Solubility of rare earth elements in carbonatite magmas, indicated by the liquidus surface in CaCO_3-Ca(OH)_2-La(OH)_3 at 1 kbar pressure

Abstract

The system CaCO_3-Ca(OH)_2(CC-CH) represents a synthetic analog to a carbonatite magma. Addition of a light rare earth (RE) component La(OH)_3 (LH) gives a simple analog of a rare earth carbonatite. Liquidus relations for two joins were studied at 1 kbar pressure; CH-LH and (CC_(55)CH_(45))-LH. CH-LH is binary with a eutectic at CH_(79)LH_(21) and 710°C. Thejoin CC_(55) + CH_(45))-LH has a liquidus piercing point between CC and LH, at (CC_(55)CH_(45))_(60)LH_(40) and 700°C. Combining the new results with known results for CC-CH allows construction of a liquidus diagram for part of the join CC-CH-LH. A ternary eutectic between the primary liquidus fields for CH, CC and LH occurs near 610°C with estimated composition CC= 33%, CH= 47%, LH= 20%. The solubility of La(OH)_3 in the synthetic carbonatite magma increases with increase of CO_2/H_2O from 20% at the eutectic to 40% at the piercing point on the join (CC_(55)CH_(45))-LH. The solubility of La in synthetic carbonatite is high compared with that of silicates. P_2O_5, and S. The results show that REE can become concentrated to high levels by fractionation of carbonatites, as long as they are not removed by high temperature crystallization of apatite and monazite.

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