Experimental studies of REE fractionation during water–mineral interactions: REE release rates during apatite dissolution from pH 2.8 to 9.2

Rare earth elements (REE) release rates were measured during the open-system dissolution of natural apatite at 25 °C, 2.8 < pH < 9.2, and at ionic strengths ranging from 0.001 to 0.02 mol/kg. All inlet solutions were Ca, REE, and P free. In agreement with previous studies, apatite dissolution rates determined from outlet solution Ca concentrations decrease monotonically with increasing pH. Outlet solution molar REE / Ca ratios differ from that of the dissolving apatite during the experiments, this behaviour is interpreted to stem from dissolution or precipitation of minor quantities of rhabdophane (REE(PO4)·nH2O). This interpretation is supported by solute speciation calculations, which indicate that REE are retained in the solid phase when the outlet fluids are supersaturated with respect to Nd-rhabdophane (Ksp ≈ 10− 24.5 ± 0.5), but preferentially released when the outlet fluids are undersaturated with respect to Nd-rhabdophane. The relative concentrations of the REE in the outlet solutions suggest that the secondary rhabdophane is slightly enriched in light REE (Ce to Eu). The results of this study suggest that rhabdophane dissolution and precipitation during apatite–fluid interaction plays a major role in controlling surface and ground water REE signatures.