Tracking magma-air interaction processes based on the analysis of oxidation textures in pyroclastic materials

2022 Fiscal Year Final Research Report

• Project/Area Number: 19K14820
• Research Category: Grant-in-Aid for Early-Career Scientists
• Allocation Type: Multi-year Fund
• Review Section: Basic Section 17040:Solid earth sciences-related
• Research Institution: National Institute of Advanced Industrial Science and Technology
• Principal Investigator: Matsumoto Keiko 国立研究開発法人産業技術総合研究所, 地質調査総合センター, 主任研究員 (10803926)
• Project Period (FY): 2019-04-01 – 2023-03-31
• Keywords: 火山 / 爆発的噴火 / 鉄酸化鉱物 / 酸化 / 大気 / 実験

Outline of Final Research Achievements

Oxidation textures in natural pyroclasts and experimental products were studied to advance our understanding of magma-air interaction processes during volcanic eruptions. Fe-oxide pseudomorphs replacing sulfide crystals were observed in pyroclasts from explosive eruptions at Sakurajima and Asama volcanoes, which reflect air-mixing processes in eruption columns. Fine red particles on the surfaces of mafic minerals were found to be the source of the red color in the 2018 Shinmoedake eruption pyroclasts. Based on comparisons with the observed eruption sequence, the formation time of the particles was estimated to be within two weeks after lava effusion. The products of heat and quench experiments showed that groundmass color reflects the amount and type of crystallized minerals and that the variation depended not on the oxygen pressure but on the starting materials. For in-situ oxidation experiments conducted on pyroxene, at least two mineral types formed within several tens of minutes.

Free Research Field

火山学，岩石学

Academic Significance and Societal Importance of the Research Achievements

噴火に際してマグマは大気により瞬時に急冷するとみなされ，これまで爆発的噴火の酸化組織の詳細な記載や形成過程はほとんど議論されてこなかった．しかし，現に赤色酸化した火山噴出物は普遍的に存在する．肉眼で認知される赤色化だけでなく，これまで認識されていない酸化組織も含めてその種類・形成条件・形成過程が明らかになれば，マグマと大気との相互作用の履歴を噴出物から制約できる．直接観測が不可能な噴火前マグマに対する大気の影響や噴煙内部の温度・大気構造の推定が実時間・過去を問わず可能となり，物質解析に基づく噴火ダイナミクスの理解につながる．