Textures of plagioclase microlite and vesicles within volcanic products of the 1914-1915 eruption of Sakurajima Volcano, Kyushu, Japan

Abstract

We classify volcanic products of the 1914-1915 emption of Sakurajima Volcano into four types according to vesicularity and the texture of plagioclase microlite. Type-1 is white Plinian fall pumice with high vesicularity (> 60 vol%) and low modal content (< 1.0 vol%) and number density (< 1014 m-3) of microlite. Type-2 is white Plinian fall pumice with high vesicularity (> 55 vol%) and intermediate modal content (1.0-11.0 vol%) and number density (1-10 × 1014 m-3) of microlite. Type-3 is slightly darker Plinian fall pumice with low vesicularity (25-50 vol%) and intermediate to high modal content (8-16 vol%) and number density (5 × 1014 -2 × 1015 m-3) of microlite. The fourth type of volcanic product is lava flows with low vesicularity (< 25 vol%) and high modal content (> 16 vol%) and intermediate number density (1-5 × 1014 m3) of plagioclase microlite. Type-1 and type-2 pumices were mainly deposited within the lower and middle parts of the Plinian fall deposit, whereas type-3 pumices were mainly deposited in the upper part. Homogeneous chemical features of preeruptive melts of all types of pumices indicate that the observed variations in number density and modal content of plagioclase microlite, as well as the decreasing vesicularity from type-1 to type-3 pumices, are related to a decrease in the decompression rate of magma from early to late stages of the Plinian phase. Residual water contents and degree of vesicularity of Plinian fall pumices suggest a change in the degassing process during the Plinian phase of eruption from minor degassing during the eruption of type-1 pumices to effective degassing during the eruption of type-3 pumices. Differences in microlite texture between type-1 and type-2 pumices of the early to middle stages of the Plinian phase and late-stage type-1, -2 and -3 pumices reflect changes in the decompression rate, perhaps related to differences between the central and marginal parts of conduits. A systematic increase in the An content of plagioclase microlites from Plinian fall pumice to Taisho Lava reflects a chemical change in the preemptive melt from silicic to basic composition related to progressive magma mixing.