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.
- Programområde 4: Mineralske råstoffer