Zircon trace element geochemistry: partitioning with garnet and the link between U–Pb ages and metamorphism

Abstract

With the aim to link zircon composition with paragenesis and thus metamorphic conditions, zircons from eclogite- and granulite-facies rocks were analysed for trace elements using LA-ICP-MS and SHRIMP ion microprobe. Metamorphic zircons from these different settings display a large variation in trace element composition. In the granulites, zircon overgrowths formed in equilibrium with partial melt and are similar to magmatic zircon in terms of high Y, Hf and P content, steep heavy-enriched REE pattern, positive Ce anomaly and negative Eu anomaly. They are distinguishable from magmatic zircon because of their low Th/U ratio. Independently of whole rock composition, metamorphic zircon domains in eclogite-facies rocks have low Th/U ratio and reduced HREE enrichment and Eu anomaly. In a low grade metamorphic vein, zircon has low Th/U ratio but is extremely enriched in Y, Nb and HREE. Petrological and geochronological data demonstrate that metamorphic zircon overgrowths crystallised at granulite-facies conditions in equilibrium with unzoned garnet. It is thus possible for the first time to calculate trace element distribution coefficients between zircon and garnet. Hf is the elements that most strongly partition into zircon. Y, Nb and REE have distribution coefficients between 90 and 0.9 with minimum values for the MREE. In eclogite-facies rocks, the HREE depletion in metamorphic zircon domains is attributed to concurrent formation of garnet under sub-solidus conditions. In one sample, the zircon/garnet trace elements partitioning indicates that metamorphic zircon formed in equilibrium with the garnet rim, i.e. at the eclogitic peak. The reduced Eu anomaly in the metamorphic zircon is interpreted as indicating absence of feldspars and thus supports zircon formation in eclogite facies. In a metamorphic vein within the eclogite-facies rocks, zircons have larger Eu anomaly with respect to high-pressure zircon. Together with geochronological evidence, the Eu anomaly suggests that these zircons formed during prograde metamorphism, before the break down of feldspars at high pressure. The REE composition of zircon can therefore relate zircon formation to specific metamorphic stages such as eclogite, granulite or greenschist facies. This allows linking zircon U–Pb ages with pressure–temperature conditions, a fundamental step in constraining rates of metamorphic processes.