Experimental simulation of bubble nucleation and magma ascent in basaltic systems: Implications for Stromboli volcano

Abstract

The ascent of H2O- and H2O-CO2-bearing basaltic melts from the deeper to the shallower part of the Stromboli magmatic system and their vesiculation were simulated from decompression experiments. A well-studied “golden” pumice produced during an intermediate- to a large-scale paroxysm was used as starting material. Volatile-bearing glasses were synthesized at an oxygen fugacity ![Formula][1] ranging from NNO−1.4 to +0.9, 1200 °C and 200 MPa. The resulting crystal- and bubble-free glasses were then isothermally (1200 °C) decompressed to final pressures P f ranging between 200 and 25 MPa, at a linear ascent rate of 1.5 m/s (or 39 kPa/s) prior to be rapidly quenched. Textures of post-decompression glasses that were characterized by X-ray computed tomography result from different mechanisms of degassing that include bubble nucleation, growth, coalescence, and outgassing, as well as fragmentation. Homogeneous bubble nucleation occurs for supersaturation pressures (difference between saturation pressure and pressure at which bubbles start to form homogeneously, Δ P HoN) ≤ 50 MPa. In the CO2-free melts, homogeneous nucleation occurs as two distinct events, the first at high P f (200–150 MPa) and the second at low P f (50–25 MPa) near the fragmentation level. In contrast, in the CO2-bearing melts, multiple events of homogeneous bubble nucleation occur over a substantial P f interval along the decompression path. Bubble coalescence occurs in both H2O- and H2O-CO2-bearing melts and is the more strongly marked between 100 and 50 MPa P f. The CO2-free melts follow equilibrium degassing until 100 MPa P f and are slightly supersaturated at 60 and 50 MPa P f, thus providing the driving force for the second bubble nucleation event. In comparison, disequilibrium degassing occurs systematically in the CO2-bearing melts that retain high CO2 concentrations. Fragmentation was observed in some CO2-free charges decompressed to 25 MPa P f and is intimately associated with the occurrence of the second bubble nucleation event. Textures of H2O-CO2-bearing glasses reproduce certain critical aspects of the Stromboli natural textures (bubble number densities, shapes, sizes, and distributions) and chemistries (residual volatile concentrations). Average bubble sizes, bubble size distribution (BSD), and bubble number density (BND) data are used together to estimate that the “golden” pumice magmas ascend from their source region in 43 to 128 min. [1]: /embed/mml-math-1.gif