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

Sphene and allanite rare-earth-element (REE) and whole-rock light-REE concentrations have been found to increase during granitoid fractionation. The REE concentrations in both sphene and allanite crystals are zoned, with relative depletion occurring at the crystal rims. The 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 granitoid. Our data suggest that crystal-melt separation controls light-REE enrichment or depletion in granitoids and that the end result depends on liquidus, accessory, and/or restite phases present during the crystal-melt separation.