Allanite U-Th-Pb geochronology by ion microprobe

摘要

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.