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Crystal-chemical constraints of U-Pb dating in monazite and xenotime from Aust-Agder pegmatites, Norway

The pegmatites of the Aust-Agder province, Norway, have been widely recognized for their numerous occurrences as well as interesting and versatile mineralogy. They formed during Sveconorwegian orogeny (around 1 Ga ago). The samples of monazite and xenotime were collected at two different pegmatite fields: the Evje-Iveland field with the localities of Eptevann and Eikeråsen, and the Froland field with the Garta locality. The collected minerals were heated at 200, 500, 800 and 1000°C at 1 atm for 24 hours, and examined by XRD and SEM to investigate their crystal-chemical properties and thermal behaviour. The ages of the monazite samples were determined using U-Pb geochronology by quadrupole LA-ICP-MS technique. The electron microprobe analysis gave a monazite-(Ce) composition for both monazite samples concerned, with Ce2O3 ranging from 22.69 to 24.86%, Y2O3 0.05 to 2.25%, ThO2 9.18 to 9.92%, and UO2 0.47 to 0.53%. The xenotime-(Y) sample contains 45.16% Y2O3, 0.31% ThO2, 0.31% UO2, and is significantly impoverished in LREE (below detection limits). The age of 868±20 Ma was obtained for the Evje-Iveland monazite from Eptevann, and 1023±27 Ma for the Kongsberg-Bamble monazite from Garta. A thorough SEM analysis using back-scattered electrons and energy-dispersive detector revealed numerous inclusions therein. Inclusions of ThSiO4 are found in the monazite samples, which are sometimes surrounded by Y-rich monazite/xenotime and apatite. The xenotime sample also contains ThSiO4 inclusions with frequent uraninite inclusions. Zircon, mica and quartz are intimately intergrown with xenotime. Annealing experiments induced a slight decrease of unit cell volume ranging 0.70-0.76% in monazite, and 0.72 % in xenotime. The unit cell decrease is greater in the monazite from Eptevann which is more enriched both in Y and Th. However, both monazite samples have unit cell volume very close to that of the pure CePO4, and when heated up to 1000°C it is approaching to the one of pure huttonite. The reduction of the unit cell volume of the xenotime sample indicates reinforcement of the xenotime structure, since any additional incorporation of Th or U from the ThSiO4/UO2 inclusions into its structure would result in a unit cell increase. This is also supported by X-ray diffraction data, which showed occurrence of a monazite-type phase at temperatures above 800°C along with the pre-existing xenotime. SEM examinations of the samples heated at 1000°C showed that the monazite, as well as the ThSiO4 inclusions, is cracked. Sparse YPO4 inclusions are still present, being positioned rather over than around ThSiO4 inclusions as in the case of the unheated samples. The crackings also appeared in the case of xenotime both in the mineral matrix and the inclusions. Generally, the crackings divide the ThSiO4 inclusions from the xenotime matrix. The idiomorphic inclusions of UO2 in ThSiO4, especially those situated close to the edges of the ThSiO4 grains, separated from the ThSiO4 by the crackings. The occurrence of the monazite-type phase in the XRD pattern of the annealed xenotime sample could be related to the ThSiO4 inclusions that re-crystallized as huttonitic phase due to the heating treatment. Also, inclusions with cheralite-type composition seem to be present in the heated xenotime, and thus contribute to the monazite-structure peaks in the XRD pattern. The thermal treatment of the monazite and xenotime samples showed their thermal stability as well as the one of the ThSiO4 inclusions, which recrystallize as huttonite in the case of both minerals. In spite of numerous ThSiO4/UO2 inclusions, the dating for both monazite samples gave ages comparable to the previous zircon ages.

U-Pb dating; monazite; xenotime

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