Igneous microstructures from kinetic models of crystallization

During crystallization, a variety of competing kinetic processes determine the evolution of the igneous microstructure, yet, the relative contribution of each process remains elusive. To this end, a stochastic algorithm is developed to yield a detailed spatial representation of the igneous microstructure during progressive crystallization. This algorithm is used to test a variety of kinetic models for nucleation and crystal growth that yield realistic igneous microstructures. The algorithm itself relies on a theoretical model of crystallization (the Avrami method) and can therefore be intemally validated in addition to being verified against natural microstructures using crystal size distributions (CSDs). The most realistic simulated igneous microstructure, in which the CSDs remain approximately log-linear throughout the crystallization interval, is produced using a kinetic model involving exponential nucleation rate and constant growth rate. An extension of the algorithm is used to simulate multiply saturated mineral growth, emulating basalt crystallization through simultaneous nucleation and growth of plagioclase and clinopyroxene. A fundamentally important feature of this procedure is the numerical production of detailed 3D representations of the microstructure that can be interrogated to study many physical and chemical processes. For example, the critical crystallinity necessary for the onset of a finite yield strength and the interstitial melt flow within the igneous microstructure depend on the percolation of the solid and melt phases, respectively. Bounds on the percolation thresholds, the critical crystallinity at which the phase of interest is connected or contiguous such that it extends across the full microstructure from one face to the opposite face, are determined for the simulated igneous microstructure and are comparable with previously published results for the percolation threshold of both the melt phase and solid network. The simulated igneous microstructure can also be used as input into other physical models to ascertain the physical properties of partially molten magmas that are otherwise difficult to estimate by experiment. In a real sense, these computed microstructures are equivalent to 3D microtomographic images of partially molten basalt within a solidification front. (c) 2006 Elsevier B.V All rights reserved.