Zircon from the Anorthosite zone II of the Stillwater Complex as a U–Pb geochronological reference material for Archean rocks

Abstract

Well-characterized reference materials are essential for precise and accurate geochronology. The growing use of in situ techniques, especially U–Pb zircon dating by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), requires the availability of natural reference materials for matrix-matched calibration, age normalization, and quality control. Although there are a number of zircon reference materials available for U–Pb LA-ICP-MS (e.g., Plešovice, 91500, Temora, FC-1), there is currently no widely available Archean zircon reference material for U–Pb geochronology. Zircon from a leucogabbro in the Anorthosite zone II (AN2) of the Neoarchean Stillwater Complex (Montana, USA) is a candidate for an Archean reference material. The AN2 leucogabbro contains abundant zircon with simple sector zoning typical of mafic igneous rocks and with no inheritance of older grains; accessory baddeleyite and secondary rutile and titanite are also present. Trace element concentrations and ratios determined by LA-ICP-MS are variable in Stillwater AN2 zircon (e.g., U=54–585ppm, Hf=5500–9800ppm, Ti=9–22ppm, Th/U=0.38–0.70, Ce/Nd=0.76–18) and consistent with crystallization from highly fractionated interstitial melt at near-solidus temperatures (Ti-in-zircon thermometry=768–864°C) in plagioclase-rich cumulates. 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±0.32Ma (n=9) by CA-TIMS to 2710.56±0.65Ma by ID-TIMS (n=4) to 2715±9Ma by LA-ICP-MS (n=58). The in situ dates are statistically identical both at a scale of tens to hundreds of microns within individual zircon grains and between grains, and there is only rare evidence for localized domains that have lost small amounts of Pb since crystallization. In contrast, baddeleyite from the AN2 leucogabbro has micron-scale overgrowths and internal replacement patches of secondary zircon that has lost Pb; the weighted mean 207Pb/206Pb date of 2709.73±0.48Ma for baddeleyite that was air-abraded to remove the secondary zircon rims is indistinguishable from the igneous zircon dates. 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.