Allanite micro-geochronology: A LA-ICP-MS and SHRIMP U-Th-Pb study

Abstract

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 the variable incorporation of common Pb into the allanite crystal structure, however, have 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). The accuracy of those procedures has been demonstrated by dating six Phanerozoic allanite samples of known age and with different FeO, REE and Th contents. Both analytical techniques require normalising factors for the measurement of208Pb/232Th and206Pb/238U, necessitating 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 32 × 32 × 20μm. A207Pb-based correction is used to estimate the fraction of common Pb in individual LA-ICP-MS analyses with a precision of 0.3-2%. Accurate (± 1-3%) and precise (1-2%, 95% confidence level) SHRIMP208Pb/232Th 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 17 × 21 × 2μm. LA-ICP-MS is an efficient technique for dating melt-precipitated allanite (e.g., from igneous or migmatitic rocks). SHRIMP analysis is preferable for samples that have a relatively small grain size, are isotopically complex or have relatively large common Pb contents (e.g., metamorphic allanite).