Darling, and 2 co-authors, reported in 2012 that the rare earth element (REE) rich epidote-group mineral allanite has great potential as a geochronometer for a wide range of geological processes. Its utilization has been hampered by several analytical challenges:
- Often-high common-Pb contents (208Pbcomon/208Pbtotal range from <1 to 48% in this study) and lack of well characterized and widely available allanite standards, and wide-ranging compositional variations that have been shown to cause matrix effects during in-situ analyses. Here the authors tested laser ablation ICPMS analytical protocols that aim to overcome these challenges, using a suite of allanite reference materials that range from ca. 30 to 420 Ma in age, for which independent U–Th–Pb isotopic data are available.
- Using a 213 nm Nd:YAG laser, coupled with an Agilent 7500 cs (quadrupole) ICPMS, analytical protocols focused upon minimizing the causes of matrix sensitivity in the laser ablation ICPMS process. This has primarily been achieved via dynamic (raster) ablation, which greatly reduces time-dependent laser induced elemental fractionation. Accordingly, a non-matrix matched external standardization approach is adopted, utilizing the zircon standards Plesovice, 91500 and GJ1. Accurate common-Pb corrections are critical to allanite geochronology, and here we advocate an approach based upon the measured intensity of 204Pb,
as it minimizes assumptions and allows for simple and robust error propagation. - For each of the allanite reference materials, mean common-Pb corrected 208Pb/232Th ages and Tera–Wasserburg regression ages are within uncertainty of reference values. Three of the samples (SISS, BONA and AVC) have significantly higher 206Pb/238U ages, which reflects incorporation of 230Th during crystallization,
and hence 206Pb excess. Such isotopic disequilibrium, together with the high Th/U ratios of allanites (up to 1000), makes the Th–Pb dating system preferable, particularly for relatively young allanites (i.e. Phanerozoic). That accurate age information has been generated from allanites of wide ranging composition, suggests that matrix effects are not significant in the authors' analyses, on the scale of uncertainties generated: 0.5 to 1.5% (2σ) on mean 208Pb/232Th ages. The final propagated uncertainty in ages is a function of the common-Pb content, uncertainty in the measurement of 202Hg/204Pb, and uncertainty in the isotopic composition of the common-Pb inherited into the mineral. Our results show that accurate U–Th–Pb geochronological information, at geologically useful levels of precision, can be determined from allanite with relatively simple analytical and data reduction protocols, and without the requirement for matrix-matched standardization.
Procedures have been defined that can produce accurate U–Th–Pb isotope data from allanite, without the requirement of primary allanite standards, at geologically useful levels of precision. This has principally been achieved by minimizing time dependent fractionation during laser ablation ICPMS analysis, via a dynamic line raster ablation
protocol and careful optimization of the ICPMS. Additional laser-sample interaction, aerosol transport and plasma processes that can lead to matrix sensitivity in LA-ICPMS must have a relatively small effect compared to analytical uncertainties. Normalization of allanite analyses to zircon standards has been shown to provide accurate
age information from allanites of wide ranging composition. This suggests that matrix effects are not significant on the scale of uncertainties generated: 0.5 to 1.5% (95% conf.) on mean 208Pb/232Th ages.
Accurate common-Pb corrections are critical to allanite geochronology, and the authors advocated an approach based upon measured 204Pb. This is shown to provide accurate and reproducible corrections on allanites ranging in 208Pb COM from >1 to 48%, with the final propagated uncertainty in ages being a function of common-Pb content, uncertainty in the measurement of 202Hg/204Pb, and uncertainty in the isotopic composition of the common-Pb inherited into the mineral.
The U–Pb system is more problematic in this regard, because the radiogenic 206Pb and 207Pb signals in Phanerozoic allanites are often swamped by common-Pb, due to the relatively low uranium concentrations. An additional problem of the U–Pb system is the tendency of allanite to incorporate 230Th during growth, an intermediary of the 238U–206Pb decay system. This leads to 206Pb excess, and older apparent 206Pb/238U ages, which has been observed for the SISS, BONA and AVC allanite samples analysed. The Th–Pb system is undoubtedly the most useful in Phanerozoic allanites, which have been the focus in this study, but unfortunately the Th–Pb system does not allow for evaluation of concordance. The allanite reference materials analysed here have consistent 208Pb/232Th, 206Pb/238U ages (except for those samples with 206Pb excess) and Tera–Wasserburg semi total U–Pb concordia intercept ages.
Although the uncertainties on the U–Pb ages are typically significantly greater than the Th–Pb ages, combining these systems offers a powerful tool for allanite geochronology. It is important to note that the analytical setup used in this study is relatively common in research institutions, with a 213 nm Nd:YAG laser coupled to a quadrupole ICPMS. The procedures defined here will hopefully enable many laboratories to produce accurate and precise geochronological data from allanite, a geochronometer with great potential in a wide range of geological settings.
Supplementary materials related to this article can be found online at doi:10.1016/j.chemgeo.2011.11.012.
Managed & Built by I2M Consulting, LLC | © 2010-
