Modeling of gas‐driven magmatic overturn: Tracking of phenocryst dispersal and gathering during magma mixing

We present a combined multiphase numerical and crystal‐tracking approach that provides a framework to investigate the transport and zoning of crystals associated with a gas‐driven mixing event. Mixing in compositionally intermediate to silicic magmatic systems is often initiated by gas exsolution in the recharging magma, causing a density inversion and subsequent overturn. The overturn is simulated for a range of bubble volume fractions ɛ bubbles and therefore indirectly for a range of Reynolds numbers Re . All simulations show chaotic flow dynamics with fast overturn timescales of minutes to hours. The large‐scale mixing is inefficient during a single overturn, resulting in a continuously stratified system with respect to bubble volume fraction. The crystal‐tracking algorithm provides us with information on the small scales, i.e., 10 cm. On this length scale we observe gathering of different crystals during the overturn that typically ranges on the order of tens of meters. Thus, a complex crystal population may arise within a single overturn. This gathering and dispersal of crystals is strongest and most uniform for high Re . For low Re , crystal populations are characterized by less gathering of crystals that originated from distal portions of the magma body. During the overturn the crystals pass through environments of changing chemical potential. We apply the Damköhler number Da , which compares the crystallization or dissolution to the advection timescale. Results show an asymmetry between crystallization and dissolution. Crystallization times are too slow during gas‐driven overturn to record transient changes in chemical potential. Crystals most likely only record their initial as well as their final chemical environment. In contrast, dissolution and advection rates are of similar order, suggesting potential dissolution during the overturn. On the basis of the results for gas‐driven overturn we expect that slower physical mixing processes may be continuously recorded in the zonation pattern of the crystal phases as long as the changes in chemical potential produce crystal growth.