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 in clusions, has been determined by ICP-MS after crush-leach extraction o f 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 N d. Fluids were released at different times from a melt, which changed composition as it underwent fractional crystallisation, and this is re flected in the concentration of REE in solution. Early formed quartz-f luorite veins, hosted by granophyre, contain the highest concentration of REE. and appear to be in equilibrium with aplite melt from which t he 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 independ ent of salinity which remains constant at approximately 80 wt% total s alts. Later veins, hosted by aplite, contain fluid derived from a porp hyritic 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 d istribution coefficients have been calculated for these minerals and s how there is a strong preference for REE to partition into the mineral s.