HIGH-PRECISION IN SITU 238U-234U-230TH ISOTOPIC ANALYSIS USING LASER ABLATION MULTIPLE-COLLECTOR ICPMS

Abstract

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 μm scale resolution, corresponding to 1-4 ng 238U, ~70-200 fg of 234U and 20-60 fg of 230Th consumed per analysis. Synthetic glass standards and naturally occurring samples of zircon and opal, with U contents of 460, 260, and ~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 [234U/238U]act of 0.17114 +/- 0.00022 and 1.0018 +/- 0.0014 (2σM), indistinguishable from the MC-ICPMS solution nebulization values of 0.17094 +/- 0.00006 and 1.0011 +/- 0.0009 (2σM), respectively. The usual within-run precision obtained for both glass and zircon is +/-3%% at the 2σM level. An additional 12 laser analyses on the opal give a mean [234U/238U]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%% (2σM). Our Nd:YAG laser, coupled with an all Ar gas system, produces large elemental fractionation effects between U and Th. Both 238U/232Th and [230Th/238U]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 238U-234U-230Th 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 [230Th/238U]act of 0.996 +/- 0.013 and 238U/232Th 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 (2σM), respectively. In an unknown sample, it is possible to determine correct values for [230Th/238U]act and 238U/232Th, with respective within-run uncertainties as good as 7%% and 2%%, by monitoring the isotopic composition of a well characterized, matrix-matched standard. For high-U material, the combined uncertainties in [234U/238U]act and [230Th/238U]act routinely translate to 2σM errors in the 238U-234U-230Th age of better than +/-2,500 years in 100,000-year-old samples.