Research Spotlight Archives

Calibrating the Archean Eon with a New Zircon Reference Material

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.

Corey’s Papers:

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.

Scoates, J.S. and Wall, C.J., 2015. Geochronology of Layered Intrusions. In: Charlier B., Namur O., Latypov R., Tegner C. (eds) Layered Intrusions. Springer Geology. Springer, Dordrecht. pp. 3-74.