High-precision in situ U-238-U-234-Th-230 isotopic analysis using laser ablation multiple-collector ICPMS

We have developed a method for the rapid, in situ measurement of U-Th isoto pic compositions at the semimicro scale using laser ablation sampling, comb ined with multiple collector ICP magnetic sector mass spectrometry (MC-ICPM S). The system uses a Q-switched and frequency quadrupled 266 nm Nd:YAG las er to ablate samples containing 100 ppm levels of U at 150 mum scale resolu tion, corresponding to 1-4 ng U-238, similar to 70-200 fg of U-234 and 20-6 0 fg of Th-230 consumed per analysis. Synthetic glass standards and natural ly occurring samples of zircon and opal, with U contents of 460, 260, and s imilar to 200 ppm, respectively, were used to assess the precision and accu racy of our laser ablation technique. Our initial experiments used argon as the plasma support gas. Thirty-seven laser analyses on the glass and 29 on the zircon give respective mean [U-234/U-238](act) of 0.17114 +/- 0.00022 and 1.0018 +/- 0.0014 (2 sigma (M)), indistinguishable from the MC-ICPMS so lution nebulization values of 0.17094 +/- 0.00006 and 1.0011 +/- 0.0009 (2 sigma (M)), respectively. The usual within-run precision obtained for both glass and zircon is +/-3 parts per thousand at the 2 sigma (M) level. An ad ditional 12 laser analyses on the opal give a mean [U-234/U-238](act) of 0. 9997 +/- 0.0034, in excellent agreement with the expected secular equilibri um value of unity and a typical within-run precision of +/-8 parts per thou sand (2 sigma (M)). Our Nd:YAG laser, coupled with an all Ar gas system, pr oduces large elemental fractionation effects between U and Th. Both U-238/T h-232 and [Th-230/U-238](act) can be measured at the per mill level, but Th ion beams are suppressed relative to U. As a result, the Th/U ratios are s ystematically lower, and the apparent U-238-U-234-Th-230 ages are systemati cally younger than the true values. The U-Th fractionation is primarily con trolled by ionization conditions in the plasma, transportation efficiency o f ablated particles, and the composition of the sample matrix. The use of h elium instead of Ar in the ablation cell significantly improves the relativ e sensitivity of Th, and entirely eliminates the elemental fractionation be tween U and Th, while retaining accuracy and precision in U isotope measure ment. With He, mean values for [Th-230/U-238](act) Of 0.996 +/- 0.013 and U -238/Th-232 Of 1.625 +/- 0.092 were determined for the zircon standard, in excellent agreement with the solution nebulization values of 1.0042 +/- 0.0 016 and 1.6288 +/- 0.0006 (2 sigma (M),), respectively. In an unknown sampl e, it is possible to determine correct values for [Th-230/U-238](act) and U -238/Th-232, with respective within-run uncertainties as good as 7 parts pe r thousand and 2 parts per thousand, by monitoring the isotopic composition of a well characterized, matrix-matched standard. For high-U material, the combined uncertainties in [U-234/U-238](act) and [Th-230/U-238](act) routi nely translate to 2 sigma (M) errors in the U-238-U-234-Th-230 age of bette r than +/-2,500 years in 100,000-year-old samples. Copyright (C) 2000 Elsev ier Science Ltd.