The Distribution and Separation of Crystals from Melt During Magmatic Evolution

Abstract:

The chemical evolution of magmas and the great compositional diversity observed in igneous rocks is ultimately driven by physical process including multiphase convection in magmatic systems and melt-crystal separation (e.g. Marsh and Maxey, 1985). Key to quantitative description of these systems is to accurately calculate the rate and timing of crystal-melt separation. This calculation requires accurately computing four separate, but closely linked, quantities: heat transfer, phase equilibria, crustal stresses and multiphase dynamics, all of which can play an important role in determining the compositional and melt fraction histories of magmatic bodies. Both heat transfer and crustal stresses influence the state variables of temperature and pressure controlling phase equilibria, and melt dynamics governs the distribution of chemical species and motion of physically distinct phases.

We use a coupled computational approach to examine the interaction of these processes in shaping measurable geochemical and geophysical processes. Previous work using this technique showed that one mechanism for the production of compositional (Daly) gaps was controlled by the joint probability of melt extraction efficiency (driven by melt-crystal dynamics) and the probability in time of the occurrence of distinct melt fractions (primarily driven by thermal processes). In the present work we expand this analysis to examine the temporal evolution of these probability distributions and consider dynamics in active reservoirs during recharge events, their relation to melt chemistry, zircon saturation, and introduce over-pressure within a framework of eruption probability. We analyse systems that span a range of sizes from oceanic rhyolites to the Fish Canyon Tuff, and examine the mixing of magmatic parcels leaving the reservoir from a crystal or crystal cluster perspective. We also examine the heterogeneity of accumulated stresses and chemistry of the residual material in the putative plutonic residue that is corollary to extracted material. In most instances the last stage of melt evolution for silicic systems requires melt extraction in a relatively narrow window of crystallinity between 50-70%.