Geochemistry of Rare Earth Elements in Carbonatites

Abstract

The geochemical behaviour of rare earth elements (REE) was investigated by studying both natural and experimental samples of carbonatites. The Cummins Range Carbonatite Complex (CRCC) lies on the southern margin of the Kimberley Craton in northern Western Australia. It comprises a sub-vertical stock, ~2 km in diameter, with a central calcite-dolomite carbonatite plug mantled by variably metasomatised pyroxenite. Geochemical characterisation of the complex suggests a petrogenetic model in which a carbonated silicate magma, highly enriched in incompatible trace elements, was emplaced at <1100 °C at relatively shallow depth. Crystallisation of the pyroxenite leads to HFSE and REE fractionation resulting in the formation of the accessory phases zirconolite and perovskite. The carbonatite segregated from the evolving carbonated silicate magma crystallising REE-rich pyrochlore and primary carbonates. Both the pyroxenite and carbonatite show similar LREE-enrichments despite their contrasting mineralogy.

Apatite, a common phase in the CRCC, is major host of the REE (ΣREE = 4950-6080 ppm) and is characterised by strongly LREE-enriched patterns in both the pyroxenite and carbonatite units. Trace-element doped synthetic (F- and Cl-) apatites were crystallised in equilibrium with carbonatite melt as function of temperature (1150-1350 °C), pressure (10-30 kbar), and melt composition. Partition coefficients of trivalent cations (REE and Y) were fit to lattice strain models and a general equation derived relating partitioning to pressure and temperature. DREE increase with 1) increasing temperature, 2) decreasing pressure and is greater in F-apatite compared to Cl-apatite. Importantly, despite the addition of charge compensating species, in all experiments the REE apatite/carbonate partition coefficients remained ~1, demonstrating that the fractional crystallisation of apatite will not enrich a carbonate melt in REE.

The REE distribution in the CRCC is strongly influenced by crystallisation of the accessory phases zirconolite and perovskite in the pyroxenite, and pyrochlore in the carbonatite. These phases contain up to ~5-10 wt.% REE. Despite their petrogenetic importance partitioning studies involving these accessory phases are limited, often because of experimental difficulties. Magnetite is a common accessory mineral for which the REE partition coefficients have not been determined. It is also experimentally difficult to produce crystals of sufficient size for the analysis of low concentrations of REE. Adopting a new experimental technique of equilibrating a single crystal of magnetite with a silicate melt reservoir, concentration profiles radiating from the magnetite-melt interface can be measured using an orientated laser spot (slit) and scanning LA-ICP-MS. Modelling these profiles using the diffusion equation enables diffusion coefficients and for the first time partition coefficients, for all the REE and Y between magnetite and silicate melt to be determined. DREE are all <0.01 and show little dependance with changing ƒO2 however diffusion rates vary from 10^-14.4 to 10^-11.96 m/s^2. Comparing measured diffusion rates to the point defect model confirms that point defects govern the diffusion of REE in magnetite. Diffusion pathways result from Fe2+/ Fe3+ reordering in the crystal lattice in response to changes in ƒO2. This is the first demonstration of the point defect model in magnetite using trace elements.