Formation of monazite and xenotime inclusions in fluorapatite megacrysts, Gloserheia Granite Pegmatite, Froland, Bamble Sector, southern Norway

Abstract

Xenotime and monazite inclusions in fluorapatite megacrysts from a granitic pegmatite, Gloserheia, Froland, Bamble Sector, southern Norway are described utilizing high contrast backscattered electron imaging of cross sections of a selection of fluorapatite crystals. Electron microprobe analysis is then used to further characterize the xenotime and monazite, as well as (Y+REE) normal and depleted regions in the fluorapatite. In the (Y+REE) normal regions Y2O3 ranges from 0.4 to 1.3 whereas it ranges from below the electron microprobe detection limit to around 0.4 in the depleted regions. Low Y values in monazite (XY = 0.01−0.05) co-existing with xenotime indicates that inclusion formation in the originally (Y+REE)-enriched fluorapatite must have occurred below 300°C. Formation of the xenotime and monazite inclusions is attributed to fluid-aided coupled dissolution-reprecipitation processes during the later stages of subsolidus cooling of the pegmatite. The fluorapatite megacrysts are hypothesized to have under gone two major fluid-induced alteration events. The first occurred sometime after crystallization was complete at temperatures below 300°C and resulted in the initial formation of the xenotime and monazite inclusions. The second occurred at some later time as the product of a relatively limited fluid infiltration, also under T < 300°C. This resulted in the formation of (Y+REE)-depleted regions along lattice and cleavage planes while at the same time promoting Ostwald ripening of the xenotime inclusions resulting in larger grains in the (Y+REE)-depleted areas.