Uranium incorporation in garnet

Uranium contents in natural garnet are typically in the range of 1–50 μg/g in andraditic garnet found in alkali-magmatic rocks and skarns, but are orders of magnitude lower in metamorphic almandine–pyrope with typically 1–100 ng/g. Nonetheless, the instrumental setup at FIERCE coupling laser-ablation with a modern MC-ICPMS system enables us to routinely extract U–Pb ages even from samples of the latter group at a geologically meaningful level of precision. However, limited knowledge of the mechanisms and systematics of U incorporation into garnet hamper the processes of sample selection and data interpretation for garnet petrochronology.

We find that U is predominantly incorporated into both andraditic and almandine–pyrope garnet in its tetravalent form and that it strongly favours the dodecahedral X site of garnet. The elbrusite substitution, which includes hexavalent U does not operate at the trace-element level of U in natural garnet. Instead, the incorporation of tetravalent U into the dodecahedral X-site dominates and is charge balanced through tetrahedral ferric Fe and/or through divalent cations in the Y-site. Many andradite samples show oscillatory zoning with a covariation of U, Th and the light REE, demonstrating that incorporation of the lanthanides and actinides may be controlled by the same crystal chemical mechanisms. Some samples additionally show clear sector zoning in U (and Th and Ce) with a 20–50 % enrichment of U in the {211} sectors compared to the {110} sectors, demonstrating a control of crystal growth rate on U incorporation.