Europium anomaly

In the case of Eu, its reduced divalent (2+) cations are similar in size and carry the same charge as Ca2+, an ion found in plagioclase and other minerals.

[2] Enrichment or depletion is generally attributed to europium's tendency to be incorporated into plagioclase preferentially over other minerals.

If the Eu-depleted magma is then separated from its plagioclase crystals and subsequently solidifies, its chemical composition will display a negative Eu anomaly (because the Eu is locked up in the plagioclase left in the magma chamber).

Conversely, if a magma accumulates plagioclase crystals before solidification, its rock composition will display a relatively positive Eu anomaly.

The darker lunar mare, consisting mainly of basalt, shows a large negative Eu anomaly.