MELT COMPOSITION AND OXYGEN FUGACITY INFLUENCE on CE AND EU ANOMALIES IN ZIRCON

Zircon chondrite normalized rare earth element (REE) profiles generally increase monotonically from La to Lu, with two notable exceptions: they often have enriched Ce abundances relative to La and Pr, and depleted Eu abundances relative to Sm and Gd. The magnitude of a zircon Ce anomaly is often attributed to an increase in Ce4+/Ce3+ of the crystallizing medium (Ce4+ is more compatible than Ce3+ in zircon), which is generally associated with more oxidizing environments. Conversely, a negative Eu anomaly is indicative of more reducing conditions, implying a higher Eu2+/Eu3+ ratio. Natural zircons may display both positive Ce anomalies and negative Eu anomalies; this observation results in an apparent contradiction because Eu should only be found in the trivalent state at oxygen fugacities where Ce4+ is stable (Robie et al. 1979).

In order to gain some insight into this apparent inconsistency and to determine whether the anomalies can be correlated with the oxygen fugacity of the host melt, zircons were synthesized in hydrous peralkaline, metaluminous, and peraluminous melts from 800-1300oC at 10 kbar. The oxygen fugacity was buffered at Mo-Mo2 (~IW), NNO, and Ru-RuO2 (HM+1) and bracketing REEs were included (e.g., La, Pr) in order to decouple changes in partition coefficients caused by melt composition.

Results show a systematic increase in zircon Ce anomalies that correlate with higher oxygen fugacities and lower crystallization temperatures; Eu anomalies are more negative at ~IW vs. NNO for the same temperature and melt composition. The magnitude of the positive (Ce) or negative (Eu) anomaly is also most pronounced in peraluminous melts (vs. other melts) at a given oxygen fugacity. Other trends are also evident; for example, with the oxygen fugacity buffered at NNO, our results suggest that zircons may have both positive Ce anomalies and negative Eu anomalies. Thus, Eu2+ and Ce4+ may co-exist in terrestrial melts; furthermore, melt depletion of Eu by plagioclase fractionation prior to (or during) zircon crystallization is not a requisite for the presence of zircon Eu anomalies. These observations highlight the importance of melt structure/composition for the determination of partition coefficients, including those of multivalent cations, which may not depend on oxygen fugacity alone.