13:45 〜 14:15
[SVC31-01] 減圧するマグマの酸化還元状態の変化：減圧実験からの制約
★招待講演
キーワード：マグマ、酸化還元状態、減圧、水、硫黄
Redox condition of magma controls stable mineral phases and coexisting chemical species of volcanic gases; hence, its information is essential to the understanding of magmatism in the crust and volcanic eruption. A recent petrological study (Waters and Lange, 2016) demonstrated that redox condition of hydrous rhyolite is not changed during magma ascent to the surface. However, this conflicts with laboratory experiments, that is, strongly oxidation of hydrous rhyolite during decompression was observed from decompression experiments (Humphreys et al. 2015). To resolve this problem, we focused on sulfur that is a major volatile component in magma but not considered in previous studies, and experimentally investigated redox evolution of sulfur-bearing hydrous rhyolite during decompression.
We performed decompression experiments for sulfur-bearing hydrous rhyolitic glasses. The glassy samples were sealed in a gold tube and heated at 800°C under 100 MPa in a cold-seal pressure vessel. The continuous decompressions to 10, 30 and 50 MPa were isothermally carried out with rates of 10 and 100 MPa/h. The run products were observed under SEM. Bulk sulfur contents of the samples and water and sulfur contents in glass parts of the samples were measured using EA and SIMS, respectively. To investigate redox evolution, the Fe K-edge XANES spectra of the glass parts were measured and ferric to total iron ratio (Fe3+/Fetotal) was determined based on the calibration using standard samples on loan from the National Museum of Natural History, Smithsonian Institution. The oxygen fugacity (fO2) was calculated from the measured Fe3+/Fetotal value and the empirical relationship between the Fe3+/Fetotal ratio and fO2 (Kress and Carmichael, 1991).
Our experimental results showed that the fO2 slightly decreases with decompression. No clear effect of decompression rate was observed. The water contents decreased with decompression and were almost consistent with equilibrium solubility. In contrast, the sulfur contents showed a slight decrease during decompression from 100 to 50 MPa and then no clear increase and decrease during additional decompression.
The results obtained were compared with the redox evolution predicted from the thermodynamic model of Burgisser et al. (2015). The model shows that magma is reduced if the existence of a pre-exsolved fluid phase is assumed, while magma is oxidized when it contains only water or no pre-exsolved fluid phases. This is because sulfur buffers the oxidation of magma through a reaction with oxygen in the fluid phase. In our experiments, pre-exsolved fluid phase existed and the experimental and theoretical redox evolutions are qualitatively consistent. Based on these results, we inferred that the redox condition of sulfur-bearing hydrous magma is not oxidized during explosive volcanism with a pre-exsolved fluid phase and closed-system degassing. In contrast, if magma contains only water or sulfur-bearing hydrous magma experiences open-system degassing, it may be oxidized. Finally, we conclude that the effects of fluid composition, pre-exsolved fluid phases, and degassing processes on redox evolution must be evaluated to understand redox evolution during decompression and thus to determine redox condition of magma reservoir.
We performed decompression experiments for sulfur-bearing hydrous rhyolitic glasses. The glassy samples were sealed in a gold tube and heated at 800°C under 100 MPa in a cold-seal pressure vessel. The continuous decompressions to 10, 30 and 50 MPa were isothermally carried out with rates of 10 and 100 MPa/h. The run products were observed under SEM. Bulk sulfur contents of the samples and water and sulfur contents in glass parts of the samples were measured using EA and SIMS, respectively. To investigate redox evolution, the Fe K-edge XANES spectra of the glass parts were measured and ferric to total iron ratio (Fe3+/Fetotal) was determined based on the calibration using standard samples on loan from the National Museum of Natural History, Smithsonian Institution. The oxygen fugacity (fO2) was calculated from the measured Fe3+/Fetotal value and the empirical relationship between the Fe3+/Fetotal ratio and fO2 (Kress and Carmichael, 1991).
Our experimental results showed that the fO2 slightly decreases with decompression. No clear effect of decompression rate was observed. The water contents decreased with decompression and were almost consistent with equilibrium solubility. In contrast, the sulfur contents showed a slight decrease during decompression from 100 to 50 MPa and then no clear increase and decrease during additional decompression.
The results obtained were compared with the redox evolution predicted from the thermodynamic model of Burgisser et al. (2015). The model shows that magma is reduced if the existence of a pre-exsolved fluid phase is assumed, while magma is oxidized when it contains only water or no pre-exsolved fluid phases. This is because sulfur buffers the oxidation of magma through a reaction with oxygen in the fluid phase. In our experiments, pre-exsolved fluid phase existed and the experimental and theoretical redox evolutions are qualitatively consistent. Based on these results, we inferred that the redox condition of sulfur-bearing hydrous magma is not oxidized during explosive volcanism with a pre-exsolved fluid phase and closed-system degassing. In contrast, if magma contains only water or sulfur-bearing hydrous magma experiences open-system degassing, it may be oxidized. Finally, we conclude that the effects of fluid composition, pre-exsolved fluid phases, and degassing processes on redox evolution must be evaluated to understand redox evolution during decompression and thus to determine redox condition of magma reservoir.