The geochemical and petrological characteristics of prenatal caldera volcano: a case of the newly formed small dacitic caldera, Hijiori, Northeast Japan

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 H2O 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. C; (2) bulk magma composition: 66 +/- 1.5 wt% SiO2; (3) bulk magmatic H2O: approximately 2.5 wt%, and variable characteristics that depend on depth; (4) crystal content: <= 57 vol%; (5) bulk modulus of magma: 0.1-0.8 GPa; (6) magma density: 1.8-2.3 g/cm(3); and (7) amount of excess magmatic H2O: 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 (2 similar to 13 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.