Evolution of Magma Plumbing System in Miyakejima Volcano: Constraints From Melting Experiments

Abstract

The evolution of the magma plumbing system of Miyakejima volcano, an island arc basaltic volcano located on the volcanic front of the Izu arc, Japan, was investigated by combining (i) melting experiments of hydrous basaltic magma, (ii) analysis of melt inclusions in the volcanic rocks, and (iii) thermodynamic modeling using the thermodynamic program (MELTS) to understand the temporal evolution of the magma plumbing system. Basalts of the Ofunato Stage (10,000–4,000 year BP) show limited compositional variation without evidence of magma mixing, and seem to be parental to volcanic rocks of later stages. Hydrous melting experiments were conducted using the Ofunato Scoria (OFS), which represents the most undifferentiated basaltic magma during the past 10,000 years. The magma chamber conditions required to crystallize the phenocryst assemblage of the OFS were constrained to ~150 MPa, ~1100 °C, and ~3 wt% H2O in melt. The constrained H2O content is consistent with H2O content of olivine‐hosted melt inclusions in the OFS. Here we propose that a simple magma plumbing system existed beneath Miyakejima volcano during the Ofunato Stage, comprising a single magma chamber. The MELTS calculations show that andesites of the Tsubota Stage (4,000–2,500 year BP; representing the later part of the Ofunato Stage after a period of dormancy of 3,000 years), can be derived from the Ofunato basalts, provided that H2O content in magma is <1 wt%. The results of modeling indicate that a new magma chamber formed at a shallower depth after the Ofunato Stage, in which the differentiation of degassed basaltic magmas occurred.

Temporal Evolution of Proto-Izu–Bonin–Mariana Arc Volcanism over 10 Myr: Constraints from Statistical Analysis of Melt Inclusion Compositions

“…We argue that (1) a new magma chamber was formed at shallower levels as the magma-plumbing system evolved (e.g. Ushioda et al, 2018) and therefore several magma chambers at different depths in the crust may have been present; (2) the melt was saturated with H 2 O as most arc melts are (e.g. Plank et al, 2013; 2−6 wt %); and (3) the saturated-H 2 O content of Cluster 3b melts decreased with decreasing pressures (e.g.…”

Hydrothermal and Magmatic System of a Volcanic Island Inferred From Magnetotellurics, Seismicity, Self‐potential, and Thermal Image: An Example of Miyakejima (Japan)

“…For this reason, we suggest that the 'dr' unit could represent a two-phase or supercritical fluid reservoir feeding the fumarolic area. We propose that this reservoir would have developed above the residual part of the shallow andesitic chamber, involved in Miyakejima eruptions since at least the 15th century (Amma-Miyasaka et al, 2005;Ushioda et al, 2018). Magmatic gases stored in this reservoir would have two origins: (i) a small portion linked to the long-term crystallization of the shallow andesitic body, and (ii) a large amount coming from intrusions of the deep basaltic source.…”