HR: 14:10h
AN: V13G-03 INVITED     [Abstracts]
TI: Volcanic Gases Emitted During Mild-Strombolian Eruptions of Villarrica Volcano, Chile
AU: * Shinohara, H
EM: shinohara-h@aist.go.jp
AF: Geological Survey of Japan, AIST, Higashi 1-1-1, Central 7, Tsukuba, Iba 305-8567 Japan
AU: Witter, J
EM: jeff.witter@arkex.co.uk
AF: ARKeX Limited, Newton House, Cambridge Business Park, Cowley Road, Cambridge, CB4 0WZ United Kingdom
AB: The composition of volcanic gas emitted during mild Strombolian eruptions at the summit of Villarrica volcano (Chile) was estimated based on plume measurements. Villarrica volcano is characterized by continuous degassing from a persistent summit lava lake of basaltic andesite magma. The volcanic gas was emitted from a central vent of 20 m diameter where mild-Strombolian eruption continues with lava spattering from the vent. The gas compositions were measured at the rim of the summit crater about 100 m downwind of the vent, by application of a Portable Multi-Sensor System, Alkaline Filter Pack and Gas Detection Tubes. The estimated composition was; 95.0 mol% H₂O, 2.0% CO₂, 2.0% SO₂, <0.01% H₂S, 0.63% HCl and 0.23% HF, that are similar with the gases emitted during continuous passive degassing of subduction related basaltic volcanoes, such as Masaya or Miyakejima. The measured gas composition agree well with the composition estimated from the difference in H₂O, S and Cl contents of a gas-rich melt inclusion and matrix glasses, indicating that the volcanic gas was supplied as dissolved volatiles in a magma from depth and separated from the magma at very low pressure. The continuous supply of the gases with the source magma is consistent with the hypothesis of degassing of a convecting magma column, which has been proposed as a mechanism responsible for the continuous passive degassing at volcanoes (Kazahaya et al., 1994). The CO₂ content in the source magma is estimated to be 1060 ppm whose saturation pressure is 260 MPa at the coexistence with 2.1 wt.% H₂O. The measured H₂O/CO₂ and CO₂/SO₂ ratios of the gas were constant for about two hours during the measurements in spite of intermittent lava spattering from the vent, indicating that the gas emitted with the spattering lava has the same composition as the continuously emitted volcanic gas. This implies the lava spattering is not driven by bubbles supplied from depth but caused by bursting of bubbles formed under equilibrium with the magma.
DE: 8400 VOLCANOLOGY
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8419 Volcano monitoring (7280)
DE: 8430 Volcanic gases
DE: 8494 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005