Theoretical versus empirical secular change in zircon composition
Résumé
We generate theoretical curves for zircon growth during cooling of tonalitic and A-type granitic magmas and compare these with empirical Ti-in-zircon populations from the Paleoarchean Pilbara Craton, Australia, Mesoarchean Akia Terrane, Greenland, and the Mesoproterozoic Musgrave Province, Australia. Our models predict variable zircon growth rates on magma cooling dependant on magma composition, crystallizing assemblage, and zircon growth process. In most modelled magma compositions, higher-temperature grains growing close to the zircon saturation temperature are more abundant, with yields decreasing continuously thereafter. However, there are important dissimilarities in the cumulative zircon growth curve for different magma compositions and whether zircon growth is by equilibrium or disequilibrium processes. For a given starting melt Zr concentration, A-type granite magmas grow zircon at higher temperatures than tonalitic magmas. This compositional distinction is most pronounced at lower starting melt Zr concentrations, and in low Zr tonalite the rate of zircon growth may even increase on cooling. The dependence of zircon growth on magma composition and crystallization process leads to predictive differences in cumulative Ti-in-zircon distributions. Greater disequilibrium growth yields more sigmoidal cumulative growth curves that are dissimilar to predictions from phase equilibrium models. When applied to Mesoarchean-aged zircon grains from the Akia Terrane, calculated Ti-in-zircon temperatures decrease over the 3100-2900 Ma interval. This magmatic episode also reveals a change in cumulative zircon growth curve topology from steeper to shallower, consistent with a reduction in the relative proportion of disequilibrium growth, greater crystal-liquid communication, and enhanced infracrustal reworking. The temporal variability in cumulative zircon growth and its implication for melt interconnectedness are powerful tools in understanding magmatic processes and indicate an important secular change point at c. 3.0 Ga in the Akia Terrane where zircon growth dynamics changed.