Model Answer
0 min readIntroduction
Rare Earth Elements (REEs), comprising the lanthanide series plus yttrium and scandium, are critical components in numerous modern technologies, from smartphones to wind turbines. The lanthanides, specifically, are a group of seventeen chemically similar metallic elements with atomic numbers 57-71, starting with lanthanum (La) and ending with lutetium (Lu). Their geochemical behavior is complex and often used to decipher the origin and evolution of magmatic systems. Understanding their characteristics and the methods used to analyze their abundances, such as chondrite normalization, is fundamental to interpreting Earth’s geological processes.
General Characteristics of Lanthanides
The lanthanides exhibit a remarkable degree of chemical similarity due to their nearly identical electronic configurations – having electrons filling the 4f orbitals. This leads to several characteristic properties:
- Ionic Radius: Lanthanide ions have a relatively constant ionic radius, decreasing slightly with increasing atomic number (Lanthanide Contraction). This contraction is due to the poor shielding of the nuclear charge by the 4f electrons.
- Oxidation State: The most common oxidation state is +3, although +2 and +4 states are also observed, particularly in cerium (+4) and europium (+2).
- Geochemical Behavior: They are generally incompatible elements in mantle minerals, meaning they do not readily fit into the crystal structures of common mantle minerals like olivine and pyroxene. This leads to their enrichment in melts.
- Coordination Number: They exhibit a high coordination number, readily forming complexes with various ligands.
- Magnetic Properties: Many lanthanides are paramagnetic, and some are ferromagnetic at low temperatures.
Chondrite Normalized Diagram for REE Abundances
Expressing REE abundances in a rock using a chondrite-normalized diagram is a standard practice in geochemistry. Chondrites, specifically carbonaceous chondrites, are considered to represent the primordial solar system material and provide a relatively constant REE pattern. The rationale behind this normalization is as follows:
- Relative Enrichment/Depletion: Normalization to chondrites allows geochemists to easily identify whether a rock is enriched or depleted in specific REEs relative to the starting material.
- Magmatic Processes: REE patterns are sensitive to various magmatic processes like partial melting, fractional crystallization, and metasomatism. The normalized diagram highlights these effects.
- Source Identification: Different mantle sources (e.g., depleted mantle, enriched mantle) have distinct chondrite-normalized REE patterns, aiding in source identification.
- Visual Representation: The diagram provides a clear visual representation of the REE distribution, making it easier to compare different rock types and identify anomalies.
The diagram plots the ratio of each REE in the rock sample to its corresponding concentration in a standard chondrite reference material (typically the average carbonaceous chondrite). A flat pattern indicates no significant fractionation, while deviations from the flat pattern reveal the effects of geological processes.
Petrogenetic Significance of Ce and Eu Anomalies
Ce and Eu anomalies are significant deviations from the typical chondrite-normalized REE pattern and provide valuable insights into the petrogenetic history of a rock.
Cerium (Ce) Anomaly
A negative Ce anomaly (low Ce/Ce* ratio, where Ce* is the interpolated value between La and Pr) is commonly observed in seawater-altered rocks and sediments. This anomaly arises due to:
- Oxidation of Ce3+ to Ce4+: Ce4+ is more soluble in seawater than Ce3+, leading to preferential removal of Ce4+ from seawater and its incorporation into marine precipitates (e.g., phosphate nodules, ferromanganese crusts).
- Hydrothermal Alteration: Hydrothermal fluids circulating through oceanic crust can also oxidize Ce, leading to its removal.
Therefore, a negative Ce anomaly indicates interaction with seawater or hydrothermal fluids.
Europium (Eu) Anomaly
A positive Eu anomaly (high Eu/Eu* ratio, where Eu* is the interpolated value between Sm and Gd) is often associated with feldspar fractionation during magmatic crystallization. This anomaly is caused by:
- Eu2+ Substitution in Plagioclase Feldspar: Eu2+ has an ionic radius similar to Ca2+ and readily substitutes into the plagioclase feldspar structure.
- Fractional Crystallization: During fractional crystallization, as plagioclase crystallizes, Eu is preferentially incorporated into the solid phase, leaving the remaining melt depleted in Eu. This results in a positive Eu anomaly in the residual melt and a negative anomaly in the crystallized plagioclase.
A positive Eu anomaly, therefore, suggests plagioclase fractionation and can be used to infer the crystallization history of a magma.
Conclusion
In conclusion, the lanthanides are a unique group of elements whose geochemical behavior is governed by their electronic structure and ionic properties. Chondrite normalization provides a crucial framework for interpreting REE abundances in rocks, revealing the effects of magmatic and sedimentary processes. Ce and Eu anomalies, in particular, serve as powerful tracers of oxidation-reduction conditions and fractional crystallization, offering valuable insights into the petrogenetic history of igneous and sedimentary rocks. Understanding these concepts is essential for unraveling the complexities of Earth’s geological evolution.
Answer Length
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