The Behaviour of Rare Earth Elements (REE) in Carbonate Melts: a Geochemical Approach

Abstract

Carbonatites are the world's primary source of REE, but it's currently unknown why this enrichment occurs. Investigating the reason why was explored through a geochemical approach by looking into how certain REE exist within carbonate melts through measuring the oxidation states and coordination environments of REE using XAS synchrotron techniques.

Cerium (Ce) and Europium (Eu) are the only REE that occur in oxidation states other than 3+ (Ce3+ and Ce4+; Eu2+ and Eu3+). Ce and Eu LIII-edge XANES spectra were recorded for three synthetic carbonate glass compositions: MgCO3-Mg2P2O7-MgO at 1400 C and 2 GPa, MgCO3-K2CO3-CaCO3 at 1300 C and 0.5 GPa, and MgCO3-K2CO3 at 1000 C and 0.5 GPa, buffered over a range of oxygen fugacities (fO2). Ce oxidation state was found to change as a function of fO2, temperature, and melt composition. For a given QFM, a carbonatite melt containing Ce formed at higher temperatures would have a higher Ce4+/Ce than a melt formed at lower temperatures. Eu oxidation state was found to change as a function of fO2 and melt carbonate concentration. For a given QFM, a melt containing Eu with a large carbonate component would have a lower Eu3+/Eu relative to a melt with a low-no carbonate component. Typical carbonatite melts have a QFM value of 1.3, which corresponds to a preference of Ce3+ and Eu3+ in these experiments. These conclusions were used to produce an expression that relates Ce4+/Ce to temperature, melt composition, and fO2, allowing fO2 to be predicted for carbonatite melts.

Lanthanum (La, LREE), gadolinium (Gd, MREE), and lutetium (Lu, HREE) represent the range of ionic radii shown in REE with La having the largest radius and Lu having the smallest. The coordination environment of these three elements were studied to determine if there were REE-O bond length differences between LREE, MREE, and HREE in carbonatite melts. La, Gd, and Lu LIII-edge XANES and EXAFS spectra were recorded for the same three synthetic carbonate glass compositions. In the MgCO3-Mg2P2O7-MgO glass, Gd-O bond length is 2.39 A and Lu-O bond length is 2.27 A. REE-O bond length in carbonatites decreases with decreasing ionic radius.

Carbonatites host a variety of REE-bearing minerals, a large portion of which are in the phosphate group. Current REE extraction practices don't use carbonatite melt proxies to help extract REE from these phosphate ores, which would decrease time spent in the extraction process. Monazite, xenotime, and fluorapatite minerals were equilibrated with two synthetic carbonate flux compositions (Yan1 and ODL2) to see if this process is more effective at extracting REE than current practices. The optimal time spent mixing the ore and flux in a furnace is 1 hour. The best carbonate flux type is Yan1. Mix ratios of 0.3 and 0.4 have near 100% extraction of REE in all minerals. The addition of a carbonate component to the REE extraction process raises the REE extraction from <10% to ~100% when compared with just acid leaching alone. Show more