Yoshihiko Tamura

Subduction transports carbon into Earth's interior, yet how subducted carbon influences arc magma ascent dynamics remains unclear. Here we combine volatile compositions of olivine-hosted melt inclusions with olivine diffusion chronometry at Pagan volcano (Mariana arc) to reveal a direct link between carbonate-capped seamount subduction and rapid magma ascent. Melt inclusions record high CO₂ contents (2000-6000 ppm), and magma storage near Moho depths (~20 km). Olivine zoning profiles constrain magma ascent from near-Moho to the surface on timescales of weeks to months. Elevated CaO/Al₂O₃ ratios (>1) in erupted volcanics, heavy δ¹³C values (~0.5‰) in monitored volcanic gases, and the occurrence of seamount carbonate in the forearc mud volcano collectively point to subducted seamount carbonates as the dominant CO₂ source feeding Pagan magmatism. We propose that early CO₂ exsolution near the Moho generates overpressure that enables hydrous arc magmas to bypass crustal stalling. These results identify CO₂ as a key driver of rapid magma ascent in arcs and highlight carbonate-capped seamount subduction as an efficient pathway for deep carbon cycling.

Nishinoshima is a volcanic island in the Ogasawara Arc that has exhibited intermittent activity since 2013, including four major eruptive episodes: 2013–2015 (Episode 1), 2017 (Episode 2), 2018 (Episode 3), and 2019–2020 (Episode 4). Previous studies reported a change in eruption style—from a Strombolian eruption with lava flows (Episode 1 to the early phase of Episode 4) to violent Strombolian eruptions (mid to late phase of Episode 4)—accompanied by a shift in the bulk chemical composition of volcanic ash from andesite to basaltic andesite. However, the number of tephra samples from Episode 4 remains limited, and additional data are required to clarify the details about changes in chemical composition during this episode. In this study, we present the major and trace element compositions of offshore tephra collected from Episode 4. We analyzed Kairei in December 2020. The bulk chemical compositions indicate that these samples were classified into andesite and basaltic andesite and were deposited during the mid‐ to late phase of Episode 4. Furthermore, their chemical compositions fall between those of the Nishinoshima andesites and the Episode 4 basalts reported previously. This suggests that the magma composition associated with the eruption shifted from andesite to basaltic andesite within a period of only 1.5 months. In addition, the samples show both vertical and spatial compositional variations. The former may reflect gravitational settling of ash particles through the water column and their floating behavior at the sea surface, whereas the latter may be influenced by eruption timing, local wind directions, or ocean currents.

In early October 2023, earthquake swarms occurred near Izu-Torishima in the southern Izu arc. On October 9, tsunamis reaching up to 60 cm were observed at Hachijojima and along the Pacific coast of the Japanese archipelago where the height of the tsunamis were disproportionate to the earthquake magnitudes (Sandanbata et al., 2023). T-wave analysis identified the epicenter near Sofu Seamount, bathymetric high west of Sofugan volcano, and subsequent bathymetric surveys by JAMSTEC and Japan Coast Guard independently revealed significant topographical changes, including a concave depression near the western summit of Sofu Seamount (Fujiwara et al., 2024; Minami & Tani, 2024). This significant bathymetric change suggests an intense deep-sea eruption even though the water depth of the pre-eruption summit was ~800 m.We conducted seafloor geological surveys using ROV and dredges during cruises KM25-02Leg2 (March 2025) and KM25-09 (September 2025) by the R/V Kaimei. Sofu Seamount is an east-west trending bathymetric, which was not previously recognized as an active volcano. Its western edifice features a caldera-like structure approximately 5 km in diameter, with a central cone in the northern section now containing a 1.5 km wide crater formed during the 2023 activity. The crater floor lies at a depth of ~1200 m, with the rim at ~900 m. Recent shallowing within the outer caldera was detected, suggesting significant tephra deposition in the proximal area. Rock samples were collected from the inside of the central cone, inside of the outer caldera, and northern slope of the seamount. Around the central cone crater, dark-colored volcanic rocks were collected, some of which featured brown-colored coatings identified as iron oxyhydroxides. Bacterial mats were observed on both sides of the northeastern crater rim, suggesting an active hydrothermal venting. White pumice (clasts to boulders) was found deposited on the upper most surface of the caldera interior, likely originating from the October 2023 event. Large dark-colored volcanic rocks were observed outside the caldera, notably lacking iron oxide coatings.The collected samples ranged from basaltic andesite to rhyolitic pumice, with SiO2 = 52 – 73 %. Mafic rocks contain olivine (Fo70-75), clinopyroxene, Ca-rich plagioclase, and rare orthopyroxene. Some dredged samples contained xenoliths with less-differentiated olivine (~Fo85) and well-preserved basaltic volcanic glass. Olivine-melt composition suggests P-T conditions of ~1050 C and ~160 MPa. In contrast, pumice samples contain clinopyroxene, orthopyroxene, and Na-rich plagioclase, with the estimated conditions of ~900 C and 135-165 MPa. Thermodynamic modelling indicates the compositional trend cannot be explained by fractional crystallization. As such, the mixing of high-temperature mafic magma and low-temperature felsic magma within a pressure range of 134-165 MPa beneath Sofu Seamount triggered an explosive eruption in October 2023.

This study investigates the origin of silicic magmas at the Kibblewhite Volcano in the Kermadec arc. Two alternative differentiation pathways are evaluated: fractional crystallization of primary basaltic magmas producing low‐Mg andesites, and fractional crystallization of primary andesitic magmas represented by magnesian andesites. Major‐ and trace‐element geochemistry, rare earth element (REE) systematics, geothermobarometric constraints, and thermodynamic modeling demonstrate that the low‐Mg andesites represent already evolved melts and cannot generate the observed dacitic to rhyolitic magmas through further crystallization. In contrast, magnesian andesites show close agreement with the silicic magmas in both major‐element and REE compositions, indicating that they provide a viable parental magma for the silicic magmas at Kibblewhite Volcano. These results indicate that two distinct differentiation pathways coexist beneath the Kibblewhite Volcano, in which basaltic and andesitic primary magmas evolve independently to produce low‐Mg andesites and dacitic to rhyolitic magmas, respectively. When considered in a broader context, parallel differentiation of basaltic and andesitic primary magmas may also contribute to the bimodal SiO ₂ distributions characteristic of the Kermadec Arc; however, this arc‐scale application should be regarded as a working hypothesis requiring further evaluation at other volcanic centers.

Subduction of oceanic plates drives carbon cycling between Earth’s interior and surface, yet the influence of subducted carbon on arc magma ascent dynamics remains poorly understood. Here we integrate volatile analyses of olivine-hosted melt inclusions and diffusion chronometry at Pagan volcano (Mariana arc) to reveal a direct link between carbonate-capped seamount subduction and rapid magma ascent. Melt inclusions record high CO₂ contents (2000-6000 ppm), and storage at Moho depths (20 km). Diffusion profiles of Fo, Ni, and Mn in olivine constrain magma ascent from the Moho to the surface to have occurred within ~100 days. Elevated CaO/Al₂O₃ ratios (>1) in melts, heavy δ¹³C values (~0.5‰) in volcanic gases, and the presence of seamount carbonate in the forearc mud volcano collectively indicate subducted seamount carbonates as the main source of CO₂-rich magma at Pagan. We propose that early CO₂ exsolution at Moho depths generates overpressure, enabling hydrous arc magmas to bypass stalling within crust. This mechanism provides a new perspective on CO₂-driven arc eruptions and highlights the role of seamount subduction in deep carbon cycling.

The water cycle between Earth’s surface and interior plays a critical role in maintaining long-term sea-level stability. The discovery of ~24-km-thick serpentinized mantle in the Pacific plate near the Mariana Trench suggests that water influx could be up to three times greater than previous estimates. However, the contributions of thick slab serpentinite to the magma genesis in the subduction zones remain uncertain. We integrate new boron isotopic data from Mariana arc volcanics and Lau Basin basalts with Sr isotopic, halogen, and trace element data. One rear arc volcano shows evidence of contributions from slab serpentinite, which otherwise has a limited role in the overall chemical variations of arc-basin magmatism. Instead, water-rich fluxes from lower crustal gabbro contribute to back-arc magmatism in both the Mariana and Tonga subduction zones. The geochemical signature of lower crustal gabbro-derived fluxes is also evident in volcanoes from the Izu rear-arc, the Lesser Antilles Arc, and the Cascades arc. Water flux calculations demonstrate that the slab crust alone provides sufficient water influx to balance outflux through Mariana magmatism, raising the question of the fate of slab serpentinite. It appears that most of the water from serpentinite does not contribute to the arc, and hence would lead to a large deep water return flux to the mantle. If applied globally, however, such subduction would imply a sea-level drop of ~500 m per 100 million years, contradicting geological evidence of long-term sea-level stability. To reconcile this discrepancy, we propose that the water budget of slab serpentinite is either overestimated or not representative of cold slabs. A globally averaged thickness of ~2 km of slab serpentinite with 15 vol% serpentinization (total water budget within 3.4 × 10⁷ Tg/Myr) is permitted to maintain the sea-level stability during the Phanerozoic.

Nearly 800 million people worldwide are exposed to volcanic hazards, with two-thirds of these areas located in subduction zones. Studying the movement of arc magma in subduction zones is particularly essential for volcanic hazard prevention. In particular, rapid trans-crustal movement of arc magma from the mantle can lead to volcanic eruptions with little warning, which would pose a high risk, yet research in this area is scarce. This paper reports for the first time the rapid trans-crustal movement of arc magma in the Izu-Mariana Arc. Systematic analyses on the volatile contents of melt inclusions in primitive olivine from the Pagan arc volcano yield exceptionally high CO2 contents of up to 6000 ppm with H2O contents of 3.7 wt%, which record magma storage at near-Moho depth of ~20 km. Diffusion chronometry of Fo, Ni and Mn in the host olivine, on the other hand, reveals that the parental magma took 15-125 days to ascend from near-Moho storage before its eruption. Such a rapid ascent of high Fo olivine from Moho to eruption is rare in arcs. That is because arc magma mostly has high water contents which would lead to viscous stalling during ascent and then delay the eruption. However, magma under Pagan volcano stored at a deeper depth than other arc volcanoes with similar water content, still ascended to eruption at a high rate. High CO2 content and exsolution in Pagan magma, compared to other arc magmas, which could provide additional buoyancy or significant overpressure through continuous CO2 degassing, and then result in rapid ascent.

Intermittent volcanic activity has continued at Nishinoshima volcano in the Izu–Bonin arc in the western Pacific since 2013, characterized mostly by effusive Strombolian eruptions (Episodes 1–3). Subsequently, the eruption style changed suddenly to explosive violent Strombolian in mid‐June 2020 (Episode 4), after which the volcano entered a period of quiescence. In 2021, volcanic activity restarted at Nishinoshima, although the eruptions were small (Episode 5). The change in eruption style between Episodes 1–3 and 4 was controlled mainly by a change in magma composition. However, the details of the redox state with respect to magma composition remain unclear. We aimed to understand the change in magma composition during Episode 4 and the subsequent activity, focusing on the redox state. Fe–K edge X‐ray absorption near edge structure (XANES) analysis and petrographic observations were performed on volcanic glass in seafloor and subaerial samples from Episodes 4 and 5. The results show that the explosive eruption of Episode 4 was characterized by the intrusion of oxidized basaltic magma into a reduced andesitic magma, which is a similar tendency to another large‐scale eruption in the Izu‐Bonin arc (Fukutoku‐Oka‐no‐Ba). Episode 5 shows different redox changes, involving remnants of the basaltic andesite magma from Episode 4, whose f _O2 had been reduced. The groundmass glass in Episode 5 subaerial ejecta has oxidized and differentiated (SiO ₂ ‐rich) composition possibly due to the subaerial cooling process. The observed geochemical and petrographic characteristics suggest that the small Episode 5 eruption occurred due to the continuous feed of the magma similar to Episode 4, but the magma plumbing system in the volcano made the provided high f _O2 magma reduced, which may mitigate the explosivity of the future eruption.

Monitoring and detecting marine volcanic activities are key for scientific understanding and disaster prevention. However, this is difficult because they are hidden under water. Near Torishima Island in the Izu Islands, Japan, intensified seismic activity was observed during October 2023, including a mysterious tsunami-triggering earthquake on October 8 (UTC), which was considered to be linked to a volcanic activity. On October 20, 2023, aerial surveys confirmed an 80-km stretch of floating pumice near Torishima Island. This study conducted a Lagrangian back-tracking drift simulation using the ocean current data and surface wind data to trace the origin of the pumice while clarifying the theoretical basis of Lagrangian back-tracking from the Bayesian perspective. Results indicate that the pumice drifted southward from around extensional back-arc basins near Myojinsho Reef and Sumisujima Island approximately 3–5 days before its discovery. These findings are consistent with independent observations such as biological traces and the geochemical characteristics of sampled floating pumice, which is considered identical to that found on October 20 by an airplane. This indicates the presence of unknown volcanic activity around back-arc basins west of the major active volcanic zone. This study demonstrates the utility of combining drift simulations with geochemical and biological data to identify the sources of marine volcanic events, particularly in regions where direct observations are limited. The results of this study contribute to our understanding of volcanic mechanisms and their potential hazards.