A guide to quantitative morphology of accessory zircon

Abstract

Traditionally, the morphological evolution of zircon crystals is presented as paths connecting morphological types arranged in rectangular grids. The present guide introduces an alternative approach overcoming the major deficiencies of the typological methods. The growth rates of individual crystal faces are normalized to a {011}-reference face and plotted as functions of the reference face's central distance. Thus, the morphological evolution of a crystal can be constrained by a set of normalized growth rate functions. The information necessary for determining these functions is obtained from the measurement of central distances and growth steps in crystallographically oriented sections, viewed with cathodoluminescence techniques. The data for individual crystals are generally not representative, but they can be joined in diagrams plotting normalized growth rates vs. central distance for a large population of crystals. In a case study, this procedure is applied to a calc-alkaline granodiorite. The normalized growth rates of the {010}-prism faces show a negative correlation with the central distance of the {011}-reference faces. This is related to the global change of a still unknown factor in the magma body. The new quantitative method should encourage further detailed correlations between morphological characteristics and petrogenetic environments. This procedure is transferable to crystals of other minerals, provided that oriented sections can be obtained and growth bands can be revealed. As a major advantage over traditional typological methods, the new method considers growth rates of individual faces which are immediately controlled by interface kinetics.