Geochemical characterisation of shallow pre-eruptive magma degassing at Marapi Volcano, Sumatra (Indonesia)

Abstract Marapi, Sumatra’s most persistently active volcano, exhibits frequent short-lived explosive activity and sporadic prolonged eruptions, often occurring with little to no detectable geophysical precursory signals. Despite advances in monitoring, our understanding of Marapi’s eruptive dynamics remains limited by a lack of information on its gas emissions. Here we present the first chemical dataset of Marapi’s volcanic gases measured using a Multi-component Gas Analyser System (Multi-GAS) from the rim of the active Verbeek crater during inter-eruptive activity in 2014 and 2018. Both datasets reveal gas compositions typical of magma-derived arc gases and a puffing degassing regime, with a periodicity of ~ 26 min in 2014 that decreased to 8–10 min in 2018. Notably, our 2018 measurements—performed just one month before two explosive outbursts on 27 April and 2 May 2018—show comparable SO₂/H₂S molar ratios (3.3 and 2.5) but a marked increase in total sulphur relative to CO 2 , resulting in a CO 2 /SO 2 ratio more than three times lower than in 2014. This chemical shift is consistent with shallower magma degassing preceding the April and May 2018 explosive activity. Our findings underscore the importance of regular, ideally continuous, gas monitoring at Marapi to better resolve magmatic processes that drive its explosive activity. We also report the first CO 2 /S T gas ratio measured for a Sumatran volcano, providing further evidence for sulphur-limited degassing along the Sunda Arc compared with other subduction zones.