Chemical (U-Th-Pb) dating of monazite: Analytical protocol for a LEO 1450VP scanning electron microscope and examples from Rogaland and Finnmark, Norway

Abstract

Monazite is a common accessory mineral and is a valuable mineral chronometer on account of its relatively high U and Th contents, negligible common Pb, and a high closure temperature. Because all the Pb present in monazite is derived from decay of U and Th, an age can be calculated based on the concentrations of U, Th and Pb. This contribution presents the analytical protocol for U-Th-Pb chemical dating of monazite using a LEO 1450VP scanning electron microscope. Using the analytical protocol described here, approximately 15 spots can be analysed in 24 hours. Instrumental drift during analytical sessions is monitored by repeated measurements of internal standards. Monazites of known age, ranging from Palaeoproterozoic, through Mesoproterozoic to Palaeozoic, yield chemical ages that are well within error of the isotopic ages. One pelite sample from the contact aureole to the Egersund anorthosite complex in the Sveconorwegian province, Rogaland contains monazite inclusions in garnet that yield amean age of 1002 \u00B117 Ma, interpreted to represent regional high-grade Sveconorwegian metamorphism, whereas matrix monazites yield a mean age of 923 \u00B119 Ma, corresponding to anorthosite magmatism and thermal overprinting. A kyanite-biotite schist from Sørøya in the West Finnmark Caledonides, the type area of the elusive Late Cambrian Finnmarkian event, contains monazites with a mean age of 416 \u00B110 Ma. This observation suggests that the high-grade metamorphism in the West Finnmark Caledonides was related to the Scandian event, and does not lend support to the existence of a Finnmarkian event in this area.