Does Zircon Shape Retain Petrogenetic Information?

Abstract

AbstractZircon shape is commonly reported during geochronology and geochemistry analyses of igneous, metamorphic, and sedimentary rocks, but the relationship of zircon shape to primary growth environmental conditions remains poorly constrained. Current models for the control on igneous zircon shape focus on the relative growth of crystal prisms and pyramids, which are not discernible in the imaging techniques used for rapid quantification of zircon shape in geochronology sample mounts. We model the relationship between whole‐rock composition and zircon 2D shape in mineral separates from 45 mafic to felsic igneous samples, representative of Archean and Proterozoic crust in Western Australia. Shape parameters are derived from semi‐automated measurement of photomicrographs of polished zircon crystals in epoxy resin mounts. Whole‐rock composition shows a statistically significant relationship to median magmatic zircon crystal area and mathematically defined “roundness.” Zircon populations show reduced median area and increased median roundness as whole‐rock silica decreases. Phase equilibrium modeling based on whole‐rock composition, and automated electron microscopy mineral maps, indicates that the compositional predisposition of zircon shape is influenced by fundamentally different physical growth environments in mafic versus felsic melts. Specifically, influential factors that differ between mafic and felsic liquids include crystallization sequence and duration—which influence unconstrained growth space—and the potential for absorption/exsolution of zirconium from the accompanying mineral assemblage. We present quantitative, explanatory models for the relationship between zircon 2D shape and whole‐rock silica and demonstrate that the relationships are adhered to across a broad spectrum of whole‐rock compositions.