Light-rare-earth-element zoning in sphene and allanite during granitoid fractionation.

The light-REE concentrations in sphene and allanite and in rocks have been found to increase during fractionation of granitic rocks. In both sphene and allanite crystals, the LREE concentrations are zoned, with relative depletion occurring at the rims. Such REE concentration variability and zoning in these minerals can be explained by changes in the melt structure that occur during differentiation and final solidification of the granitic rocks. During the final stages of solidification the sufficiently enriched F breaks bridging oxygen bonds and thus increases the ratio of nonbridging oxygen to tetrahedral cation (NBO/T), thereby decreasing sphene and allanite partition coefficients. The data suggest that crystal-melt separation controls LREE enrichment or depletion in granitic rocks and that the end result depends on liquidus accessory and/or restite phases present during the crystal-melt separation. -L.di H.