Oddo-Harkins Effect: Why We Normalize REEs

 The foundations of the universe were created during the Big Bang event (13.7 GA), where an infinite amount of energy was condensed to a single point, then exploded outwards. About 98% of the universe's elemental abundance was formed within the first three minutes of this event in a process called Big Bang nucleosynthesis, and the remaining 2% was synthesized later by stars[5]. Nucleosynthesis is the process of creating new atomic nuclei by combining neutrons and protons (fusion) or breaking larger atomic structures apart (fission and radioactive decay)[3]. During stellar nucleosynthesis, fusion processes such as proton chains and the CNO cycle often result in the perpetual combination of even numbered elements (helium binds with continuous amounts of helium). The synthesis of odd numbered elements is less common, as it can only be done using leftover hydrogen elements or during supernovae[4]. This phenomena is referred to as the Oddo-Harkins effect which states that elements with an even atomic number are more abundant than their adjacent odd-numbered elements; for example, carbon (atomic number of 6) is more enriched than boron (atomic number of 5) and nitrogen (atomic number of 7).

Figure 1. A logarithmic elemental abundance chart displaying the "zig-zag" Oddo-Harkins effect. Image Credit: Wikipedia 

The overabundance of even elements creates a "distracting" zig-zag effect in elemental abundance graphs, making pattern recognition and direct comparisons between elements challenging. This is especially true in regards to the analysis of rare earth elements (REEs, a.k.a. the lanthanides) which are commonly used to constrain geological processes[2]. To resolve this dilemma, geochemical studies often normalize elemental abundance charts using CI chondrites to minimize the Oddo-Harkins effect. 

Chondrites are the primitive remains of cosmic sediments that did not accrete into planets. CI chondrites are a rare, stony type of meteorite that are rich in volatiles, but they are highly regarded for their unique chemical composition. The elemental ratios within CI chondrites are remarkably similar to that of the Sun in our  Solar System (see figure below). Thus, this link between CI abundance and the solar photosphere has been used as a geochemical standard to "cancel" or "normalize" the effects of stellar biases (e.g. the Oddo-Harkins effect). With the removal of "pre-solar system processes", elemental analyses are readily examined for the trends and behaviors which stem from later, secondary processes[1,2].

Figure 2. An elemental abundance chart displaying the 1:1 ratio between a CI chondrite (Orugueil) and the Solar Photosphere.

References

1. Aleon, J. Meteorites and the physico-chemical conditions in the early solar nebula. Physics and Astrophysics of Planetary Systems, Les Houches. 2018. DOI:10.1051/eas/1041020
2. Cornell, D. H. Rare earths from supernova to superconductor. Pure and Applied Chemistry. Vol 65, No. 12, 2453-2464. https://doi.org/10.1351/pac199365122453
3. Helmenstine, A. Nucleosynthesis-How Elements Are Made. Science Notes. 2023. https://sciencenotes.org/nucleosynthesis-how-elements-are-made/
4. Inglis-Arkell, E. The Oddo-Harkins Rule shows the universe hates the odd. Gizmodo. Published October 17, 2013. https://gizmodo.com/the-oddo-harkins-rule-shows-the-universe-hates-the-odd-1446581327
5. Schramm, David, "The Big Bang Creation of the Universe", in Quarks, Quasars and Quandries, Ed. Gordon Aubrecht, Amer. Assoc of Physics Teachers, 1987.