Dynamics of magma chamber replenishment under buoyancy and pressure forces

Abstract

Active magma chambers are periodically replenished upon a combination of buoyancy and pressure forces driving upward motion of initially deep magma. Such periodic replenishments concur to determine the chemical evolution of shallow magmas, they are often associated to volcanic unrests, and they are nearly ubiquitously found to shortly precede a volcanic eruption. Here we numerically simulate the dynamics of shallow magma chamber replenishment by systematically investigating the roles of buoyancy and pressure forces, from pure buoyancy to pure pressure conditions and across combinations of them. Our numerical results refer to volcanic systems that are not frequently erupting, for which magma at shallow level is isolated from the surface (â\eurooeclosed conduitâ\euro? volcanoes). The results depict a variety of dynamic evolutions, with the pure buoyant endmember associated with effective convection and mixing and generation of no or negative overpressure in the shallow chamber, and the pure pressure end-member translating into effective shallow pressure increase without any dynamics of magma convection associated. Mixed conditions with variable extents of buoyancy and pressure forces illustrate dynamics initially dominated by overpressure, then, over the longer term, by buoyancy forces. Results globally suggest that many shallow magmatic systems may evolve during their lifetime under the control of buoyancy forces, likely triggered by shallow magma degassing. That naturally leads to long-term stable dynamic conditions characterized by periodic replenishments of partially degassed, heavier magma by volatile-rich fresh deep magma, similar to those reconstructed from petrology of many shallow-emplaced magmatic bodies.