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 U Pb 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. U Pb data are regressed to determine both the crystallization age and initial 207 Pb/ 206 Pb 0 composition. These results are compared to the U Pb age of co-genetic zircon and the 207 Pb /206 Pb 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 207 Pb/ 206 Pb 0 results is governed by the proportion of initial Pb, f( 206 Pb 0 ), especially its minimum (for age)/maximum (for 207 Pb/ 206 Pb 0 ) value, while precision is chiefly controlled by the centroid of f( 206 Pb 0 ) distribution. Calculations using published allanite/melt partition coefficients show that a 230 Th 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 207 Pb/ 206 Pb 0 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 U Pb data for constraining the radiogenic character of crustal magma sources. • Both age and initial Pb in allanite can be obtained with low-volume, low-fluence ablation and zircon as reference material. • A Pb-anchored regression of U–Pb data hardly affects age accuracy, but might assume an incorrect initial Pb composition. • The 207 Pb/ 206 Pb 0 of magmatic allanite is similar to that of coexisting feldspar and gives insight on the magma source. • Allanite in Al-rich magmatic rocks preserves REE zoning whereas that in Al-poorer ones has a nearly constant ΣREE content.