マグマの脱ガス : 熱水流体の放出過程  [in Japanese] Release of hydrothermal fluid from a degassing magma  [in Japanese]

マグマから放出される熱水流体の組成・量について簡単なモデル計算を行ない, その結果を基に火山ガスの脱ガス過程および火山性熱水系の進化についての考察を行なった。モデル計算はH_2O-CO_2およびH_2O-NaClの二つの系について行ない, 脱ガス過程としては減圧による離溶-結晶化に伴う離溶および閉鎖系-開放系の組み合わせによる四つの極端な場合について行ない, CO_2の初期における離溶などが結論された。火山ガスの脱ガス過程としては, 2段階脱ガス, 1段階脱ガス, マグマ対流を伴う脱ガス, 順次脱ガス, 火山山体を通じての脱ガスについて計算結果との対比を行ないつつ概説した。火山性熱水系の進化に及ぼす, 熱水流体の組成変化および脱ガス過程・機構の変化についても概説した。

Degassing process of magma is discussed on the basis of model calculation. The model calculation is performed for the degassing of H_20-CO_2 and H_2O-NaCl system with following extreme cases; 1) closed degassing with pressure decrease, 2) closed degassing with crystallization, 3) open degassing with pressure decrease, and 4) open degassing with crystallization. The model calculation showed the variation of composition and amount of the hydrothermal fluid exsolving from a magma as a function of pressure or degree of crystallization. For example, CO_2 is concluded to exsolute from magma alwaysearlier than H_2O, therefore the release of CO_2-rich hydrothermal fluids can be released only at very early stage of the degassing. Several degassing processes of volcanic gases, including two-stage degassing, single-stage degassing, degassing of convective magma, continuous degassing and diffusive degassing from volcanic edifice, are reviewed and compared with the result of model calculation. The implication of the changes in composition and degassing mechanism with time is discussed as possible causes of an evolution of a magmatic-hydrothermal system.