RARE-EARTH ELEMENTS IN APATITE - UPTAKE FROM H2O-BEARING PHOSPHATE-FLUORIDE MELTS AND THE ROLE OF VOLATILE COMPONENTS

The partitioning of rare earth elements (REEs) between fluorapatite (F Ap) and H2O-bearing phosphate-fluoride melts has been studied at about 700 and 800 degrees C and 0.10-0.15 GPa. REE uptake patterns, i.e., p lots of D(REE:FAp/melt), are convex upwards and peak near Nd for singl e-REE substituted FAp at minor (0.03-0.25 wt% REE2O3) abundances, and binary(LREE + HREE)-substituted FAp, and hexa-REE-substituted FAp at m inor to major (0.25-7.8 wt% REE2O3) abundances. Partition coefficients for minor abundances of REE and depolymerized phosphate melts are abo ut 5, 8, and 1 for La, Nd, and Lu, respectively and broadly comparable to those for early fluorapatite in the fractionation of melts of basa ltic composition. The Ca2 site exerts marked control on the selectivit y of apatite for REE because it preferentially incorporates LREE and i ts effective size varies with substitution of the A-site volatile anio n component (F, Cl, OH). Using simple crystal-chemical arguments, melt (or fluid) normalized REE patterns are predicted to peak near Nd for f luorapatite and be more LREE-enriched for chlorapatite. These predicti ons are consistent with data from natural rocks and laboratory experim ents. The wide variation in D(REE:apatite/melt) in nature (from <1 for whitlockite-bearing lunar rocks to about 100 for evolved alkalic rock s) is attributed largely to the influence of the volatile components. Copyright (C) 1997 Elsevier Science Ltd.