CHARACTERIZING A NEW STANDARD ZIRCON FOR U-PB AND LU-HF

Zircons are the most robust geochronometers the scientific community has, but their accuracy is ultimately influenced by a combination of factors including analytical precision, homogeneity of the standard, relative age of the standard, and its concordance. There are many zircons in use as standards, but none are Archean. This presents a problem because most U-Pb analyses are made on mass spectrometers equipped with ion counters, especially multi-collector ICP-MS instruments used for laser ablation analyses. Under conditions of simultaneous collection, the dynamic range required can only be met by collecting some ion beams on a Faraday collector and some on an ion counter. Because ion counters have both deadtime and linear limits to the count rates they can accept, trying to use a Paleozoic zircon standard for an Archean zircon is difficult. The difficulty arises because the ablation conditions must be first maximized for the standard, and then those ablation parameters are applied to analyses of the unknowns. In general, younger zircons will require larger spot sizes to generate sufficiently accurate analyses with high precision. When those parameters are applied to Archean unknowns, the count rates often exceed the linear limits of the ion counters. We have identified specific lithologies in the Archean Stillwater layered complex in Montana, USA that have been suggested to contain magmatic zircons free of common Pb and of high concordance based on previous work. The Stillwater Complex is comprised of distinguishable layers ranging from ultramafic to anorthositic and hosts the country’s only Pt and Pd reserves, which are currently being mined. Zircons from five samples of four different rock types have been collected: leucogabbro, anorthositic gabbro, gabbroic anorthosite, and gabbro. Zircons from these samples are being characterized for their suitability as an Archean age standard and a Lu-Hf isotopic standard.