Geochemistry of Rare Earth Elements in Carbonatites
Date
2018
Authors
Tolley, James Paul
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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.
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Keywords
Carbonatites, Rare Earth Elements, Apatite, Magnetite, Cummins Range, Partitioning, Diffusion
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