LGC-1: A zircon reference material for in-situ (U-Th)/He dating

Abstract

A pairwise in-situ (U-Th)/He dating method has been proposed for mitigating matrix-related bias in U and Th measurements using synthetic reference materials. This method requires a natural zircon reference material whose (U-Th)/He age should be homogeneous on the scale (~10-100µm) to be used in such dating experiments. A newly characterized zircon LGC-1 megacryst fulfils this requirement. This pale-yellowish, flawless Sri Lanka gem specimen is about 1.2*0.8*0.8cm in size. Optical microscopy, cathodoluminescence-imaging, X-ray elemental mapping, and Raman spectroscopy on a large number of random shards did not reveal any detectable textural and compositional heterogeneity. Laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) analyses on a large number of randomly selected fragments yield 266 measurements of U, Th and Pb concentrations, which are within the corresponding experimental uncertainties. The weighted mean U, Th and Pb concentrations are 357.7±1.8ppm, 740.9±5.0ppm, and 39.06±0.18ppm, respectively, with a weighted mean Th/U ratio of 2.07±0.01, indistinguishable from Isotope Dilution ICP-MS (ID-ICP-MS) and Thermal Ionization Mass Spectrometry (ID-TIMS) results. ID-TIMS U/Pb ages are concordant within uncertainties of decay constants, with a concordia age of 541.70±0.70Ma. Conventional (U-Th)/He dating on 28 random shards from the crystal in different laboratories gives a central age of 476.4±5.7Ma. Six in-situ (U-Th)/He analyses yield consistent 4He concentrations and ages with weighted mean values of 1248±46nmol/g and 462±21Ma, respectively. Fractions of this zircon have been shared with several laboratories in the Australia, China, UK and US, and are expected to serve as a reference for both in-situ and conventional (U-Th)/He analyses. The combination of analytical methods used to characterize LGC-1 zircon may be used as a template for future age reference calibration.