Abstract
Evaluating the magma depth and its physical properties is critical to conduct a better geophysical assessment of magma chambers of caldera volcanoes that may potentially cause future volcanic hazards. To understand pre-eruptive conditions of a magma chamber before its first appearance at the surface, this paper describes the case of Hijiori caldera volcano in northeastern Japan, which emerged approximately 12,000 years ago at a place where no volcano ever existed. We estimated the depth, density, bulk modulus, vesicularity, crystal content, and bulk H_2O content of the magma chamber using petrographic interpretations, bulk and microchemical compositions, and thermodynamic calculations. The chemical mass balance calculations and thermodynamic modeling of the erupted magmas indicate that the upper portion of the Hijiori magmatic plumbing system was located at depths between 2 and 4 km, and had the following characteristics: (1) pre-eruptive temperature: about 780 ^{circ }C; (2) bulk magma composition: 66 ± 1.5 wt% SiO_{2}; (3) bulk magmatic H_2O: approximately 2.5 wt%, and variable characteristics that depend on depth; (4) crystal content: le57 vol%; (5) bulk modulus of magma: 0.1–0.8 GPa; (6) magma density: 1.8–2.3 g/cm3; and (7) amount of excess magmatic H_2O: 11–32 vol% or 48–81 mol%. The range of melt water contents found in quartz-hosted melt inclusions (2–9 wt%) suggests the range of depth phenocrysts growth to be wide (2sim13 km). Our data suggest the presence of a vertically elongated magma chamber whose top is nearly solidified but highly vesiculated; this chamber has probably grown and re-mobilized by repeated injections of a small amount of hot dacitic magma originated from the depth.
Highlights
According to the Japan Meteorological Agency, 110 volcanoes are currently listed as active volcanoes in the country
Broken plagioclase phenocryst faces are sometimes covered by vesicular matrix glass, suggesting that multiple magma fragmentation events took place before the final event that produced the fragments of pumice in the analyzed samples
The chemical mass balance calculations and MELTS modeling undertaken during this study indicated that the erupted upper portion of the Hijiori magmatic plumbing system was located at depths of 2–4 km and had the following characteristics: (1) bulk magmatic H2O was approximately 2.5 wt%, (2) crystal content was ≤57 vol%, (3) bulk modulus was about 0.1–0.8 GPa, (4) density was between 1.8 and 2.3 g/cc, and (5) excess magmatic H2O was 11–32 vol% or 48–81 mol% (Table 3)
Summary
According to the Japan Meteorological Agency, 110 volcanoes are currently listed as active volcanoes in the country. An active volcano is defined as having erupted in the last 10,000 years or that currently exhibits active fumarolic vents (JMA 2013). These volcanoes have potential to erupt again in the future. To understand initial behavior of volcanoes in a region that is volcanically inactive, we focused on some newly formed explosive volcanoes in the northeastern part of Japan. These include Ichinome-gata volcano at 39◦57′33′′N 139◦44′ 25E (Kano et al 2011; Katsui et al 1979), Hijiori volcano
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