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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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