Re-evaluation of the role of volatiles in the rupture of magma chambers and the triggering of crystal-rich eruptions

The role of volatile exsolution in triggering volcanic eruption is re-evaluated using numerical simulations of a growing and solidifying magma chamber. The modelled magma chamber is silicic, emplaced at about 2 Kbar, and H2O-saturated. The rate of volume additions to the chamber by the release of volatiles is controlled by cooling and solidification and is significantly lower than magma infilling rates. Thus, the increase in volume due to volatile exsolution is unlikely to be the primary cause of magma chamber failure unless the magma chamber is already on the brink of failure. However, the formation of buoyant H2O rich layers within the mush that sur-rounds the magma chamber makes the system gravitationally unstable. If the host rock is not permeable and volatiles accumulate, their buoyancy might overcome the strength of the wall resulting in dyke injection and possibly an eruption. Eruptions that are triggered or facilitated by volatiles exsolution are associated to the presence of mush in the chamber and are expected to be crystal-rich. Triggering of eruption by volatiles buoyancy explains the common occurrence of large ranges of crystals ages and protracted crystal residence times at low temperature.