User:Georgiapeach5/Cerium anomaly

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The cerium anomaly, in geochemistry, is the phenomenon whereby cerium (Ce) concentration is either depleted or enriched in a rock relative to the other rare-earth elements (REEs)^[1]. A Ce anomaly is said to be "negative" if Ce is depleted relative to the other REEs and is said to be "positive" if Ce is enriched relative to the other REEs^[1].

Cerium Oxidation States

Cerium is a rare-earth element (lanthanide) characterized by two different redox states: III and IV. Unlike other lanthanide elements, which are only trivalent (with the notable exception of Eu²⁺), Ce³⁺ can be oxidized by atmospheric oxygen (O₂) to Ce⁴⁺ under alkaline conditions^[2].

The cerium anomaly relates to the decrease in solubility, which accompanies the oxidation of Ce(III) to Ce(IV). Under reducing conditions, Ce³⁺ is relatively soluble, while under oxidizing conditions CeO₂ precipitates^[1]. Sediments deposited under oxic or anoxic conditions can preserve the geochemical signature of Ce³⁺ or Ce⁴⁺ as long as no early diagenesis occurs^[1].

Cerium Anomalies in Zircon

Zircon (ZrSiO₄) is commonly found in felsic igneous rock^[3]. Because both Ce³⁺ and Ce⁴⁺ can substitute for Zirconium, Zircon often has a positive Ce anomaly. Ce⁴⁺ substitutes with Zr much more easily than Ce³⁺ because Ce⁴⁺ (ionic radius 0.97Å) has the same charge and a similar ionic radius as Zr⁴⁺ (ionic radius 0.84Å)^[1]. Therefore, the oxidation state of the magma is what determines the Ce anomaly in Zircon^[3]. If the oxygen fugacity is high, more Ce³⁺ will oxidize to Ce⁴⁺ and create a larger positive Ce anomaly in the zircon structure. At lower levels of oxygen fugacity, the level of Ce anomaly will also be lower^[3].

Cerium Anomalies in Coal

Negative Cerium Anomalies

Cerium in coal is typically weakly negative, meaning that it is present at slightly lower concentrations than the other rare-earth elements^[2]. Cerium anomalies in coal are influenced by the sediment source region^[2]. Coal mined from mafic regions dominated by basalts, such as the location of the Xinde Mine in China, does not have a Ce-anomaly^[2]. In contrast, coal mined in felsic rock regions, such as Guxu Coalfield in in China, does have weakly negative Ce-anomalies^[2]. Negative Ce-anomalies can also be attributed to the weathering and oxidation of the coal-mining region^[1]. During oxidation, Ce³⁺ precipitates out as CeO₂, leaving less Ce in the coal^[2].

Positive Cerium Anomalies

While cerium anomalies in coal are usually negative, they can rarely be positive as well^[4]. This can occur during volcanic eruptions when volcanic ash is weathered into mafic tuffs with positive Ce-anomalies^[1]. The Pavlovka deposit in Far East Russia has large positive Ce-anomalies in its Fe-Mn oxyhydroxide ores^[4]. Because cerium is one of only two REEs that can obtain an oxidation number of +4, Ce⁴⁺ is absorbed into Mn(IV) oxides instead of other REEs and this results in a positive Ce-anomaly^[4].

References

Category:Geochemistry Category:Lanthanides

^ ^a ^b ^c ^d ^e ^f ^g ^h Thomas, J. B., Bodnar, R. J., Shimizu, N., Chesner, C. A. (2003) Melt inclusions in zircon. Reviews in Mineralogy and Geochemistry, 53 (1), 63–87.
^ ^a ^b ^c ^d ^e ^f Dai, Shifeng; Graham, Ian; Ward, Colin (2016). "A review of anomalous rare earth elements and yttrium in coal". International Journal of Coal Ecology. 159: 82–95.
^ ^a ^b ^c Zhong, Shihua; Seltmann, Reimar; Qu, Hongying; Song, Yingxin (2019-12-01). "Characterization of the zircon Ce anomaly for estimation of oxidation state of magmas: a revised Ce/Ce* method". Mineralogy and Petrology. 113 (6): 755–763. doi:10.1007/s00710-019-00682-y. ISSN 1438-1168.
^ ^a ^b ^c Laveuf, C.; Cornu, S. (2009). "A review on the potentiality of Rare Earth Elements to trace pedogenetic processes". Geoderma. 154 (1–2): 1–12. doi:10.1016/j.geoderma.2009.10.002. ISSN 0016-7061.

[:0-1] ^ ^a ^b ^c ^d ^e ^f ^g ^h Thomas, J. B., Bodnar, R. J., Shimizu, N., Chesner, C. A. (2003) Melt inclusions in zircon. Reviews in Mineralogy and Geochemistry, 53 (1), 63–87.

[:2-2] ^ ^a ^b ^c ^d ^e ^f Dai, Shifeng; Graham, Ian; Ward, Colin (2016). "A review of anomalous rare earth elements and yttrium in coal". International Journal of Coal Ecology. 159: 82–95.

[:3-3] Zhong, Shihua; Seltmann, Reimar; Qu, Hongying; Song, Yingxin (2019-12-01). "Characterization of the zircon Ce anomaly for estimation of oxidation state of magmas: a revised Ce/Ce* method". Mineralogy and Petrology. 113 (6): 755–763. doi:10.1007/s00710-019-00682-y. ISSN 1438-1168.

[:1-4] Laveuf, C.; Cornu, S. (2009). "A review on the potentiality of Rare Earth Elements to trace pedogenetic processes". Geoderma. 154 (1–2): 1–12. doi:10.1016/j.geoderma.2009.10.002. ISSN 0016-7061.

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