Redox state of magma recorded in volcanic glass from an ash-forming eruption at Bromo volcano, Indonesia: Insights into the degassing process

We quantified the redox state of magma recorded in volcanic glass particles from an ash-forming eruption at Bromo volcano, Indonesia, to obtain insights into its degassing process. An ash sample was collected by real-time sampling from the ash-forming eruption on 24 March 2011. The ash sample contains juvenile glass particles with a brown-colored, transparent appearance. Detailed observations of texture under a field-emission–electron probe micro analyzer show that the brown glasses lack nanoscale crystals in their groundmass. The compositions of groundmass glass (trachy-andesite) and phenocrysts in the brown glass particles indicate a pre-eruptive magma temperature of 1060 ± 20 °C. The Fe 3+ /ΣFe ratios of the brown glasses were determined to be 0.15–0.24 using synchrotron-based Fe–K edge micro X-ray absorption near-edge structure spectroscopy. From the chemical composition, temperature, and Fe 3+ /ΣFe data, the oxygen fugacity of the magma is estimated to be in the range of 10 −10.3 to 10 −8.9 , yielding a redox state of ΔQFM = 0.57 ± 0.70. 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 1060 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. The comparison between the redox states of volcanic glass and gas reported in this study gives valuable insights into the degassing process driving volcanic activity.