An improved U-Pb age dating method for zircon and monazite using 200/266 nm femtosecond laser ablation and enhanced sensitivity multiple-Faraday collector inductively coupled plasma mass spectrometry

We present an improved U-Pb age dating method for zircon and monazite crystals using 193 nm excimer laser ablation and 200/266 nm femtosecond laser ablation (200/266FsLA) multiple-Faraday collector inductively coupled plasma-mass spectrometry (MFC-ICP-MS). Optimization of a 266 fs laser beam enabled an analysis of Pb-207/Pb-206 and Pb-206/U-238 ratios with an in-run precision of 1-2% from a crater of dimensions 50 mu m x 10 mm (diameter x depth) at a repetition rate of 2 Hz for 30 s. The same in-run precision was obtained from a 30 mu m x 20 mm crater using a 200 fs laser beam of 20 mm in diameter rastered along the circumference of a circle with a 7 mu m radius at 25 Hz for 15 s. With an enhanced sensitivity ion interface, the sensitivity for the total amount of Pb was similar to 2 mV ppm(-1) or similar to 125 000 cps ppm(-1) using the above crater setup. The use of high gain amplifiers equipped with a 10(12) Omega register enabled the determination of the U-Pb age of zircon and monazite crystals with an internal and intermediate precision comparable to that obtained from sensitive high resolution ion microprobe (SHRMP) techniques. We analysed standard zircon crystals using a 91500 zircon crystal (1065.4 +/- 0.6 Ma determined by isotope dilution thermal ionization mass spectrometry (ID-TIMS)) as a bracketing standard. Ages determined from TEMORA2, Pre. sovice, and OD-3 zircons compared very well with their reference ages determined by ID-TIMS and/or SHRIMP. Thompson Mine and Monangotory standard monazites, dated using a 44069 monazite crystal (424.9 +/- 0.4 by ID-TIMS) as a standard, also reproduced the U-Pb ages determined by ID-TIMS/LA-MFC-ICP-MS, but at a sample volume four times smaller than that required for zircons. Zircon and monazite ages are accurate given the small offsets from ID-TIMS ages, 0.15-0.7% for zircons and 0.2-0.7% for monazite well within internal precision from the primary standard in the analytical session and competitive with an internal precision of 0.43-0.6% for zircon and 0.2-0.8% for monazite. More easily obtaining high resolution age data is useful for the precise determination of the U-Pb age.