Dating of allanite

The accessory mineral allanite occurs in a wide range of igneous and metamorphic rocks and contains appreciable amounts of trace elements including the REEs, Sr, Th and U. The high degree of compositional substitution and variable incorporation of common lead into the allanite crystal structure however has limited its use for U-Th-Pb dating. Procedures have now been developed for the isotopic dating of allanite using Laser Ablation ICP-MS and the Sensitive High Resolution Ion Microprobe (SHRIMP). Both techniques require the use of external matrix-matched standards.

The wide range of Th/U in allanite from single samples makes it possible to use multiple LA-ICP-MS analyses to construct Th-Pb isochrons from which ages can be calculated with a precision of 1.4-5.8 % (95 % confidence level) at a spatial resolution of 32x32x20µm. Accurate (±1-3 %) and precise (1-2 %, 95 % confidence level), SHRIMP 208 Pb- 232 Th ages can be measured directly on allanite samples with REE+Th>0.5 atoms per formula unit, without additional matrix corrections at a spatial resolution of 17x21x2µm.

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