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\euroœclosed
conduitâ\euro? volcanoes). The results depict a variety of dynamic
evolutions, with the pure buoyant end-member 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.