Bromo activity over the last decade: consistent passive degassing and source magma evolution

Abstract Bromo is among the very active volcanoes in Indonesia and is known for its recurrent and long-lasting eruptive manifestations. Past volcanic gas studies have revealed Bromo as one of the principal sources of volcanic degassing in Indonesia. This high degassing from Bromo volcano is further characterized in this work, based on more than 10 years of intermittent ground-based gas measurements, combined with daily SO2 mass, captured by the OMI sensor. Over the past decade, Bromo has released 0.7 Tg of SO2 into the atmosphere, representing 3% of the volcanic degassing budget of Indonesia and 0.3% of the global volcanic SO2 emission budget outside eruptive periods. Results also reveal that 18.8 Tg of H2O, 2.0 Tg of CO2, 0.1 Tg of H2S, and 0.005 Tg of H2 were released from the Bromo volcano in one decade. About 81% of these gases are released passively between eruptive events. The chemistry of the eruptive products, sampled between 2001 and 2019, indicate that Bromo volcanic activity is sustained by a basaltic-andesite to basalt trachy-andesite magma source with a transition from medium-K to high-K composition. Such an evolution associated to a C-rich gas likely resulted from a low partial melting and sediment contribution to the genesis of the source magma. New magma injections into the reservoir and fractional crystallization have further amplified the changes of magma composition. Finally, we speculate that the shallow reservoir replenishment, in response to the continuous strong degassing is the driving mechanism behind the Bromo frequent eruptive events.