The Europium anomaly is the phenomenon whereby the europium concentration in a mineral is either enriched or depleted relative to some standard, commonly a chondrite or mid-ocean ridgebasalt. In geochemistry a europium anomaly is said to be "positive" if the Eu concentration in the mineral is enriched relative to the other rare-earth elements, and is said to be "negative" if Eu is depleted relative to the other REEs. While all lanthanides form relatively large trivalent ions, Eu and cerium have additional valences, europium forms 2+ ions, and Ce forms 4+ ions, leading to chemical reaction differences in how these ions can partition versus the 3+ REEs. In the case of Eu, its reduceddivalent cations are similar in size and carry the same charge as Ca2+, an ion found in plagioclase and other minerals. While Eu is an incompatible element in its trivalent form in an oxidizing magma, it is preferentially incorporated into plagioclase in its divalent form in a reducing magma, where it substitutes for calcium. Enrichment or depletion is generally attributed to europium's tendency to be incorporated into plagioclase preferentially over other minerals. If a magma crystallizesstable plagioclase, most of the Eu will be incorporated into this mineral, causing a higher than expected concentration of Eu in the mineral versus other REE in that mineral. The rest of the magma will then be relatively depleted in Eu with a concentration of Eu lower than expected versus the concentrations of other REEs in that magma. If the Eu-depleted magma is then separated from its plagioclase crystals and subsequently solidifies, its chemical composition will display a negative Eu anomaly. 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 is seen on the Moon. REE analyses of the Moon's light-colored lunar highlandsshow a large positive Eu anomaly due to the plagioclase-rich anorthosite comprising the highlands. The darkerlunar mare, consisting mainly of basalt, shows a large negative Eu anomaly. This has led geologists to speculate as to the genetic relationship between the lunar highlands and mare. 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.
