Magma Plumbing System at Izu-Oshima Volcano, Japan: Constraints From Petrological and Geochemical Analyses

The Izu-Oshima volcano is one of the most active volcanoes in Japan, and has generated relatively large-scale eruptions every 30–40 years for the past 200 years. As more than 30 years have passed since the last eruptions in 1986–87, volcanic activity is expected to resume in the near future. To help elucidate the current and future state of the volcano’s magma system, the temporal evolution of the recent magma plumbing system was investigated through a petrological and geochemical analysis of its basaltic lavas and pyroclastics (<∼53 wt.% of SiO2) that were erupted during the last ∼1.5 kyr. The basaltic products have variable phenocryst contents, ranging from ∼0 to ∼20 vol.%, and phenocryst-bearing samples commonly contain plagioclase, orthopyroxene, and clinopyroxene phenocrysts. The whole-rock compositions are significantly scattered in the Harker variation diagrams, suggesting that the compositional diversity was established by at least two independent magmatic processes. The application of principal component analysis on the whole-rock major element data suggests that one magmatic process was crystal fractionation of crystal-poor magmas, and the other process was either plagioclase accumulation or mixing of plagioclase-rich magmas. Based on this observation, and combined with the petrological analysis and previous geophysical studies, we propose that aphyric magmas, stored in an 8–10 km-deep magma chamber, progressively differentiated over time from the 7th to 20th century. Furthermore, the compositional variations in basalts resulted from the mixing of the differentiating aphyric magmas with variable proportions of porphyritic magmas derived from a 13–18 km-deep magma chamber. Because recent eruptions have been triggered by the ascent of porphyritic magma from the 13–18 km-deep magma chamber, and its injection into the 8–10 km-deep magma chamber, it is important to monitor the deeper magma chamber to predict future volcanic activity

Buoyant hydrous mantle plume from the mantle transition zone

Abstract Magmatism at some intraplate volcanoes and large igneous provinces (LIPs) in continental areas may originate from hydrous mantle upwelling (i.e. a plume) from the mantle transition zone (MTZ) at 410–660 km depths in the Earth’s deep interior. However, the ultimate origin of the magmatism, i.e. why mantle plumes could have been generated at the MTZ, remains unclear. Here, we study the buoyancy of a plume by investigating basalts from the Changbaishan volcano, beneath which a mantle plume from the hydrous MTZ is observed via seismology. Based on carefully determined water contents of the basalts, the potential temperature of the source mantle is estimated to be 1310–1400 °C, which is within the range of the normal upper mantle temperature. This observation suggests that the mantle plume did not have a significant excess heat, and that the plume upwelled because of buoyancy resulting from water supplied from the Pacific slab in the MTZ. Such a hydrous mantle plume can account for the formation of extremely hydrous LIP magmatism. The water was originally sourced from a stagnant slab and stored in the MTZ, and then upwelled irrespective of the presence or absence of a deep thermal plume

マグマ溜まりの熱物質進化 : 火山噴出物からの微分的情報による制約

University of Tokyo (東京大学

Intensive Hydration of the Wedge Mantle at the Kuril Arc-NE Japan Arc Junction : Implications from Mafic Lavas from Usu Volcano, Northern Japan

The generation and evolution of basaltic magmas at Usu volcano, located at the junction between the NE Japan arc and the Kuril arc, have been investigated. The mafic products, which form the somma edifice of the volcano, consist of basalt (49 center dot 6-51 center dot 3 wt % SiO2) and basaltic andesite (52 center dot 0-54 center dot 9 wt % SiO2) lavas. The basaltic lavas show relatively tight compositional trends, and Sr-87/Sr-86 ratios tend to decrease with increasing whole-rock SiO2 content. The water content of the basaltic magmas was determined to be similar to 4 center dot 8 wt % based on plagioclase-melt thermodynamic equilibrium. Using this information and an olivine maximum fractionation model, the water content of the primary Usu magma was estimated to be 3 center dot 9 wt %. Multi-component thermodynamic calculations suggest that the primary magma was generated by similar to 23% melting of the source mantle with similar to 0 center dot 94 wt % H2O at similar to 1300A degrees C and similar to 1 center dot 4 GPa. The 0 center dot 94 wt % water content of the source mantle is significantly higher than that beneath volcanoes in the main NE Japan arc (generally < 0 center dot 7 wt % H2O); this implies that the wedge mantle at the arc-arc junction is intensively hydrated. The temperature of the wedge mantle of similar to 1300A degrees C at similar to 1 center dot 4 GPa is also significantly higher than that of the mantle in the main NE Japan arc. Unlike the basaltic lavas, the whole-rock compositions of the basaltic andesite lavas are scattered in Harker variation diagrams. This observation suggests that the compositional diversity was produced by at least two independent processes. To elucidate the processes responsible for this compositional diversity, principal component analysis was applied to the major element compositions of the samples. This suggests that 47% of the diversity of the whole-rock compositions can be explained by mixing with partial melts of lower crustal materials, 25% is explained by redistribution of plagioclase phenocrysts, and 16% is explained by fractionation of accessory minerals

Pre-eruption magmatic processes and magma plumbing system at Hachijo-Nishiyama volcano, Izu–Bonin arc, Japan

Abstract Nishiyama volcano on Hachijojima Island is an active basaltic volcano located in the Izu–Bonin arc. In this study, petrological and geochemical analyses were conducted on mafic lavas and pyroclastics to understand the magma plumbing system and pre-eruption magmatic processes. Whole-rock major element compositions show significant variations (49.4–54.9 wt.% SiO2), and the samples contain variable amounts of plagioclase phenocrysts (1–40 vol.%). The whole-rock Sr, Nd, and Pb isotopic compositions of samples from the youngest volcanic stage (< 0.7 ka) are homogeneous, whereas some samples from the older stage (3–1 ka) have relatively low Pb isotopic ratios. This observation suggests that the younger magmas were derived from a single parental magma, but another parental magma with distinct geochemical features was involved in the magmatic system before 1 ka. The temporal variation in the FeO*/MgO ratios of the volcanic products is complex and is considered to reflect the intermittent injection of primitive magmas into the main magma chamber in which fractional crystallization occurred. Two-pyroxene geobarometry suggests that the main magma chamber was located at a depth of 9–12 km. The core region of some plagioclase phenocrysts consists of a glass inclusion-free inner core and an inclusion-rich outer mantle, suggesting that some plagioclase crystallized in the main magma chamber, which was followed by overgrowth during magma ascent because of increasing liquidus temperatures due to decompression-induced water exsolution from the melt. The whole-rock compositions of some eruption units with different Al2O3/MgO ratios exhibit distinct plagioclase-controlled trends, which negates the possibility that plagioclase accumulation occurred in a stable magma chamber. In addition, the density of plagioclase was higher than that of the melt during the magma ascent to the surface. From these observations, it is suggested that the accumulation of plagioclase phenocrysts occurred in ascending magmas as the plagioclase settled relative to the surrounding melt. The estimated depth of 9–12 km for the main magma chamber coincides with the depth range over which earthquake swarms occurred in 2002, suggesting that the magma chamber is still active, and that the earthquake swarms may reflect the injection of primitive magma into the magma chamber. Graphical Abstrac

A new concept for the genesis of felsic magma : the separation of slab-derived supercritical liquid

Felsic magmas produced at subduction zones have played an important role in the generation and evolution of the continental crust. For the origin of felsic magmas, processes such as fractional crystallization of mafic magmas, partial melting of crustal materials, partial melting of subducting slabs, and partial melting of pyroxenitic mantle wedge components have been proposed. Recent experimental studies have predicted that felsic melt can also be produced in the mantle wedge by the separation of slab-derived supercritical liquid beyond depths corresponding to the critical point. To date, however, the presence of felsic magma of this origin has not yet been reported. In this study, we investigated dacitic lavas and preceding calc-alkaline andesite lavas from the Rishiri Volcano, located at the rear of the Kuril arc. We show that hydrous felsic melt and aqueous fluid were separated from slab-derived supercritical liquid in the mantle wedge. The former erupted as dacitic magma whilst the aqueous fluid induced the generation of primary basaltic magma involved in creating calc-alkaline andesite magma. We infer that slab-derived supercritical liquid is an efficient transport medium for moving silicate-rich components from subducting slabs to the Earth's surface, and that this process may have contributed to the growth of the continental crust