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Competitive diffusion-controlled growth of porphyroblasts

Published online by Cambridge University Press:  05 July 2018

William D. Carlson
William D. Carlson*
Affiliation:
Department of Geological Sciences, University of Texas at Austin, Austin TX, 78713, U.S.A.
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Abstract

In a diffusion-controlled process of nucleation and growth, adjacent porphyroblasts compete with one another for nutrients. When the effects of this competition are evaluated quantitatively for garnet porphyroblasts in pelitic rocks from the Picuris Range of New Mexico (U.S.A.), significant correlations arise between crystal sizes and the volumes of the domains from which the crystals drew their nutrients. These correlations strengthen the conclusion drawn from earlier work on spatial dispositions, zoning patterns, and crystal size distributions that the kinetics of intergranular diffusion governed the crystallisation of these porphyroblasts.

Computer simulations indicate that competition for nutrients during diffusion-controlled growth may have small but detectable effects on crystal size frequency distributions. Diffusional competition therefore introduces relatively minor inaccuracies into attempts to extract quantitative information on crystallisation processes from size distributions using models for the growth of isolated porphyroblasts. In contrast, the effects of diffusional competition on patterns of compositional zoning may be substantial, especially for porphyroblasts in rocks for which chemical inhomogeneity of the precursor leads to strongly clustered spatial dispositions. In such rocks, clustering may alter the patterns of compositional zoning in ways that obscure evidence for diffusion-controlled growth.

Keywords

metamorphic crystallisationintergranular diffusionreaction kinetics
Type
Research Article
Information
Mineralogical Magazine , Volume 55 , Issue 380 , September 1991 , pp. 317 - 330
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1991

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