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Zircon M257 - a Homogeneous Natural Reference Material for the Ion Microprobe U-Pb Analysis of Zircon

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Year:
2008
Type of Publication:
Article
Keywords:
zircon, reference material, M257, U-Pb, geochronology, SIMS, secondary ion mass spectrometry
Authors:
Nasdala, Lutz; Hofmeister, Wolfgang; Norberg, Nicholas; Martinson, James M.; Corfu, Fernando; Dörr, Wolfgang; Kamo, Sandra L.; Kennedy, Allen K.; Kronz, Andreas; Reiners, Peter W.; Frei, Dirk; Kosler, Jan; Wan, Yusheng; Götze, Jens; Häger, Tobias; Kröner, Alfred; Valley, John W.
Journal:
Geostandards and Geoanalytical Research
Volume:
32
Number:
3
Pages:
247-265
ISSN:
1751-908X
BibTex:
Abstract:
We introduce and propose zircon M257 as a future reference material for the determination of zircon U-Pb ages by means of secondary ion mass spectrometry. This light brownish, flawless, cut gemstone specimen from Sri Lanka weighed 5.14 g (25.7 carats). Zircon M257 has TIMS-determined, mean isotopic ratios (2s uncertainties) of 0.09100 ± 0.00003 for 206pb/238U and 0.7392 ± 0.0003 for 207pb/235U. Its 206pb/238U age is 561.3 ± 0.3 Ma (unweighted mean, uncertainty quoted at the 95% confidence level); the U-Pb system is concordant within uncertainty of decay constants. Zircon M257 contains ∼ 840 μg g−1 U (Th/U ∼ 0.27). The material exhibits remarkably low heterogeneity, with a virtual absence of any internal textures even in cathodoluminescence images. The uniform, moderate degree of radiation damage (estimated from the expansion of unit-cell parameters, broadening of Raman spectral parameters and density) corresponds well, within the “Sri Lankan trends”, with actinide concentrations, U-Pb age, and the calculated alpha fluence of 1.66 × 1018 g−1. This, and a (U+Th)/He age of 419 ± 9 Ma (2s), enables us to exclude any unusual thermal history or heat treatment, which could potentially have affected the retention of radiogenic Pb. The oxygen isotope ratio of this zircon is 13.9%o VSMOW suggesting a metamorphic genesis in a marble or calc-silicate skarn.

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