Akihiko Tomiya

Magma discharge variations during the 2011 eruptions of Shinmoe-dake volcano, Japan, revealed by geodetic and satellite observations

We present precise geodetic and satellite observation-based estimations of the erupted volume and discharge rate of magma during the 2011 eruptions of Kirishima-Shinmoe-dake volcano, Japan. During these events, the type and intensity of eruption drastically changed within a week, with three major sub-Plinian eruptions on January 26 and 27, and a continuous lava extrusion from January 29 to 31. In response to each eruptive event, borehole-type tiltmeters detected deflation of a magma chamber caused by migration of magma to the surface. These measurements enabled us to estimate the geodetic volume change in the magma chamber caused by each eruptive event. Erupted volumes and discharge rates were constrained during lava extrusion using synthetic aperture radar satellite imaging of lava accumulation inside the summit crater. Combining the geodetic volume change and the volume of lava extrusion enabled the determination of the erupted volume and discharge rate during each sub-Plinian event. These precise estimates provide important information about magma storage conditions in magma chambers and eruption column dynamics, and indicate that the Shinmoe-dake eruptions occurred in a critical state between explosive and effusive eruption.

Melt contribution to partitioning of trace element between plagioclase and basaltic magma of Fuji volcano, Japan

Abstract Ion microprobe analyses of trace elements in plagioclase crystals apply to partitioning between plagioclase phenocrysts and basaltic magma of Fuji volcano. The partitioning of trace elements of plagioclase is governed not only by the site preference of ions in the crystal as it is widely accepted, but also by the components of melt. The observed logarithmic count ratios of the ion microprobe analyses between trace and host elements in plagioclase, is linear with XAn (anorthite composition of plagioclase), in the range of XAn (0.55–0.85). The observed ratio is proportional with K′-value (apparent exchange partition coefficient) because of nearly constant composition of the basalt. We separate the K′-value into two terms: plagioclase component controlled term and non-plagioclase component controlled term. The second term mainly characterizes the pattern of the Onuma PC-IR-diagram in a magmatic system.