Abstract
We modeled 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 channeled 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 crystallizes 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 depend on whether the water is further released by fracturing. Our results account 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.
Highlights
Magmas are mixtures of melt, crystals, and volatiles
3.1 Distribution of exsolved H2O In the simulation, the exsolved water is arrested at the interface between mush and melt-rich magma ( c0.7, bubbles are trapped by crystals)
In the limiting case where the emplacement rate is too low for a magma chamber to form at all over the duration of the simulation, the concentration of water is limited by the size of the sills (Fig. 6 g-i)
Summary
Magmas are mixtures of melt, crystals, and volatiles. At depth, they contain various dissolved volatile species, including H2O, CO2, and SO2. Magma decompression or crystallization induces volatiles exsolution and formation of an independent phase of supercritical fluid. Analytical and thermodynamical models have been used to investigate the conjugate effect of crystallisation and volatiles exsolution on chamber pressure (Blake, 1984; Boichu et al, 2008; Tait et al, 1989) and magma fragmentation (Fowler and Spera, 2008) in order to evaluate the role of volatiles in triggering eruptions. We tested the influence of magma chamber growth rate and magma composition, including initial water content, on exsolved volatile spatial and temporal distributions. At the pressures and temperatures inside the magma body, the exsolved volatiles are supercritical fluids; for simplicity, we still use the terms bubbles. We will use the term water independently of its state
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