THE GENERATION OF OVERPRESSURE IN FELSIC MAGMA CHAMBERS BY REPLENISHMENT

Abstract

Evidence of magma mixing is common in the products of explosive felsic eruptions and it is generally accepted as a common mechanism for triggering such events. In order to quantify the potential for magma mixing to trigger explosive eruptions, we have developed a simple analytical model, based on previous models of magma chamber replenishment, which considers the injection of volatile-rich mafic magma into a chamber occupied by a homogeneous, volatile-rich felsic magma. We assume that the overpressure caused by the injection of new magma is not sufficient to trigger an eruption, for which additional overpressure is required. Two mixing-related mechanisms have traditionally been considered to have the potential to generate the additional overpressure: (1) exsolution of volatiles from the felsic magma during forced convection, and (2) exsolution of volatiles from the mafic magma by oversaturation during cooling and subsequent crystallization. Our calculations suggest that exsolution of volatiles from the felsic magma is not an effective mechanism to generate additional overpressure. However, significant overpressure can be achieved by volatile exsolution from the mafic magma during its cooling and crystallization. The time scale between intrusion and eruption considered in our model is of the order of a few days to a few months, which coincides with petrological and geophysical evidence obtained from magma mixing related eruptions. We also suggest that replenishment of shallow felsic magma chambers by mafic magma in most cases does not lead to large scale mixing, as eruption will occur before thermal equilibrium between the two magmas is reached, so that density and viscosity contrasts between the two magmas will remain significant.