REE COMPOSITION OF AN AQUEOUS MAGMATIC FLUID - A FLUID INCLUSION STUDY FROM THE CAPITAN PLUTON, NEW-MEXICO, USA

The REE content of aqueous magmatic-derived fluids trapped in fluid inclusions, has been determined by ICP-MS after crush-leach extraction of the fluids in 4 samples. The total REE concentration varies between 200 and 1300 ppm and is dominated by the LREE, especially La, Ce and Nd. Fluids were released at different times from a melt, which changed composition as it underwent fractional crystallisation, and this is reflected in the concentration of REE in solution. Early formed quartz-fluorite veins, hosted by granophyre, contain the highest concentration of REE. and appear to be in equilibrium with aplite melt from which the fluid was inferred to have been derived since calculated fluid/melt distribution coefficients are in broad agreement with experimentally derived values. Variation in the REE content of the fluids is independent of salinity which remains constant at approximately 80 wt% total salts. Later veins, hosted by aplite, contain fluid derived from a porphyritic melt and have lower REE concentrations, reflecting the greater incorporation of REE into mineral phases crystallising from the melt: titanite and allanite occur in these later veins. REE mineral/fluid distribution coefficients have been calculated for these minerals and show there is a strong preference for REE to partition into the minerals.