The Gloserheia granite pegmatite, situated in the Precambrian Kongsberg-Bamble formation, contains eight different zones (core, intermediate zones I-V, wall zone, and border zone). Apatite occurs in the core and in zones I and III. Xenotime has been found included in apatite from all three zones, and in other parageneses within these zones. The apatite close to xenotime inclusions is depleted in REE. Zoning of xenotime is characterized by an enrichment of the light REE relative to the heavy REE, from the center towards the rim of the cryslal. Xenotime crystals included in apatite from the outer zones show less relative enrichment in the light REE than do those in the inner zones of the pegmatite. No such trend in REE fractionation is shown by xenotimes from other parageneses. The rare earth minerals included in apatite are considered to have formed by exsolution, possibly accompanied by metasomatism. Fractionation in the apatite-xenotime system was caused by processes bearing a simple relation to the crystallization of the zones where these minerals occur. In contrast, the REE fractionation in the other xenotimes probably has a more complex relation to the sequence of zones in the pegmatite.

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