The role of volatiles exsolution in magma chambers failure

Manuscript Preprint by Annen and Pinel

Abstract:

One fundamental question in volcanology is how magma chambers fail and feed volcanic eruptions. For a magma chamber to fail, the overpressure in the chamber must exceed the strength of the solid wall rocks. It has been proposed that this overpressure could be caused by the strain induced by an increase in the chamber volume due to magma degassing. With numerical simulations of volatile exsolution from a growing and solidifying magma chamber, we show that the rate of volume increase due to volatile exsolution is too low to be the cause of magma chamber failure. Our simulations show that water transfer through the mush results in water accumulation at the periphery of the chamber. Water-rich layers get trapped in the roof and walls of the body as it solidifies; their buoyancy exerts a pressure that can exceed the rock yield strength and trigger eruptions. We expect the resulting erupted product to be crystal-rich. Magma buoyancy has been evoked as a mechanism to trigger eruption from magma chambers that are too large to fail because of magma influx. Our results show that, for a water-saturated magma, if volatiles are not continuously lost from the system, their buoyancy can be significant larger than the magma buoyancy. Bradyseismic crisis that are not followed by eruptions can be explained if volatiles accumulate and the wall rocks fail but the resulting fractures do not connect to an eruptible magma chamber.