by Corey Wall
U-Pb zircon geochronology is one of the most precise techniques for resolving the rates of processes through geologic time. As a result, it has become a major contributor to our understanding of the timescales that Earth’s processes occur over. An important aspect of any analytical technique is the availability of well-characterized natural reference materials to allow for direct comparison of results between laboratories, for determining the accuracy of unknowns, and for assessment of systematic errors.
Laser ablation-inductively coupled-mass spectrometry (LA-ICP-MS) is now the most widely used technique for determining U-Pb dates in zircon. However, the use of naturally occurring reference materials for matrix-matched age calibration, downhole fractionation correction, and quality control are required. Most of the widely available zircon reference materials are younger than 1 Ga and as a result, there are currently no materials that are age-matched for much of the geologic timescale.
To initially characterize a natural reference material, there must be an established method to determine the true value of the material under question, whether for U-Pb geochronology, isotopic composition, or elemental concentration determination. Chemical abrasion-isotope dilution-thermal ionization mass spectrometry, or CA-ID-TIMS, has become the gold standard technique for U-Pb geochronology.
U–Pb dates for Stillwater AN2 zircon were established by several different methods, including (1) conventional isotope dilution–thermal ionization mass spectrometry (ID-TIMS) of grains that were air-abraded, (2) chemical abrasion-ID-TIMS or CA-TIMS where grains were annealed and acid-leached prior to dissolution, and (3) LA-ICP-MS.
The U–Pb results by all methods for Stillwater AN2 zircon are concordant and yield indistinguishable concordia and weighted mean 207Pb/206Pb dates, with the latter ranging in relative precision from 2710.44 ± 0.32 Ma (n = 9) by CA-TIMS to 2710.56 ± 0.65 Ma by ID-TIMS (n = 4) to 2715 ± 9 Ma by LA-ICP-MS (n = 58).
Consequently, it is concluded that the Stillwater AN2 zircon grains represent a suitable geochronological reference material that can be used for a variety of U–Pb zircon analytical methods, especially for LA-ICP-MS dating of Archean zircon, and for comparison of U–Pb zircon dating results between different analytical laboratories worldwide.
Wall, C.J., Scoates, J.S., and Weis, D., 2016. Zircon from the Anorthosite zone II of the Stillwater Complex as a U–Pb geochronological reference material for Archean rocks, Chemical Geology, 436, 54–71.
Wall, C.J. and Scoates, J.S., 2016. High-Precision U-Pb Zircon-Baddeleyite Dating of the J-M Reef Platinum Group Element Deposit in the Stillwater Complex, Montana (USA), Economic Geology, 111, 771–782.