Volcanic gas composition changes during the gradual decrease of the gigantic degassing activity of Miyakejima volcano, Japan, 2000-2015

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• 作者：Hiroshi Shinohara ; Nobuo Geshi ; Nobuo Matsushima ; Genji Saito…
• 关键词：Degassing ; Multi ; GAS ; Volcanic gas composition ; Miyakejima volcano ; Conduit magma convection
• 刊名：Bulletin of Volcanology
• 出版年：2017
• 出版时间：February 2017
• 年：2017
• 卷：79
• 期：2
• 全文大小：
• 刊物主题：Geology; Geophysics/Geodesy; Mineralogy; Sedimentology;
• 出版者：Springer Berlin Heidelberg
• ISSN：1432-0819
• 卷排序：79

文摘

The composition of volcanic gases discharged from Miyakejima volcano has been monitored during the intensive degassing activity that began after the eruption in 2000. During the 15 years from 2000 to 2015, Miyakejima volcano discharged 25.5 Mt of SO₂, which required degassing of 3 km3 of basaltic magma. The SO₂ emission rate peaked at 50 kt/day at the end of 2000 and quickly decreased to 5 kt/day by 2003. During the early degassing period, the volcanic gas composition was constant with the CO₂/SO₂ = 0.8 (mol ratio), H₂O/SO₂ = 35, HCl/SO₂ = 0.08, and SO₂/H₂S = 15. The SO₂ emission rate decreased gradually to 0.5 kt/day by 2012, and the gas composition also changed gradually to CO₂/SO₂ = 1.5, H₂O/SO₂ = 150, HCl/SO₂ = 0.15, and SO₂/H₂S = 6. The compositional changes are not likely caused by changes in degassing pressure or volatile heterogeneity of a magma chamber but are likely attributed to an increase of hydrothermal scrubbing caused by large decrease of the volcanic gas emission rate, suggesting a supply of gases with constant composition during the 15 years. The intensive degassing was modeled based on degassing of a convecting magma conduit. The gradual SO₂ emission rate that decrease without changes in volcanic gas composition is attributed to a reduction of diameter of the convecting magma conduit.