The Europium anomaly, in geochemistry, is the phenomenon whereby Europium (Eu) concentration is either depleted or enriched in a rock relative to the other rare earth elements (REEs). An Eu anomaly is said to be "negative" if Eu is depleted relative to the other REEs and is said to be "positive" if Eu is enriched relative to the other REEs.
Contrary to all other lanthanides which are trivalent elements (3+) (apart from Ce4+), europium is an exception and can also exist in the reduced (2+) state under very reducing conditions. Eu is normally an incompatible element in its trivalent form (Eu3+) in an oxidizing magma, but it is preferentially incorporated into plagioclase in its divalent form (Eu2+) in a reducing magma where it substitutes for Strontium (Sr2+) and sometimes for calcium (Ca2+).[1]
Depletion or enrichment is generally attributed to europium tendency to be readily incorporated into plagioclase preferentially over other minerals. If a magma crystallizes plagioclase, most of the Eu will be incorporated into this mineral. The rest of the magma will then be relatively depleted in Eu. If this Eu-depleted magma then gets separated from its plagioclase crystals and subsequently solidifies, its chemical composition will display a negative Eu anomaly (because most of the Eu is locked up in the plagioclase, which was "left behind" by the magma, so to speak). Conversely, if a magma accumulates plagioclase crystals before solidification, its rock composition will display a relatively positive Eu anomaly.
A well known example of the Eu anomaly takes us to the moon. REE analyses of the moon's light-colored lunar highlands would show a large positive Eu anomaly due to the plagioclase in anorthosite, which is what comprises the highlands. The darker lunar mare, largely consisting mainly of basalt, shows a large negative Eu anomaly. This has led geologists to speculate as to the genetic relationship between the lunar mare and the lunar highlands. It is possible that much of the moon's Eu was incorporated into the earlier, plagioclase-rich highlands, leaving the later basaltic mare strongly depleted in Eu.[2]