Hydrothermal Controls on the Formation of REE Deposits: Insights from In Situ XAS Study of REE (Nd, Gd, Yb) Solubility and Speciation in High Temperature Fluids (T < 600 °C)

The effect of temperature, composition and pH on the solubility and speciation of REE in high temperature fluids has been determined by in situ X-ray absorption measurements (XAS) up to 600 degrees C and 100 MPa to investigate the potential role of hydrothermal fluids in the transport, segregation and deposition of REE in carbonatitic and peralkaline igneous provinces. First experiments investigated the behaviour of Yb up to 350 degrees C and 50 MPa in fluids involving F, Cl and P. Results indicate that 10 to 100 ppm Yb can be dissolved in acidic fluids (pH < 2), most probably as Yb[H2O)(7,8)](3+) species. Both pH increase and addition of P trigger the systematic precipitation of Yb compounds, confirming that buffering of orthomagmatic fluids by fluid-rock interactions/meteoritic fluids and leaching of P from host rocks must play a critical role in the development of REE enriched horizons. New experiments investigating the speciation and solubility of Nd, Gd and Yb up to 600 degrees C and 100 MPa are currently under way and should provide additional constraints on the factors controlling the LREE/HREE fractionation in magmatic-hydrothermal environments.