Nanoscale features revealed by a multiscale characterisation of discordant monazite highlight mobility mechanisms of Th and Pb

International audienceUnderstanding radionuclides mass transfer mechanisms in monazite (LREEPO4) and the resulting features, from the micro- to the nanoscale, is critical to its use as a robust U-Th-Pb geochronometer. A detailed multiscale characterisation of discordant monazite grains from a granulite which records a polymetamorphic history explores the mechanisms of Th and Pb mobility in crystals. Some monazite grains display Th-rich linear features (0.1-1 µm thick) forming a regular network throughout the grain. They are interpreted as resulting from fluid ingress along crystallographically controlled pathways. Nanoscale features termed `clusters' (Ø 2O3). They are associated with a chemically varied suite of amorphous silicate (± Al, Mg, Fe) phases or sulphur (e.g. FeS). They are interpreted as precipitates within monazite crystals. They are formed during replacement mechanism of monazite through fluid interactions. Two generations of Pb*-bearing nanophases exist supported by previous geochronological data. The shielding effect of garnet and rutilated quartz (host minerals), limiting fluid access, induces plentiful Pb*-bearing nanophases precipitation (fluid saturation enhanced) and limits Pb*-loss at the grain scale. This multiscale study provides new insights for interpretations of meaningless geochronological information, thanks to nanoscale investigations