Non-Henry's Law behavior of REE partitioning between fluorapatite and CaF2-rich melts: Controls of intrinsic vacancies and implications for natural apatites

The partitioning of rare-earth elements (REEs: Gd and multiple REEs), Sr, and Mn between fluorapatite and CaF2-rich melts was investigated over a wide range of REE concentrations (i.e., from 0.8 +/- 0.1 to 25,000 +/- 2600 ppm Gd in fluorapatite) in two different sample assemblies (i.e., tightly covered Pt crucibles and sealed Pt capsules) at 1220 degreesC and atmospheric pressure. Attainment of equilibrium is indicated by selected reversal experiments. The partition coefficient D(Gd) decreases from similar to2 to similar to0.5 with increasing Gd in fluorapatite, hence a marked non-Henry's Law behavior, but becomes independent of composition at and above similar to5000 and similar to1000 ppm Gd for experiments in Pt crucibles and Pt capsules, respectively. Non-Henry's Law behavior is also observed in experiments involving multiple REEs. All REE patterns are convex upward in shape with maxima between Nd and Gd, and D(La)/D(Nd) and D(Nd)/D(Yb) decrease systematically with increasing total REEs in fluorapatite, suggesting that REE fractionations are partly related to non-Henry's Law behavior. These experimental results and local structural data from previous electron paramagnetic resonance spectroscopic studies suggest that the non-Henry's Law behavior of REE partitioning between fluorapatite and melt is controlled by intrinsic Ca2+ vacancies in the c-axis channels. The D(Sr) and D(Mn) values are independent of composition and, therefore, do not deviate from the Henry's Law in their respective compositional ranges investigated in this study. Nonstoichiometry, such as Ca2+ and F- vacancies in the c-axis channels, is well known in natural apatites, particularly in biogenic apatites. Therefore, the observed non-Henry's Law behavior of REE partitioning is expected to have important implications for REE geochemical modeling involving apatites and for the uptake of REES by natural apatites. Particularly, the non-Henry's Law behavior of REE partitioning is at least partly responsible for the commonly observed, bell-shaped REE patterns in fossil biogenic apatites. Copyright (C) 2003 Elsevier Science Ltd.