Hendrasto, M.

The composition and fluxes of volcanic gases released by persistent open-vent degassing at Bromo Volcano, east Java (Indonesia), were characterised in September 2014 from both in-situ Multi-GAS analysis and remote spectroscopic (dual UVcamera)measurements of volcanic plumeemissions. Our results demonstrate that Bromo volcanic gas is water-rich (H2O/SO2 ratios of 56–160) and has CO2/SO2 (4.1 ± 0.7) and CO2/Stot (3.2 ± 0.7) ratios within the compositional range of other high-temperature magma-derived gases in Indonesia. H2/H2O and H2S/SO2 ratios constrain a magmatic gas source with minimal temperature of ~700 °C and oxygen fugacity of 10-17–10-18 bars. UV camera sensing on September 20 and 21, 2014 indicates a steady daily mean SO2 output of 166 ± 38 t d−1, which is ten times higher than reported from few previous studies. Our results indicate that Bromo ranks amongst the strongest sources of quiescent volcanic SO2 emission measured to date in Indonesia, being comparable to Merapi volcano in central Java. By combining our results for the gas composition with the SO2 plume flux, we assess for the first time the fluxes of H2O (4725 ± 2292 t d−1), CO2 (466 ± 83 t d−1), H2S (25 ± 12 t d−1) and H2 (1.1 ± 0.8) from Bromo. Our study thus contributes a new piece of information to the still limited data base for volcanic gas emissions in Indonesia, and confirms that much remain to be done to fully assess the contribution of this very active arc region to global volcanic gas fluxes.