Linking experimental and natural vesicle textures in Vesuvius 79AD white pumice

Vesicle populations in volcanic pumice provide a partial record of shallow magma ascent and degassing. Here we compare pumice textures from the well-characterized 79AD Vesuvius eruption to those generated during isothermal decompression experiments. Three series of experiments were conducted using starting material from the first two phases of the eruption (eruptive units EU1 and EU2). Samples were decompressed from 100 or 150 MPa to final pressures of 10–25 MPa using conditions appropriate for simulating eruption conditions (T = 850 °C, dP/dT = 0.25 MPa/s). The experiments differed not only in starting material but also in temperature at which samples were annealed prior to decompression, which determined the initial number of crystals present in the melt. Results show that experiments approach the vesicle number densities and sizes of pumice samples, but show narrower size distributions. The wider size range of pumice samples suggests continuous, rather than instantaneous nucleation, which may reflect non-linear rates of decompression. All experiments exhibited equilibrium degassing, a process that was probably aided by heterogeneous bubble nucleation on oxide microlites. We conclude that delayed bubble nucleation cannot explain the explosivity of the Vesuvius eruption, which instead appears to require high rates of magma decompression.