Allanite U-Th-Pb Geochronology by Ion Microprobe - 2020

Rare Earth Deposits & Mining

Liao, and 4 co-authors, reported in 2020 that allanite, an epidote group mineral, occurs as a common accessory mineral in igneous, metamorphic rocks and hydrothermal deposits. It contains radioactive elements Th and U, making it a valuable U–Th–Pb geochronometer for various geological processes. It all contains REE. Due to the structural and compositional complexity of allanite, in situ dating method is a better choice when the allanite is heterogeneous or has crystallized in multi-stage events. The wide variation of chemical compositions, however, raises the potential for matrix effects and complicates the use of in situ methods for allanite U–Th–Pb chronology. To address this issue, we made a comprehensive investigation on allanite U–Th–Pb chronology using Secondary Ion Mass Spectrometry (SIMS).

Five allanite samples (CAP, Daibosatsu, SQ-51, Toba OTT, TARA allanite) with variable elemental compositions and common lead concentrations have been used. Allanites with variations in FeO (from 12.8 to 16.1 wt%) and similar ThO₂ contents (around 1–2%) do not show significant matrix effects, as suggested by a previous study. While, obvious changes in calibration parameters (UO₂+/U+, ThO₂ +/Th+) were noticed for allanites with a large range of Th contents (from 300 ppm to 2.0 wt%). By employing power law relationships between Pb+/U+ versus UO₂ +/U+, Pb+/Th+ versus ThO₂ +/Th+ with suitable exponentials, the authors did not observe obvious matrix effects with ThO₂ concentrations variations.

The authors concluded that allanite can be a good geochronometer for multi-stage mineralization of hydrothermal deposits, providing valuable complementary information to zircon and monazite:

1) By applying power law relationships between Pb+/U+ versus UO₂+/U+, Pb+/Th+ versus ThO₂+/Th+ and suitable exponentials, SIMS U–Th–Pb dating on allanites with a range of FeO (from 12.8 to 16.1 wt%) and ThO₂ (from 300 ppm to 2.0 wt%) exhibit no obvious matrix effects.

2) Allanite has a propensity to incorporate large and variable amounts of common Pb, challenging the accuracy and precision of U–Th–Pb dating. ²⁰⁴Pb-based common lead correction method works well for low Th allanites, but would be problematic for high Th allanite. 207Pb-based method is suitable for older sample without obvious excess ²⁰⁶Pb. For young samples, ²⁰⁷Pb⁺ can be regarded as all common Pb and used for common Pb correction for single-spot analyses or via an isochron regression method.

3) The accuracy of allanite U–Th–Pb dating is predominantly controlled by the common Pb proportion (_f²⁰⁶ and _f²⁰⁸). For Phanerozoic allanites with high Th and relative low common Pb (_f²⁰⁶ 20%, _f²⁰⁸ < 1%), the precision of weighted average U–Pb and Th–Pb ages both can be achieved within 2% (at 95% confidence). For hydrothermal allanite with high U and low common Pb (_f²⁰⁶, _f²⁰⁸ < 1%), the precision can be better than 1.5% (at 95% confidence).

4) Hydrothermal allanites in IOCG deposits has very low common Pb and high U, which simultaneously makes precise U–Pb, Th–Pb and Pb–Pb ages feasible. The dating results exhibit robust radioactive systems of multi-stage crystallized allanites. Allanite U–Th–Pb dating can potentially be applied to more hydrothermal deposits.