Zircon M257 - a homogeneous natural reference material for the ion microprobe U-Pb analysis of zircon

Zircon M257 - a homogeneous natural reference material for the ion microprobe U-Pb analysis of zircon

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...

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Bibliographic Details
Main Authors: Nasdala, L., Hofmeister, W., Norberg, N., Mattinson, J. M., Corfu, F., Dörr, W., Kamo, S. L., Kennedy, Allen K., Kronz, A., Reiners, P. W., Frei, D., Kosler, J., Wan, Y., Götze, J., Häger, T., Kröner, A., Valley, J. W.
Format: Journal Article
Published: Blackwell publishing 2008
Subjects:
secondary ion mass spectrometry
geochronology
M257
SIMS
zircon
U-Pb
reference material
Online Access:http://www3.interscience.wiley.com/cgi-bin/abstract/121427453/ABSTRACT
http://hdl.handle.net/20.500.11937/47515
Description
Summary: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 x 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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