Abstract:
During crystallization of silicate magmas, silica polymers advect into the reaction zone carrying other chemical species with them. If some species forming minerals are more abundant relative to Si in the liquid than in the minerals, then the species become more concentrated in the reaction zone as crystallization progresses. This enrichment may cause mineral growth rates to increase, thereby accelerating the advection of silica and further increasing the concentrations. This positive feedback may be slowed down by the depletion of other reactant species that have lower concentration ratios to Si in the liquid than in the minerals. A quantitative transport-reaction model that incorporates these competing effects, mass-action mineral-growth rate laws, and diffusion plus advection, shows that the feedback can cause oscillatory crystallization. The model represents a plausible mechanism for the origin of repetitive igneous layering. It predicts the repeated layering itself, its occasional occurrence in general, its common occurrence in alkaline rocks, and paired layering. Also, it accounts directly for a significant property of microrhythmic layering: that the oscillatory modes of two or more minerals are spatially staggered.