Ce in zircon: a proxy for the oxidation state of crustal magmas

The distribution of Yb between different sectors of a synthetic zircon crystal (40 µm diameter), imaged by NanoSIMS

Excursions in Ce and Eu concentrations in the otherwise smooth REE pattern of zircon arise due to the variable oxidation states of Ce (3+ and 4+) and Eu (2+ and 3+) relative to the exclusively trivalent state of the other REE, resulting in preferential uptake of Ce4+ in zircon and exclusion of Eu2+. The magnitude of these anomalies in zircon depends on the absolute concentrations of Ce and Eu in the silicate melt and the relative proportions of Ce3+ and Ce4+, and Eu2+ and Eu3+, which is primarily a function of the oxygen fugacity (fO2) of the system but is also influenced by temperature, pressure and the composition of the melt. The abundance of Eu in the magma may be depleted by the crystallisation of minerals, which favour Eu2+, such as plagioclase and so the resulting negative Eu anomaly is convolved with the signal arising from the incompatibility of Eu2+ in zircon. This is not the case for Ce and as a result, Ce anomalies preserved in zircon, which is resistant to alteration, are a powerful potential indicator of magmatic processes. Zircon-melt partitioning experiments allow the magnitude of Ce anomalies to be calibrated as a function of fO2, temperature, pressure and melt composition, and the results from over 350 experiments covering a wide range of geological conditions, which are applicable to natural samples are presented. This dataset was modelled using a multi-component regression method to produce an expression that relates the magnitude of the Ce anomaly in zircon to the fO2 of a silicate melt. This is an important geochemical tool, and when applied to natural zircons, this provides insights into (1) the controls on porphyry-Cu deposit formation and (2) crustal conditions on the Hadean Earth.