Heterogeneous bubble nucleation and disequilibrium H2O exsolution in Vesuvius K-phonolite melts

This study focuses on constraining bubble nucleation and H2O exsolution processes in alkalic K-phonolite melts, using “white pumice” of the 79 AD eruption of Vesuvius as starting material. The first set of experiments consisted of H2O solubility runs at 1153 to 1250 K and pressures between 50 and 200 MPa, to constrain equilibrium water concentrations along the decompression pathways. The decompression experiments were equilibrated with H2O at 150 MPa and 1173 and 1223 K, and then decompressed at 3 to 17 MPa/s before rapid quenching. Experiments nucleated bubbles within the first 50 MPa pressure drop, producing maximum bubble number densities (NV), corrected to melt volume, of 3.8 × 1014 m− 3 at 1173 K and 4.3 × 1013 m− 3 at 1223 K. Most bubbles were not visibly attached to crystals, except for a subset attached to pyroxenes primarily in the 1173 K experiments. When compared with prior bubble nucleation studies, the reduced nucleation ΔP and relatively low NV observed indicate predominantly a heterogeneous nucleation mechanism. Melt–vapor–crystal wetting angles measured in 1173 K experiments from bubbles attached to pyroxene crystals are 36 to 69°, which are similar to those measured on titanomagnetite crystals in calc-alkaline dacite melts. The 1223 K experiments have porosities and water concentrations that largely track equilibrium, despite the rapid decompression rate. The 1173 K experiments deviate strongly from equilibrium trends in both porosity and water concentration, and slower H2O diffusion rates are likely the cause of the inhibited bubble growth. Bubble number densities from 79 AD Vesuvius natural EU2 pumice are relatively high (2 to 4 × 1015 m− 3; [Gurioli, L., Houghton, B.F., Cashman, K.V., Cioni, R., 2005. Complex changes in eruption dynamics during the 79 AD eruption of Vesuvius. Bull. Volcanol. 67: 144–159.]) when corrected to vesicularity. In comparison, corrected NV's from homogeneous and heterogeneous bubble nucleation experiments from this study and prior work are at least factor of 5 lower, indicating perhaps that the natural magmas initially nucleated bubbles in the presence of CO2. The disequilibrium H2O exsolution seen in the 1173 K experiments indicates that inhibited bubble growth could lead to delayed exsolution in the conduit in cooler K-phonolite magmas.