Water and halogens in volcanic clasts: tracers of degassing processes during Plinian and dome-building eruptions
B. Villemant, G. Boudon, S. Nougrigat, S. Poteaux, A. Michel, 2003. "Water and halogens in volcanic clasts: tracers of degassing processes during Plinian and dome-building eruptions", Volcanic Degassing, C. Oppenheimer, D. M. Pyle, J. Barclay
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Magma degassing may occur either with no significant gas escape from the magma column, which corresponds with typical Plinian type eruptions, or with gas loss, which corresponds with typical effusive (dome-building) eruptions. Magma degassing may also lead to melt crystallization, which modifies the residual melt composition and, in turn, may significantly affect the degassing conditions. We propose a method for modelling these processes for H2O-rich rhyolitic melts through measurements of volatiles (H2O, CI, Br) in the microcrystalline matrix and glass of erupted volcanic clasts (pumice and dome clasts). This method is applied to two Plinian and dome-building eruptions at Mount Pelee (Martinique) and Santa Maria (Guatemala) volcanoes. Extreme magma degassing and crystallization during dome-building eruptions may explain the contrasts in halogen and H2O contents of residual melts of dense volcanic clasts: they display very large ranges of CI contents (few ppm to thousands of ppm), whereas the ranges of H2O contents are much narrower, and lower than 1%. This method allows prediction of the evolution of volcanic gas chemistry (as HC1 content or HC1/HF ratio) as a function of the degassing style of magma at shallow depth.
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Humans have long marvelled at (and feared) the odorous and colourful manifestations of volcanic emissions, and, in some cases, have harnessed them for their economic value. The degassing process responsible for these phenomena is now understood to be one of the key factors influencing the timing and nature of volcanic eruptions. Moreover the surface emissions of these volatiles can have profound effects on the atmospheric and terrestrial environment, and climate. Even more fundamental are the relationships between the history of planetary outgassing, differentiation of the Earth’s interior, chemistry of the atmosphere and hydrosphere, and the origin and evolution of life. This book provides a compilation of 23 papers that investigate the behaviour of volatiles in magma, the feedbacks between degassing and magma dynamics, and the composition, flux, and environmental, atmospheric and climatic impacts of volcanic gas emissions.