Apatite/fluid partitioning of rare-earth elements and strontium: Experimental results at 1.0 GPa and 1000°C and application to models of fluid-rock interaction

Abstract

Apatite/aqueous fluid partition coefficients have been measured for Ce, Gd, Yb and Sr at 1.0 GPa and 1000°C using radiotracers and gamma-ray spectroscopy. Analysis of clean apatite separates from 3–5-day piston cylinder experiments allowed calculation of K D's from mass balance. Partition coefficients from forward experiments with doped fluids and reversal experiments with doped crystals show good agreement. Partition coefficients for apatite H 2 O range from 15 to 33 in the order 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 Eu 2+ should behave similarly to REE and any (-) Eu anomalies in fluid precipitated/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 clean 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 2–3 ×, implying complexing of REE + Sr with silicates dissolved in the fluid. Apatite H 2 O K D's are similar to apatite/silicate melt K D's, except that in the latter case Eu (Sr) is less compatible than other REE. At 1.0 GPa and 1000°, the high K D-values for REE and Sr combined with the insolubility of apatite in near-neutral pH aqueous fluids suggest that apatite may strongly influence the behavior of these elements during metasomatism by an aqueous fluid. Mass-balance modeling using measured partition coefficients and solubilities demonstrate this, and also show that compatible elements do not reach equilibrium with the primary fluid until the mass ratio fluid rock ∠ 1 . Thus, caution is required when inferring compositions of primary fluids from solid mineral assemblages.