Allanite U-Th-Pb geochronology by laser ablation ICPMS

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 (Pb-208(comon)/Pb-208(total) from <1 to 48% in this study), a 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 we test 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 7500cs (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 Pb-204, as it minimizes assumptions and allows for simple and robust error propagation. For each of the allanite reference materials, mean common-Pb corrected Pb-208/Th-232 ages and Tera-Wasserburg regression ages are within uncertainty of reference values. Three of the samples (SISS, BONA and AVC) have significantly higher Pb-206/U-238 ages, which reflects incorporation of Th-230 during crystallization, and hence Pb-206 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 our analyses, on the scale of uncertainties generated: 0.5 to 1.5% (2 sigma) on mean Pb-208/Th-232 ages. The final propagated uncertainty in ages is a function of the common-Pb content. uncertainty in the measurement of Hg-202/Pb-204, 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. (C) 2011 Elsevier By. All rights reserved.