Cerium anomaly

The cerium anomaly, in geochemistry, is the phenomenon whereby cerium (Ce) concentration is either depleted or enriched in a rock relative to the other rare-earth elements (REEs).
A Ce anomaly is said to be "negative" if Ce is depleted relative to the other REEs and is said to be "positive" if Ce is enriched relative to the other REEs.
Cerium is a rare-earth element (lanthanide) characterized by two different redox states: III and IV. Contrary to other lanthanide elements, which are only trivalent (with the notable exception of Eu2+), Ce3+ can be oxidized by atmospheric oxygen (O2) to Ce4+ under alkaline conditions.
The cerium anomaly relates to the decrease in solubility, which accompanies the oxidation of Ce(III) to Ce(IV). Under reducing conditions, Ce3+ is relatively soluble, while under oxidizing conditions CeO2 precipitates. Sediments deposited under oxic or anoxic conditions can preserve on the long term the geochemical signature of Ce3+ or Ce4+ upon reserve that no early diagenetic transformation altered it.
Cerium can occur in nature as a 3+ or 4+ ion and is a compatible element (at 4+ valency) in zircon and less commonly in silica. Thomas "et al.", (2003) state that “terrestrial zircons commonly show a positive Ce anomaly due to the incorporation of Ce4+ into zircon, which is because Ce4+ has the same charge and a similar ionic radius than Zr4+ (Ce4+ = 0.97 Å; Zr4+ = 0.84 Å)”. As such, Ce4+ is incorporated into zircon much more easily than the larger Ce3+ (ionic radius = 1.143 Å). 
This shows that both Ce3+ and Ce4+ are present and that the Ce4+ being compatible in zircon is causing the anomaly. 