In-situ observations of bubble growth in basaltic, andesitic and rhyodacitic melts

Bubble growth strongly affects the physical properties of degassing magmas and their eruption dynamics. Natural samples and products from quench experiments provide only a snapshot of the final state of volatile exsolution, leaving the processes occurring during its early stages unconstrained. In order to fill this gap, we present in-situ high-temperature observations of bubble growth in magmas of different compositions (basalt, andesite and rhyodacite) at 1100 to 1240 °C and 0.1 MPa (1 bar), obtained using a moissanite cell apparatus. Experiments show that heterogeneous nucleation of bubbles occurs in one single event and at very small degrees of supersaturaturation, due to the formation of crystal nuclei. Melt degassing is the driving mechanism of bubble growth at early stages, whereas coalescence becomes increasingly important as exsolution progresses. Ostwald ripening is less efficient and occurs only at late stages, if heterogeneous bubble size distributions (BSD) are achieved. The average bubble growth rate (GR) ranges from 3.4*10-6 to 5.2*10-7 mm/s, with basalt and andesite showing faster growth rates than rhyodacite. The bubble number density (NB) at nucleation ranges from 7.9*104 mm-3 to 1.8*105 mm-3 and decreases exponentially over time, because of bubble coalescence. The coalescence rate controls the evolutions of the BSD, with higher rates increasing the heterogeneity of BSDs by adding multiple modes to the distribution. In natural samples, such BSD´s may be misinterpreted as resulting from several separate nucleation events. Incipient crystallization upon cooling of a magma may allow bubble nucleation already at very small degrees of supersaturation and could therefore be an important trigger for volatile release and explosive eruptions.