The europium anomaly is the phenomenon whereby the

europium Europium is a chemical element with the symbol Eu and atomic number 63. Europium is the most reactive lanthanide by far, having to be stored under an inert fluid to protect it from atmospheric oxygen or moisture. Europium is also the softest lan ...

(Eu) concentration in a mineral is either enriched or depleted relative to some standard, commonly a

chondrite A chondrite is a stony (non-metallic) meteorite that has not been modified, by either melting or differentiation of the parent body. They are formed when various types of dust and small grains in the early Solar System accreted to form pr ...

or mid-ocean ridge basalt (MORB). In

geochemistry Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing th ...

a europium anomaly is said to be "positive" if the Eu concentration in the mineral is enriched relative to the other

rare-earth elements The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the lanthanides (yttrium and scandium are usually included as rare earths), are a set of 17 nearly-indistinguishable lustrous silve ...

(REEs), and is said to be "negative" if Eu is depleted relative to the other REEs. While all

lanthanide The lanthanide () or lanthanoid () series of chemical elements comprises the 15 metallic chemical elements with atomic numbers 57–71, from lanthanum through lutetium. These elements, along with the chemically similar elements scandium and yt ...

s form relatively large trivalent (3+) ions, Eu and

cerium Cerium is a chemical element with the symbol Ce and atomic number 58. Cerium is a soft, ductile, and silvery-white metal that tarnishes when exposed to air. Cerium is the second element in the lanthanide series, and while it often shows the +3 ...

(Ce) 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 reduced divalent (2+) cations are similar in size and carry the same charge as Ca²⁺, an ion found in

plagioclase Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more p ...

and other minerals. While Eu is an

incompatible element In petrology and geochemistry, an incompatible element is one that is unsuitable in size and/or charge to the cation sites of the minerals of which it is included. It is defined by the partition coefficient between rock-forming minerals and melt b ...

in its trivalent form (Eu³⁺) in an oxidizing

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natura ...

, it is preferentially incorporated into plagioclase in its divalent form (Eu²⁺) in a reducing magma, where it substitutes for

calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...

(Ca²⁺). Enrichment or depletion is generally attributed to europium's tendency to be incorporated into plagioclase preferentially over other minerals. If a magma crystallizes stable 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 (a positive anomaly). 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 (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. A well-known example of the Eu anomaly is seen on the Moon. REE analyses of the Moon's light-colored

lunar highlands The geology of the Moon (sometimes called selenology, although the latter term can refer more generally to " lunar science") is quite different from that of Earth. The Moon lacks a true atmosphere, which eliminates erosion due to weather. It does ...

show a large positive Eu anomaly due to the plagioclase-rich

anorthosite Anorthosite () is a phaneritic, intrusive igneous rock characterized by its composition: mostly plagioclase feldspar (90–100%), with a minimal mafic component (0–10%). Pyroxene, ilmenite, magnetite, and olivine are the mafic minerals most ...

comprising the highlands. The darker

lunar mare The lunar maria (; singular: mare ) are large, dark, basaltic plains on Earth's Moon, formed by ancient asteroid impacts on the far side on the Moon that triggered volcanic activity on the opposite (near) side. They were dubbed , Latin for 'seas' ...

, consisting mainly of

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

, 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.{{cite book , chapter-url = https://books.google.com/books?id=385nPZOXmYAC&pg=PA456 , chapter = Composition of the Lunar Mantle and Core , pages = 456–460 , title = Cosmochemistry , isbn = 978-0-521-87862-3 , author1 = Harry Y. Mcsween, Jr. , author2 = Huss, Gary R. , date = 2010-06-30

References

Lanthanides Geochemistry

See also

References