V020-0010
Magma Storage and Gas Source Depth Revealed by Melt Inclusion from Aso Volcano, Japan
Thursday, 10 December 2020
Poster
Masataka Kawaguchi1,2, Kenneth T Koga2, Estelle F Rose-Koga3, Toshiaki Hasenaka4, Atsushi Yasuda5, Kenji Shimizu6 and Takayuki Ushikubo6, (1)Kumamoto University, Kumamoto, Japan, (2)Université Clermont Auvergne, CNRS, IRD, OPGC, Laboratoire Magmas et Volcans, Clermont-Ferrand, France, (3)Université Clermont Auvergne, CNRS, IRD, OPGC, Laboratoire Magmas et Volcans, Aubiere, France, (4)Center for Water Cycle, Marine Environment and Disaster Management, Kumamoto University, Kumamoto, Japan, (5)Earthquake Research Institute, The University of Tokyo, Tokyo, Japan, (6)Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology, Kochi, Japan
Abstract:
Volcanic gas emission is one of the commonly observed activities even during quiescent period. It is interpreted as reflecting magma degassing beneath volcanoes. We have investigated phenocryst-hosted melt inclusions (olivine, clinopyroxene, orthopyroxene, and plagioclase from Holocene basaltic eruption products of Aso volcano, Japan), and compared their volatile element characteristics with reported gas measurements. This study identifies the composition of a volatile-rich primitive magma supplying the plumbing system, and described the process of persistent degassing originating from multiple depths of magma storage beneath an active volcano.
Melt inclusion compositions varied from 46.0 to 65.8 wt.% SiO2. High sulfur concentrations, up to 3745 ppm, were found in mafic melt inclusion hosted by ~Fo81 olivine phenocrysts. Indications of magma mixing were abundantly present: reverse mineral zoning and disequilibrium texture. Pre-eruptive storage depths based on volatile concentrations were estimated to be 2 km and 4 km deep, for the Strombolian and the sub-Plinian eruption, respectively. One endmember of the erupted mixed magma is a volatile-rich primitive magma that originated from at least 10 km depth. Its initial volatile concentrations were determined using multiple constraints: more than 4.5 wt. % H2O, 250 - 500 ppm CO2, 5050 ppm S, 1210 ppm Cl and 330 ppm F. All magma reservoir depths agree with previously reported geophysical observations.
Modeled gas evolution during magma ascent showed that observed high CO2/SO2 gas is in equilibrium with the deep primitive basaltic magma. The observed variation of gas composition was best explained by mixing of the gases originating from deep and shallow magma reservoirs.
