11:30 〜 11:45
[SVC29-08] Comparison of redox states of volcanic glass and gas at Bromo volcano, Indonesia: Implications for the degassing process
キーワード：酸化還元状態、火山ガラス、火山ガス、XAENS、ブロモ火山
Degassing process driving volcanic activity is strongly influenced by the redox state of magma. The redox state has been represented by the oxygen fugacity (fO2). The fO2 of magma has been evaluated from both eruptive products and volcanic gas. Comparison of the fO2 of the melt and gas phases in the same volcano is expected to provide insights into the degassing process. However, few studies have compared fO2 values.
We examined the redox state of magma recorded in juvenile glass particles from an ash-forming eruption at Bromo volcano, Indonesia, to obtain the implications for its degassing process. An ash sample was collected by real-time sampling from the ash-forming eruption on 24 March 2011, and contains brown colored juvenile glasses. Detailed observations of texture under a FE-EPMA show that the brown juvenile glasses lack nanolites in their groundmass. The compositions of groundmass glass and phenocrysts in the brown glass particles indicate a pre-eruptive magma temperature of 1000 ± 40 °C. The Fe3+/ΣFe ratios of the brown glasses were determined to be 0.15–0.24 using synchrotron-based Fe–K edge micro XANES spectroscopy. From these data, the oxygen fugacity of the magma is estimated to be in the range of 10−11.2 to 10−9.8, giving a redox state of ΔQFM = 0.58 ± 0.45. The redox state of magma estimated from the brown glasses is more reduced than that of volcanic gas emitted during the open-vent degassing phase (~700 °C and ΔQFM of 1.8). The low temperature and oxidized condition of the volcanic gas can be explained by closed-system cooling from 1000 to 700 °C in the gas phase after outgassing, suggesting that the magma head was located deeper during the open-vent degassing phase than during the ash-forming eruption.
We examined the redox state of magma recorded in juvenile glass particles from an ash-forming eruption at Bromo volcano, Indonesia, to obtain the implications for its degassing process. An ash sample was collected by real-time sampling from the ash-forming eruption on 24 March 2011, and contains brown colored juvenile glasses. Detailed observations of texture under a FE-EPMA show that the brown juvenile glasses lack nanolites in their groundmass. The compositions of groundmass glass and phenocrysts in the brown glass particles indicate a pre-eruptive magma temperature of 1000 ± 40 °C. The Fe3+/ΣFe ratios of the brown glasses were determined to be 0.15–0.24 using synchrotron-based Fe–K edge micro XANES spectroscopy. From these data, the oxygen fugacity of the magma is estimated to be in the range of 10−11.2 to 10−9.8, giving a redox state of ΔQFM = 0.58 ± 0.45. The redox state of magma estimated from the brown glasses is more reduced than that of volcanic gas emitted during the open-vent degassing phase (~700 °C and ΔQFM of 1.8). The low temperature and oxidized condition of the volcanic gas can be explained by closed-system cooling from 1000 to 700 °C in the gas phase after outgassing, suggesting that the magma head was located deeper during the open-vent degassing phase than during the ash-forming eruption.