka, 1.8 ka, and 1.6 ka, indicating major eruptions occurred at these ages. The tephra from the documented 871 CE eruption was also identified. Componentry analysis of ash was carried out for these five eruption deposits. The changes in tephra facies and ash components within an unbroken series of tephra layers indicate a shift from hydrothermal-dominant phreatic or phreatomagmatic eruption to magma-dominant eruptions in a single episode. Common eruption sequences were identified based on the combination of tephra facies variation and records of witnessed eruptions. Every volcanic activity begins with precursory activity of seismicity, fumaroles, and snow melting for weeks to months, then onset hydrothermal-dominant eruption happens. Then, the eruption evolves to a magma-dominant eruption, or alternatively, the hydrothermal-dominant eruption persistently continues until cessation. The eruption sizes are VEI 2 or more minor. Lahar can occur at any stage of the eruption, resulting in damage to the residential area at the base of the volcano. The eruption patterns and the extent of hazard risks elucidated by this study will be utilized to hazard mitigation plans.

Papua New Guinea is one of the least studied regions in the Southwest Pacific, and large areas of the country, such as the Fly Plat-form, remain poorly understood due to limited exposure and access constraints. This study presents the first documentation of basaltic volcanism on the Fly Platform, based on new field discoveries at Mea-hill and Yemsigi, two areas located approximately 25 km apart. Inte-grated field observations, petrography, mineral chemistry, and whole-rock geochemistry show that both basalt suites were derived from a similar magma source but record contrasting emplacement histories. Meahill basalts, which include welded tuffs and highly ve-sicular basalt units, reflect rapid magma ascent, vigorous degassing, and locally explosive activity. In contrast, the massive, less vesicular porphyritic basalts at Yemsigi preserve a quieter emplacement history, but with more extensive post-magmatic alteration. Geochemical sig-natures from least altered rocks of both suites support an intraplate origin with similarities to Pliocene-Pleistocene lava fields of Northeast Queensland. The origin of the intra-plate basaltic magmatism is enig-matic, but both young volcanic provinces correlate spatially with a lower mantle anomaly that may represent residual slab material and a seated-seated magma source. These findings provide further insight into the tectono-magmatic evolution of the Fly Platform region and highlight the need for continued geological investigation in this underexplored district.