An electron microprobe study of the U–Th–Pb systematics of metamorphosed monazite: the role of Pb diffusion versus overgrowth and recrystallization

Abstract

An electron microprobe (EMP) was used to obtain U–Th–Pb chemical ages from monazite that underwent an amphibolite facies metamorphic overprint in the Monashee complex, southern Canadian Cordillera. U–Pb isotope dilution dating of single grains yielded discordant ages that plot in linear arrays on a concordia diagram; upper intercepts of the arrays indicate primary growth in the Proterozoic and Cretaceous, and lower intercepts are coeval with a Tertiary metamorphic event. The microprobe's high spatial resolution (∼2 μm) and ability to quickly differentiate between domains with greatly contrasting U, Th, and Pb concentrations made it the ideal tool for investigating the cause of the U–Pb isotopic discordance. The microprobe dating shows that Proterozoic monazite lies directly adjacent to generally small (<20 μm thick) Tertiary domains that exist along embayments into the Proterozoic domains, in inclusion-rich zones, and in cracks. Tertiary monazite is typically U- and Th-richer than the Proterozoic monazite, although it locally has normal to slightly lower U and Th concentrations. U-rich domains also rim Cretaceous grains, but low Pb concentrations preclude differentiation between Cretaceous and Tertiary domains. The uniformity of the Proterozoic primary growth ages and the sharpness of the boundary between Proterozoic and Tertiary domains require that Pb diffusion was negligible, despite the attainment of substantial temperatures (600°C–650°C) in the Tertiary (based on the metamorphic mineral assemblage in overlying schists). The presence of Tertiary domains in locations that are susceptible to fluid interaction suggests that fluids locally dissolved Proterozoic monazite and precipitated Tertiary monazite during the overprint; this mechanism caused most of the U–Pb isotopic discordance. We use Pb diffusion data gathered by previous studies to contend that the absence of Pb diffusion was due to a short duration of elevated temperatures, perhaps only a few million years.