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

摘要

We have developed a method for the rapid, in situ measurement of U-Th isotopic compositions at the semimicro scale using laser ablation sampling, combined with multiple collector ICP magnetic sector mass spectrometry (MC-ICPMS). The system uses a Q-switched and frequency quadrupled 266 nm Nd:YAG laser to ablate samples containing 100 ppm levels of U at 150 mum scale resolution, corresponding to 1-4 ng U-238, similar to 70-200 fg of U-234 and 20-60 fg of Th-230 consumed per analysis. Synthetic glass standards and naturally occurring samples of zircon and opal, with U contents of 460, 260, and similar to 200 ppm, respectively, were used to assess the precision and accuracy 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 solution 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 additional 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 equilibrium value of unity and a typical within-run precision of +/-8 parts per thousand (2 sigma (M)). Our Nd:YAG laser, coupled with an all Ar gas system, produces large elemental fractionation effects between U and Th. Both U-238/Th-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 systematically lower, and the apparent U-238-U-234-Th-230 ages are systematically younger than the true values. The U-Th fractionation is primarily controlled by ionization conditions in the plasma, transportation efficiency of ablated particles, and the composition of the sample matrix. The use of helium instead of Ar in the ablation cell significantly improves the relative sensitivity of Th, and entirely eliminates the elemental fractionation between U and Th, while retaining accuracy and precision in U isotope measurement. 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 +/- 0.0016 and 1.6288 +/- 0.0006 (2 sigma (M),), respectively. In an unknown sample, 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 per 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) routinely translate to 2 sigma (M) errors in the U-238-U-234-Th-230 age of better than +/-2,500 years in 100,000-year-old samples. Copyright (C) 2000 Elsevier Science Ltd. [References: 38]