HR: 09:15h
AN: V11A-06    [PDF]
TI: On the Relation Between Texture, Crystallinity, Nucleation, and Growth in Basaltic Rocks: A Numerical Approach
AU: * Hersum, T G
EM: hersum@jhu.edu
AF: Earth & Planetary Sciences, Johns Hopkins University, Baltimore, MD 21218
AU: Marsh, B D
EM: bmarsh@jhu.edu
AF: Earth & Planetary Sciences, Johns Hopkins University, Baltimore, MD 21218
AB: One of the most challenging problems in discovering the fundamentals of magma crystallization is finding a proper metric or standard state to which to compare theoretical models of crystallization. This stems from the more basic problem of quantifying rock textures themselves. The critical information is the size, shape, number, and position in local 3D space of all crystals in a sample and then also the variation of these parameters globally within the thermal regime space of the pluton itself. Fortunately, it appears from the Crystallization Axiom (in essence, magmas become holocrystalline regardless of the thermal regime) that the kinetic problem can be separated from the thermal problem (i.e., large Avrami number). CSDs quantitatively link crystal size and number, leaving shape and position, including nucleation and growth reactions with neighbors, yet to be quantified. What we attempt here is to produce textures that first satisfy observed CSDs and then to compare the resulting spatial features of the derived texture to that of the real rock. A stochastic algorithm is used to generate a discretized three-dimensional spatial representation of simultaneous nucleation and crystal growth of randomly orientated and positioned crystals. The algorithm is simplified by assuming large Avrami number, thus allowing crystallinity to be calculated as a function of time using the Avrami method (i.e., JMA equation) and a kinetic model for crystal nucleation and growth. To begin with, we consider a simple kinetic model of exponential nucleation rate and constant crystal growth rate that reconcile observed batch CSD trends in natural samples. The crystallinity function (i.e., bulk crystal content as a function of time) and crystal growth rate model can uniquely determine the number and size of crystals during a simulation and a numerical nucleation rate is calculated as an output variable to compare with the analytical result as a condition for model acceptance. For a given time step during crystallization, having begun with a burst of nucleation, the crystallinity must be satisfied by first allowing growth of randomly chosen, pre-existing crystals and second, if and only if crystal mass is still available (i.e., growth having not satisfied the bulk crystallinity constraint), allowing creation of randomly located nuclei. Results of monomineralic simulations with the same crystallization parameters (i.e., total crystallization time, crystal growth rate, domain length) show that the number and mean sizes of crystals are sensitive to the degree of spatial and temporal discretization within the model. For a particular crystallization parameter group, a running average of the number and mean sizes of crystals with increasing realizations converges and a histogram of those metrics approximate a Gaussian distribution. For some time steps during most simulations, the crystallinity is completely satisfied by crystal growth and no additional nuclei are generated, which suggests that the kinetic model of steady exponential nucleation rate and constant growth rate is incorrect at the detailed level, even though it may appear true in the bulk product. With an increased exponential nucleation rate, however, this occurrence is less common and the numerically calculated CSD approaches a linear trend as naturally observed. The algorithm is also extended to multiphase crystallization, which simulates the simultaneous crystallization of plagioclase and clinopyroxene in tholeiitic basalt. Overall, reasonable adjustments nucleation and growth can lead to realistic histories of crystallization, even though the detailed processes of growth and nucleation may be, in reality, much more involved.
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting