Modeling water exsolution from a growing and solidifying felsic magma body

International audienceWe modelled water exsolution from a growing and crystalizing felsic magma body using a conductive thermal model with simplified physical mechanisms of volatile transport. Magma solidification leads to the release of the water initially dissolved in the melt. Solidification behaviour depends, in turn, on dissolved water content. The water is channelled where the magma is solidified enough to form a crystal network and rapidly ascends until it encounters solid rock or liquid-rich magma. The rate of water exsolution depends on the basal magma emplacement rate, the cooling rate, and the magma initial water content. Water-rich layers form and are eventually trapped in the solid rock as cooling and crystallization proceeds. We ran our model with a quasi-eutectic granite composition and a non-eutectic monzogranite composition. The non-eutectic magma crystallises on a wider range of temperature than the eutectic magma, produces more mush, and is more prone to the formation of water-rich lenses. Exsolved water accumulates on the sides of the chamber for both tested compositions and also above the top of the chamber for the non-eutectic composition. The actual presence of these water-rich layers in nature and their lifetime depends on whether the water is further released by fracturing. Our results accounts for the observed decoupling of volatiles from magma and for the episodic nature of volcano deformation. It has implications for the interpretation of magma chambers tomography as water-rich lenses would be difficult to distinguish from melt-rich lenses