Precise and accurate in situ U–Pb dating of zircon with high sample throughput by automated LA-SF-ICP-MS

Abstract

We present an automated method for U–Pb age dating of zircon by single collector laser ablation-magnetic sectorfield-inductively coupled plasma-mass spectrometry (LA-SF-ICP-MS). The high sensitivity of SF-ICP-MS allows routine analysis with spot diameter of 20 to 30 μm and ablation time of 30 s, resulting in an ablation crater depth of ∼ 15–20 μm (∼ 35 to ∼ 65 ng of zircon). Zircon consumption is therefore limited to < 3% of a typical crystal and only by a factor of 6–10 times larger compared to secondary ion mass spectrometry (SIMS) based techniques. Precision and accuracy has been assessed using a number of well-characterised zircons as secondary quality control standards and are comparable to those of laser ablation- and ion-microprobe based techniques that use more complex and time-consuming approaches. Average measurement uncertainties (2σ, mean analytical uncertainty) based on 402 analyses of the Plešovice zircon standard were 2.2%, 3.1% and 2.1% for the 206Pb/ 238U, 207Pb/ 235U and 207Pb/ 206Pb ratios, respectively, comparable with those attainable by SIMS. The weighted mean of all 402 analyses yielded a 206Pb/ 238U age of 338 ± 1 Ma, which is in excellent agreement with the ID-TIMS 206Pb/ 238U age of 337.1 ± 0.4 Ma reported for the Plešovice zircon. Data acquisition is done in automated mode for up to 16 h/day with analytical points pre-set with only minimal operator presence during the data acquisition. Individual U–Pb zircon analysis last ≤ 75 s, and sample throughput is more than an order of magnitude higher (∼ 1000 U–Pb ages/day) compared to SIMS techniques and 3–5 fold higher compared to conventional LA-ICP-MS techniques. The methods presented here are therefore expected to have significant impact on many aspects of zircon U–Pb geochronology, with particular benefits for studies on the Archean crustal evolution and the provenance of detrital zircon crystals from clastic sediments where a large number of high-quality stand-alone in situ zircon U–Pb age determinations are needed.