Crystal/liquid partitioning of REE in basaltic systems: Extreme fractionation of REE in olivine

Abstract

Partition coefficients for Nd, Sm, Gd, and Yb between olivine and synthetic basaltic melts were determined using the percent level doping technique and special microprobe analytical techniques for REE in olivine. These techniques permit much more accurate background measurement and thus greater sensitivity than conventional techniques. Spurious results were obtained if REE contents were measured in olivine at locations closer than about 100 micrometers to crystal margins, because of detection of REE x-rays generated in surrounding glass. For results that avoid this “edge” effect, average olivine/liquid distribution coefficients are Nd, 0.00007 ± 3; Sm, 0.00058 ± 7; Gd, 0.00102 ± 8; and Yb, 0.0194 ± 3 for synthetic lunar mare basaltic melts; and Sm, 0.00079 ± 9; Gd, 0.00144 ± 13; Yb, 0.027 ± 3 for synthetic lunar highlands basaltic melts. These values for the light REE are much lower than those of previous experimental or phenocryst/matrix studies. The higher LREE values, and consequently flatter patterns obtained in earlier experimental studies probably resulted from measurement of spuriously high LREE contents in olivines, because of small crystal size. Flatter patterns from phenocryst/matrix studies probably result from small amounts of contamination in olivine mineral separates. The steep slope of the new olivine/liquid pattern is consistent with olivine/clinopyroxene patterns previously obtained from inclusion-free olivines separated from peridotite nodules.