Allanite U-Th-Pb geochronology by ion microprobe

• Published in: Journal of analytical atomic spectrometry 2020, Vol.35 (3), p.489-497
• Main Authors: Liao, Xin, Li, Qiu-li, Whitehouse, M. J, Yang, Yue-heng, Liu, Yu
• Format: Article
• Language: English
• Subjects: Chemical composition; Crystallization; Dating techniques; Den föränderliga jorden; Geochronology; Metamorphic rocks; Monazite; Radioisotopes; Secondary ion mass spectrometry; The changing Earth; Thorium; Thorium oxides; Uranium dioxide; Zircon
• ISSN: 0267-9477; 1364-5544; 1364-5544
• DOI: 10.1039/c9ja00426b

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. 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 ThO 2 contents (around 1-2%) do not show significant matrix effects, as suggested by a previous study. While, obvious changes in calibration parameters (UO 2 + /U + , ThO 2 + /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 UO 2 + /U + , Pb + /Th + versus ThO 2 + /Th + with suitable exponentials, we do not observe obvious matrix effects with ThO 2 concentrations variations. This study demonstrates that allanite can be a good geochronometer for multi-stage mineralization of hydrothermal deposits, providing valuable complementary information to zircon and monazite. No matrix effect from Fe and Th variation was detected on SIMS allanite U-Th-Pb dating.