Conduit pre-charge during repeated Plinian eruptions at Sakurajima volcano

Abstract:

Understanding the magmatic conditions before the historic Plinian eruptions at the Sakurajima volcano (AD 1471, 1779, and 1914) is crucial to assess future eruption risks. We determined the depth of the pre-eruptive magma storage based on the volatile content in the phenocryst-hosted melt inclusions and in the groundmass glass that was equilibrated with plagioclase rims. The water content of the melt inclusions, determined by FT-IR reflectance spectroscopy (Yasuda, 2014), was 1.4–3.5 wt%, consistent with the estimates based on the plagioclase-melt hygrometer model of Waters and Lange (2015). The CO₂ content of the inclusions was 29 ppm at maximum but mostly below the detection limit (~10 ppm). Assuming volatile saturation, we estimated the magma storage depths just before the historic Plinian eruptions to be 0.9–3.2 km. This depth is even shallower than the uppermost magma reservoir located beneath the Sakurajima volcano, as estimated using geodetic and seismological observations (Iguchi et al., 2013). We conclude that before every Plinian eruption, the Plinian magmas were fed from a shallow and thick conduit that was pre-charged from deeper reservoirs. This process may require the widening of the dike-shaped conduit, preserving the possibility that significant precursory phenomena could be detected before forming an eruption column. On the other hand, the magma could reach the surface quickly from the shallow conduit. Moreover, a thick dike-shaped conduit enables a high magma discharge rate (Costa et al., 2009), thereby facilitating vigorous explosive eruptions.