Petrology, magma mixing, mush mobilization, and timescales of the 1874 A.D. Meiji Eruption of Miyakejima, Japan

Abstract:

To understand the magmatic processes generating fissure eruptions on island arc volcanoes, we investigated the products of the 1874 A.D. Meiji eruption of Miyakejima volcano of the Izu-Bonin arc, Japan. The compositional characteristics of silicate melt inclusions and their host phenocrysts (olivine, pyroxene, plagioclase) were used to study the structure of the magmatic system, the componentry of the melt storage regions, and the processes which generated the products of this eruption. Olivine diffusion chronometry was utilized to constrain timescales associated with this eruption. Silicate melt inclusions were distinguished based on their major and trace element composition into a primitive Group 1 and a more evolved Group 2. Olivines contain both Group 1 and 2 melt inclusions while the pyroxenes and plagioclase contain Group 2 melt inclusions. Olivine with Group 1 melt inclusions have Fo86-66 core compositions while olivines with Group 2 melt inclusions have core compositions that range from Fo76-67. Olivine core-to-rim zonation patterns range from abrupt normal, abrupt reverse-to-normal, and a diffuse normal zonation. The melt inclusion and phenocryst compositions indicate that the magmatic system that fed the Meiji eruption consists of a deeper primitive and a shallow more evolved reservoir. These results indicate that hot primitive melts entered into the system and mixed with the resident melt of a deep magma reservoir which triggered their ascent into the shallower reservoir. Disequilibrium between the carrier melts and the minerals indicate that some crystals formed at earlier stages of melt evolution and we were stored as crystal-rich mushes before being mobilized and entrained during the eruption. Timescales associated with magma mixing and eruption are on the order of 1-5 days. These results indicate that small-volume flank eruptions can involve inputs of recharging primitive melts into the shallow plumbing systems, which cause interactions between reservoirs located at different levels within the magmatic system. The interactions entrain mush and generate heterogeneous crystal zonation records. This study reveals that eruptions can be triggered on very short timescales and that they can proceed quickly after the onset of activity.