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. Moreover, Zircon (ZrSiO4) is commonly found in felsic igneous rock. Because both Ce3+ and Ce4+ can substitute for zirconium, Zircon often has a positive Ce anomaly. Ce4+ substitutes with Zr much more easily than Ce3+ because Ce4+ (ionic radius 0.97Å) has the same charge and a similar ionic radius as Zr4+ (ionic radius 0.84Å). Therefore, the oxidation state of the magma is what determines the Ce anomaly in Zircon. If the oxygen fugacity is high, more Ce3+ will oxidize to Ce4+ and create a larger positive Ce anomaly in the zircon structure. At lower levels of oxygen fugacity, the level of Ce anomaly will also be lower.
