Smye, and 6 co-authors, reported in 2014 that allanite has the potential to be a useful chronometer of crustal evolution, forming in response to a wide spectrum of metamorphic and magmatic conditions and incorporating weight-percent concentrations of LREE, Th and U. Despite its growing use in in situ U–Th–Pb geochronology,allanite reference materials lack sufficient U–Th–Pb isotopic characterization and little is known concerning the response of U–Th–Pb systematics of allanite to hydrothermal alteration and self-irradiation.
This paper presents the results of a combined ID-TIMS and LA-ICPMS U–Th–Pb study on a suite of five allanite crystals, spanning ~ 2.6 Ga and including three commonly-used allanite reference materials: the Siss, Bona and Tara allanites. Siss and Bona allanites preserve an inherited ca. 1 Ga Pb component, consistent with the presence of xenocrystic allanite cores or the presence of zircon micro-inclusions. Tara allanite yields concordant U–Pb ages (407–430 Ma), but is affected by Th/U fractionation, likely caused by hydrothermal alteration. Additionally, the tendency for Th to become mobilized post-crystallization is further evidenced by two Precambrian allanite megacrysts, LE40010 (ca. 2.8 Ga) and LE2808 (ca. 1.1 Ga), that both exhibit discordant Th/Pb analyses, linked to the formation of thorite micro-inclusions along hydration pathways.
Self-irradiation dose versus discordance relationships show that a percolation threshold is present in allanite at cumulative dose values close to 3 X 1017 a-decay g-1, an order of magnitude smaller than zircon. Collectively, the presence of common-Pb and excess-206Pb, its susceptibility to incur Th/U fractionation and hydrothermal Pb-loss complicates the use of allanite as a geochronometer. These factors explain dispersion of ~ 4% in the isotopic compositions of Siss and Tara allanites measured by LA-ICPMS, providing a fundamental limit on the accuracy of the allanite chronometer using these reference materials.
Using both ID-TIMS and LA-ICPMS techniques, the authors have characterized the U–Th–Pb systematics of five allanite samples, including the Siss, Bona and Tara allanites, currently in use as standards for microbeam allanite U–Th–Pb geochronology. Both Siss and Bona allanites are found to include an inherited Pb component attributable to either the presence of xenocrystic core domains, or the presence of zircon microinclusions. The presence of old allanite cores in allanite from the Bergell intrusion is consistent with previous observations that solid-state Pb diffusion through the allanite lattice must operate over length scales shorter than ~ 300 um at temperatures and timescales less than 700–800 °C and 65 million years, respectively. Despite being strongly retentive of Pb against volume diffusion, two pegmatite hosted allanite megacrysts, LE40010 and LE2808, exhibit disturbed U–Th–Pb isotope systematics, reflecting interaction with crustal fluids. Allanite megacryst LE40010 experienced extreme Th/U fractionation to yield apparent 208Pb/232Th ages > 9 Ga. Allanites Tara and LE2808 also exhibit evidence for mobilization of Th over length scales comparable to grain diameters (300–500 um). Micrometer-scale precipitates of thorite, and rare uraninite provide physical evidence for dissolution– precipitation reactions during fluid-driven alteration.
Dose–discordance relationships exhibited by allanites LE40010, LE2808 and Tara are consistent with the explanation that increasing structural disorder in the allanite lattice elevates the susceptibility of the grain to dissolution-driven Th/U fractionation and facilitates faster rates of diffusive Pb-loss. These calculations also suggest that allanite is affected by a percolation point at ~ 3 X 1017 a-decay g-1, an order of magnitude smaller than determined for zircon.
The authors concluded that Pb/Pb, U/Pb or Th/Pb ratios obtained from single grains vary by > 4% (2δ) for each of the allanites considered. This provides a limiting uncertainty in the use of these materials as allanite standards for in situ geochronology. Furthermore, the U–Th–Pb isotopic data presented suggest that allanite rarely fulfill the commonly- assumed condition of U–Th–Pb concordancy, meaning that it is important to evaluate both U–Pb and Th–Pb ages before correcting for common-Pb. The authors have confirmed the importance of matrix composition on the magnitude of laser induced elemental fractionation for allanite and show that Pb/U fractionation factors are consistently smaller than those derived for Pb/Th fractionation.
The authors also concluded that allanite LE40010 is a suitable primary reference material for allanite U–Th–Pb geochronology. However, its effective use is critically dependent on screening individual shards for chemical alteration. Tara allanite is favored over Siss and Bona allanites as a secondary U–Pb reference material or a primary reference material provided that Pb/ Pb fractionation factors can be derived from zircon. Siss allanite is best suited for use as a Th–Pb reference material.
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