APATITE FLUID PARTITIONING OF RARE-EARTH ELEMENTS AND STRONTIUM - EXPERIMENTAL RESULTS AT 1.0 GPA AND 1000-DEGREES-C AND APPLICATION TO MODELS OF FLUID-ROCK INTERACTION/

Apatite/aqueous fluid partition coefficients have been measured for Ce , Gd, Yb and Sr at 1.0 GPa and 1000 degrees C using radiotracers and g amma-ray spectroscopy. Analysis of clean apatite separates from 3-5-da y piston cylinder experiments allowed calculation of K-D's from mass b alance. Partition coefficients from forward experiments with doped flu ids and reversal experiments with doped crystals show good agreement. Partition coefficients for apatite/H2O range from 15 to 33 in the orde r Gd>Ce>Sr>Yb, corresponding to a convex-up REE pattern characteristic of natural apatites. If Sr serves as an analog of EU(2+), then Eu2+ s hould behave similarly to REE and any (-) Eu anomalies in fluid precip itated/equilibrated apatites must result from inheritance rather than from apatite fractionation. The elements Be and Cs partition strongly into the fluid, so that low levels in dean apatite separates permitted only estimates of maximum K-D's. Addition of dissolved albite to the fluid decreased K-D's of REE and Sr 2-3X, implying complexing of REESSr with silicates dissolved in the fluid. Apatite/H2O K-D's are simila r to apatite/silicate melt K-D's, except that in the latter case Eu (S r) is less compatible than other REE. At 1.0 GPa and 1000 degrees, the high K-D-values for REE and Sr combined with the insolubility of apat ite in near-neutral pH aqueous fluids suggest that apatite may strongl y influence the behavior of these elements during metasomatism by an a queous fluid. Mass-balance modeling using measured partition coefficie nts and solubilities demonstrate this, and also show that compatible e lements do not reach equilibrium with the primary fluid until the mass ratio fluid/rock angle 1. Thus, caution is required when inferring co mpositions of primary fluids from solid mineral assemblages.