EVIDENCE OF PASSIVE DEGASSING THROUGH OPEN VOLCANIC SYSTEM FOR BARREN ISLAND VOLCANO (ANDAMAN SEA, NE INDIAN OCEAN) FROM FUMAROLIC GAS COMPOSITION

During the field work in February 2003 and March 2008, Barren Island (hereafter BI) volcano (Andaman Sea, NE Indian Ocean) was in a quiescent stage, with profuse fumarolic activity in the nested craters of the cinder cone located in the central part of a caldera formed by the collapse of the cone of the BI proto-volcano. The difficulties in finding suitable sampling sites (with high flux and safe) have enabled the collection of only one fumarolic gas sample. It is mainly composed of water vapour (931,321 μmol/mol), CO₂ (35,356 μmol/mol) and N₂ (15,113 μmol/mol). Highly acidic compounds (viz., SO₂, HCl and HF), commonly found in active volcanic systems, were below the detections limits (0.01 μmol/mol), indicating no direct contribution from magmatic source. Even the high H₂ content (5752 μmol/mol), normally associated with magmatic fluids, can also be due to hydrothermal-type reducing conditions. Typical hydrothermal gas compounds (viz., CH₄ and H₂S) have relatively high contents (338 and 45 μmol/mol, respectively), whereas CO, which in shallow aquifers easily hydrolyzes to produce formic acid, was not detected. In contrast, the isotopic ratios of both helium (R/Ra = 5.1) and carbon (δ¹³C–CO₂ = −2.94‰ PDB) have a clear mantle signature. Relatively low R/Ra value (5.1) can be due to dilution with a shallow hydrothermal system or mixing with a crustal component. Geothermometry based on the temperature dependence of both fH₂ and the CO₂–CH₄ pair, under the control of the FeO–Fe₂O₃ redox buffer, indicates an equilibrium temperature of about 300 °C. Relatively low discharge temperature and the dominance of H₂O–CO₂ gas mixture, with significant contents of atmospheric gases, suggest typical phenomenon of passive degassing from open volcanic systems. Thus, a well-developed hydrothermal system can be deciphered at relatively shallow depth, promoting secondary interactions capable of masking the magmatic signatures and establishing the chemical features of the discharged fluids.