The absence of Ocean Island Basalts (OIBs) in the Archean rock records is commonly attributed to the higher melting degrees associated with mantle plumes. However, this explanation raises questions about the presence of carbonatites during the Archean, as carbonatites are typically associated with lower degrees of mantle melting. The oldest carbonatites date back to around 3000 Ma. Additionally, the presence of lighter Ca isotopes in carbonatites globally suggests a significant crustal component in their formation, even in the oldest examples. Could this mean that carbonatites are actually the melting product of carbonated eclogites (as one popular model suggests)? And if so, does this imply that mantle plumes may have a lesser role in the formation of carbonatites compared to the melting of carbonated eclogites?
A key factor in the formation of carbonatites is metasomatism, which occurs both in the mantle, giving rise to alkaline silicate and carbonatite associations, and in the crustal host rocks through which carbonatite melt ascends. The plume's role, besides providing heat, is to supply C, O, and H to metasomatized lherzolites-peridotites, resulting in the formation of alkaline melts. Primary carbonatite liquids can form through decompression of these alkaline rocks. However, the presence of alkaline silicate melts in the asthenosphere or lithosphere is not necessarily associated with plumes. Subduction processes can enrich the mantle with volatiles. Without volatiles, lherzolitic-peridotite melts produce tholeiitic, picritic, or alkaline basaltic magmas. Volatiles are crucial for the formation of exotic melts. Therefore, I believe that the presence of volatiles is crucial for the formation of carbonatites. Crustal contamination can also significantly influence their formation. For example, interactions between carbonatites and granitic hosts can create pyroxenites, which may later evolve into "silicocarbonatites."
I believe that these studies can contribute to the discussion:
- Metasomatic ijolite, glimmerite, silicocarbonatite, and antiskarn formation: carbonatite and silicate phase equilibria in the system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–O2–CO2
Michael Anenburg & Jesse B. Walters
https://doi.org/10.1007/s00410-024-02109-0
- Carbonatites: Classification, Sources, Evolution, and Emplacement
Gregory M. Yaxley, Michael Anenburg, Sebastian Tappe, Sophie Decree, and Tibor Guzmics
https://doi.org/10.1146/annurev-earth-032320- 104243
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