Hello, Mr.Vafadar, can you recommend one or some document(s)? Thanks.

Hi Saleh, yes of course, they can bear positive anomalies. This can have several explanations. One is that zircon saturation was achieved prior to feldspar fractionation. It is well known that zircons can reside protracted periods of time in the magma and record the geochemical evolution of the system. In some cases zircons can be isolated from the system and be reintegrated during reheating and remelting. Consequently, you can have an slightly older zircon population (part of the same system) that are not in gecochemical equilibrium to the host matrix (which could be much more evolve at that time). Other possiblity is that they could be xenocryst? Zircons are very resistant minerals to physical and chemical processes and usually not affected by alteration processes. 

Hi Saleh,  As you know, zircons typically have a negative Eu anomaly of varying magnitude. A positive Eu anomaly is possible, but is most often associated with zircon alteration, melts effected by strong sphene fractionation, or, in rare cases, when the melt resorbs feldspar. The Eu anomaly reflects the total feldspar history of the melt, from partial melting in the source and source mineralogy, to feldspar fractionation before and after zircon saturation (Hoskin and Ireland, 2000; Poller et al., 2001; Hoskin and Schaltegger, 2003). 

It is hard to understand a positive Eu anomaly in zircon. As suggested by Mr. León and Moscati, Eu anomaly in zircon depends on fractionation of paragenetic mineral which mostly is feldspar. However,  recent experments demonstrate that Eu and Ce anomalies in zircon are mainly controlled by oxygen fugacity (Trail et al., 2012). Terrestrial zircons crystallized from a normal magma should have a negative or no Eu anomaly which is affected by the Eu2+/Eu3+ratios. The similar geochemistry of  Eu3+ to Sm3+and Gd3+controlled by charge and radius  indicate no Eu anomaly in zircon even under an extermely oxydic condition and/or without feldspar fractionation. Therefore, a positive Eu anomaly in zircon can only be resulted from a nonequilibrium process, such as metamorphism or alteration. However, similar case is rarely reported...