Investigating the crystallization age and initial lead composition of Phanerozoic magmatic allanites by LA-MC-ICP-MS

Allanite is a REE-bearing, epidote-group mineral that incorporates Th, U, but also initial Pb. This results in UPb isotopic analyses defining a mixing trend between an initial and radiogenic Pb component. So far, only the latter has been given attention because it can reveal the age of various geological events, whereas the significance of the former has been neglected. We explore both the age and initial Pb isotopic composition of nine magmatic samples (volcanic, plutonic, pegmatitic) with emplacement ages that cover nearly the entire Phanerozoic (∼ 540–19 Ma). We first characterize the major-element composition of allanite by EPMA, then measure its (Th–)U–Pb isotopic composition in situ by LA-MC-ICP-MS using zircon as reference material. UPb data are regressed to determine both the crystallization age and initial 207Pb/206Pb0 composition. These results are compared to the UPb age of co-genetic zircon and the 207Pb/206Pb ratio of coexisting feldspar from the same samples or localities, respectively. Their good agreement (mainly less than ∼4 % and 1 % offset, respectively) supports the validity of our analytical protocol. The accuracy of age and 207Pb/206Pb0 results is governed by the proportion of initial Pb, f(206Pb0), especially its minimum (for age)/maximum (for 207Pb/206Pb0) value, while precision is chiefly controlled by the centroid of f(206Pb0) distribution. Calculations using published allanite/melt partition coefficients show that a 230Th disequilibrium correction based on the whole-rock Th/U is not only appropriate for magmatic samples, but also necessary to improve accuracy. In four samples the 207Pb/206Pb0 ratios obtained from allanite are lower than values predicted by terrestrial Pb evolution models at the time of emplacement, showing that anchoring a regression line to a fixed Pb composition is not always valid. Although it does not affect age determination, it overlooks the potential of allanite UPb data for constraining the radiogenic character of crustal magma sources.