Merapi Volcano, Central Java, Indonesia: A case study of radionuclide behavior in volcanic gases and its implications for magma dynamics at andesitic volcanoes

For more than 20 years, volcanic gases have been regularly collected at Merapi Volcano, Indonesia, and have been subsequently analyzed for their ( 210 Pb), ( 210 Bi), and ( 210 Po) activities and SO 2 contents. These new results show the high volatility of the three radionuclides in andesitic gases, although their emanation coefficients (0.94%, 3.5%, and ≤53% for 210 Pb, 210 Bi, and 210 Po, respectively) are significantly lower than those observed at basaltic volcanoes. This emphasizes the major role of magma temperature on the degassing of these metals, which are mainly transported in volcanic gases as Pb‐chloride compounds and as Bi‐ and Po‐metallic species. Radioactive disequilibria between 210 Pb, 210 Bi, and 210 Po in the gas phase are characteristic of degassing processes and gas paths within the edifice. Gases released at both Gendol and Woro fumarolic fields are of magmatic origin, but their radionuclide content is strongly altered by secondary processes (chemical reactions with surrounding brines and deposition of sublimates in the ground). High‐temperature gases collected in the summit gas plume are of pure primary magmatic origin and are likely directly tapped in the degassing reservoir. Gases arising from the growing domes are strongly depleted in the most volatile isotopes and gas species. This suggests that magma mostly degasses at depth in open system, explaining why the volcano usually undergoes no explosive eruptions. The gas flux at Merapi is sustained by a deep magma supply which is twice the extrusion rate of lava. Unerupted degassed magma thus progressively accumulates beneath the summit, yielding an overpressure which can trigger dome collapse without precursor.