U-PB DATING OF ZIRCONS WITH LA-ICP-MS

U-Pb dating of zircon is widely used to address many tectonic problems. TIMS (Thermal Ionization Mass Spectrometer) or SHRIMP (Sensitive High Resolution Ion Microprobe) are the dominant techniques but in recent years, laser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS) has begun to be used for U-Pb dating of zircon. This method is fast, relatively inexpensive, and has a spatial resolution (30-60 µm) intermediate between the TIMS and SHRIMP methods.

In the GeoAnalytical Lab at Washington State University, U-Pb dating of zircon is carried out with a New Wave UP-213 (Nd-YAG 213 nm) laser system coupled with a ThermoFinnigan Element2 magnetic sector ICP-MS. Prior to analysis, zircons are mounted with standards in and polished. The laser is operated at 10 Hz repetition rate, and mostly with a 60% output that produces a fluency of ~ 11 J/cm². The beam size is normally 40 µm, but 30-µm beams can be used if the zircon contains high U and radiogenic Pb. Signals are collected for 28.7 seconds in 100 sweeps with a counting efficiency of 87% for every spot. The crater produced is typically ~ 25 µm deep. External standardization is applied with natural zircons of known ages. A blank is measured before every analysis, and a standard is measured every 5-6 analyses. Laser induced time-dependant elemental fractionation is corrected off-line using the regression line method (Sylvester and Ghaderi, 1997; Horn et al., 2000; Kosler et al., 2002). Inclusions in zircons, as well as signal fluctuations are eliminated by removing outliers from the regression line. Non-stoichoimetric fractionation, mass bias, machine drift, and other variation are corrected by the external standards. Accuracy of the U-Pb determinations, based on replicate analyses of “known” zircons, is generally 2-3%. Internal precision of the measurements is typically better than 2%.

The LA-ICP-MS method of zircon dating has clear applications for addressing a range of important geological problems. In addition to the U-Pb dating of detrital zircon populations (e.g., Kosler et al., 2002), this method has use in reconnaissance investigations in geologically complex areas such as exist in much of western North America.