Abstract
We present a novel method for the determination of Th and U isotopic ratios by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) applicable to samples with U concentrations as low as 0.4 mg g(-1). While previous in situ U-series determinations have used multi-collector (MC) ICP-MS, we use a single-collector sector-field ICP-MS connected to a 213 nm Nd:YAG laser. Precision and accuracy were determined for different matrices: NIST SRM 612, MPI-DING (T1-G, and ATHO-G) and USGS (BHVO-2G, NKT-1G, and BCR-2G) reference glasses, 91500 zircon, and a travertine. We used a laser beam with a diameter of 110 mm and tracks up to 1800 mm long in order to obtain adequate precision on samples with the lowest abundances of Th-230 and U-234. We applied corrections for background, tailing of the Th-232, U-235 and U-238 beams on Th-230 and U-234, instrumental mass discrimination, and elemental sensitivity. Mass discrimination and relative elemental sensitivities for Th and U are similar for igneous rock glasses, zircon, and calcium carbonate under the experimental conditions used; synthetic, transparent NIST SRM 612, however, yields different values. Therefore, we used BCR-2G (Th/U = 3.507 +/- 0.018 (1 SE)) for calibration of relative elemental sensitivities. The internal precision of three line scans obtained for Th-230/U-238, U-234/U-238, and Th-230/Th-232 ratios varies between 0.1 and 2.8% (relative standard error, 1 RSE), limited by counting statistics on the minor isotopes. For repeated measurements of BCR-2G and the travertine, we obtained activity ratios (+/-2SD) of (Th-230/U-238) = 1.000 +/- 0.006 and (U-234/U-238) = 0.994 +/- 0.014, and (Th-230/U-238) = 1.493 +/- 0.032 and (U-234/U-238) = 1.369 +/- 0.011, respectively, concordant within 2% of TIMS values. This consistency indicates that LA-ICP-SF-MS is a suitable method for U-series analyses. Regarding future studies, it also provides the advantage of combining U-Th isotope ratio determinations with additional multi-element and isotope analyses, respectively, during a single line scan.