Petrologic Regime Diagrams: Parameterizing Kinetic Controls on Vesiculation and Crystallization

Regime diagrams are commonly employed in geophysical fluid dynamics to classify experimental results and, ideally, to define non-dimensional parameters that allow those results to be applied to natural systems. Petrologic experiments, in contrast, are typically run to mimic a specific natural system, and to infer conditions of magma storage, cooling or decompression. This approach has produced important insight into specific volcanoes, but the results are difficult to generalize. Additionally, very few experimental studies evaluate the vesiculation, crystallization and degassing histories of the same sample suite, an omission that is understandable given the time-consuming nature of the experiments and analysis, but which leaves important gaps in our general understanding of the interplay between gas exsolution, crystal formation and eruption dynamics.

One way to bridge these gaps is to construct a regime diagram for conditions of vesiculation and crystallization. As both are controlled by the effective supercooling experienced by the magma during cooling or decompression, one key parameter is supersaturation, although in practice, decompression rate (cooling rate) are commonly used as proxies for supersaturation. Vesiculation and crystallization are also modulated by diffusion (dependent on individual species and melt viscosity), which can be simply approximated by melt composition. Using these parameters and published data for water-saturated decompression experiments, the following fields can be (partially) defined: (1) non-equilibrium volatile exsolution, (2) equilibrium volatile exsolution, and (3) exsolution accompanied by crystallization. Melt compositions, volatile contents and crystal textures of natural samples can be measured, and thus related (crudely) to the regime diagram. Additional information required for fully linking experiments and volcanic pyroclasts includes phase proportions (crystallization efficiency), pyroclast textures (phase change conditions) and gas loss (closed- or open-system degassing). Although simplified, this approach helps to provide a kinetic framework for predicting threshold behaviors (such as transitions from explosive to effusive, or steady to pulsatory) that characterize volcanic activity.