Titel: Carbonatites do not exist in vacuum: carbonatite–rock interactions from experiment and nature, and implications for REE mineralisation

Michael Anenburg

Australian National University, Australia

Veranstaltung: GeoKarlsruhe 2021

Datum: 2021

DOI: 10.48380/dggv-9tgy-8s83

The igneous nature of carbonatites has been controversial for decades until experimental work in the 1960s and 1970s conclusively showed that carbonatite melts can exist in geologically reasonable conditions. The observation of natrocarbonatite eruptions at Ol Doinyo Lengai further confirmed their igneous nature. However, these lines of evidence for igneous carbonatites have been a red herring: many ideas, processes, and terms deriving from silicate magma systems were inaccurately projected into carbonatite systems. Instead, carbonatites should be considered as hybrid metasomatic and magmatic cumulate rocks. The low viscosity and efficient wetting of carbonatite melts makes them behave more like hydrothermal fluids, preventing formation of magma chambers. Their high chemical reactivity and disequilibrium with their host rocks leads to rapid and substantial chemical exchange with their surroundings. Therefore, the chemical composition of their host rocks imparts a first order effect on the mineral assemblage observed in solidified carbonatite rocks. Rare earth elements are typically incompatible during carbonatite melt fractionation. Whether REE mineralisation is observed in Ca, Mg, or Fe-dominated mineral assemblages is strongly dependent on the degree and nature of silicate contamination. In special cases, REE can be highly compatible and mineralisation forms early rather than late. Enigmatic light REE mineralisations in fenite-like assemblages can likewise be explained as an end-member of carbonatite–rock interaction.

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