Gas bubbles form as magmas ascend in the crust and exsolve volatiles. These bubbles evolve chemically and physically as magma decompression and crystallization proceed. It is generally assumed that the gas remains in thermal equilibrium with the melt but the relationship between gas and melt redox state is debated. Here, using absorption spectroscopy, we report the composition of gases emitted from the lava lake of Kīlauea Volcano, Hawaii, and calculate equilibrium conditions for the gas emissions. Our observations span a transition between more and less vigorous-degassing regimes. They reveal a temperature range of up to 250 °C, and progressive oxidation of the gas, relative to solid rock buffers, with decreasing gas temperature. We suggest that these phenomena are the result of changing gas bubble size. We find that even for more viscous magmas, fast-rising bubbles can cool adiabatically, and lose the redox signature of their associated melts. This process can result in rapid changes in the abundances of redox-sensitive gas species. Gas composition is monitored at many volcanoes in support of hazard assessment but time averaging of observations can mask such variability arising from the dynamics of degassing. In addition, the observed redox decoupling between gas and melt calls for caution in using lava chemistry to infer the composition of associated volcanic gases.
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This work was supported by the Natural Environment Research Council (through the Centre for the Observation and Modelling of Volcanoes, Earthquakes and Tectonics and grant NE/N009312/1) and LabEx VOLTAIRE (ANR-10-LABX-100-01). Y.M. received additional support from the Leverhulme Trust. We thank P. Kelly (US Geological Survey) for his review of the pre-submission manuscript. We are grateful to V. Tsanev for discussion on gas radiation at high temperature and pressure.
The authors declare no competing interests.
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Frontiers in Earth Science (2019)
Measuring SO2 Emission Rates at Kīlauea Volcano, Hawaii, Using an Array of Upward-Looking UV Spectrometers, 2014–2017
Frontiers in Earth Science (2018)