Interpreting Europium Anomalies in Zircon: The Importance of Source Redox

Abstract

The sources of granitoids are variably oxidized due to the diversity of environments in which they form. This environmental and consequent chemical variability leads to differences in mineral assemblages, proportions, and compositions of ferric and ferrous iron-bearing phases in these sources and in the resultant granitoids. Accessory minerals that grow or recrystallize during high-temperature metamorphism and/or melt crystallization can record such redox variations through the incorporation of redox-sensitive trace elements, notably europium. Here, we use petrological modeling to explore how variations in ferric/ferrous iron ratios of a metapelite and a hydrated (meta)basite influence the speciation of Europium (Eu3+ vs. Eu2+) in the melt and resultant Europium anomalies in zircon using phase equilibrium modeling and mass balance. Europium anomalies in zircon sourced from metapelites are generally insensitive to the proportions of ferric to ferrous iron, except at very reducing conditions. Europium anomalies in zircon sourced from metabasites are influenced by the proportion of ferric iron in the source, and more so in the absence of residual garnet. The amounts of plagioclase—which is commonly linked to pressure—play a relatively minor role in the Europium anomalies of zircon in the metabasite. Hence, Europium anomalies in zircon may not be an appropriate tool on their own to unravel past tectonic processes, including the thickness of Earth's continental crust.