Europium Anomaly
Encyclopedia
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+).
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.
Geochemistry
The field of geochemistry involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks, water, and soils, and the cycles of matter and energy that transport the Earth's chemical components in time and space, and...
, is the phenomenon whereby Europium
Europium
Europium is a chemical element with the symbol Eu and atomic number 63. It is named after the continent of Europe. It is a moderately hard silvery metal which readily oxidizes in air and water...
(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+
Cerium anomaly
The Cerium anomaly, in geochemistry, is the phenomenon whereby Cerium concentration is either depleted or enriched in a rock relative to the other rare earth elements ....
), 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
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
, but it is preferentially incorporated into plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
in its divalent form (Eu2+) in a reducing magma where it substitutes for Strontium (Sr2+) and sometimes for calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
(Ca2+).
Depletion or enrichment is generally attributed to europium tendency to be readily incorporated into plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
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
Anorthosite
Anorthosite is a phaneritic, intrusive igneous rock characterized by a predominance of plagioclase feldspar , and a minimal mafic component...
, which is what comprises the highlands. The darker lunar mare
Lunar mare
The lunar maria are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. They were dubbed maria, Latin for "seas", by early astronomers who mistook them for actual seas. They are less reflective than the "highlands" as a result of their iron-rich compositions, and...
, largely consisting mainly of basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
, 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.
See also
- CeriumCeriumCerium is a chemical element with the symbol Ce and atomic number 58. It is a soft, silvery, ductile metal which easily oxidizes in air. Cerium was named after the dwarf planet . Cerium is the most abundant of the rare earth elements, making up about 0.0046% of the Earth's crust by weight...
- Cerium anomalyCerium anomalyThe Cerium anomaly, in geochemistry, is the phenomenon whereby Cerium concentration is either depleted or enriched in a rock relative to the other rare earth elements ....
- EuropiumEuropiumEuropium is a chemical element with the symbol Eu and atomic number 63. It is named after the continent of Europe. It is a moderately hard silvery metal which readily oxidizes in air and water...
- LanthanideLanthanideThe lanthanide or lanthanoid series comprises the fifteen metallic chemical elements with atomic numbers 57 through 71, from lanthanum through lutetium...
s - Rare earth elementRare earth elementAs defined by IUPAC, rare earth elements or rare earth metals are a set of seventeen chemical elements in the periodic table, specifically the fifteen lanthanides plus scandium and yttrium...
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