In situ230Th–232Th–234U–238U analysis of silicate glasses and carbonates using laser ablation single-collector sector-field ICP-MS

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 µg 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 µm and tracks up to 1800 µm long in order to obtain adequate precision on samples with the lowest abundances of 230Th and 234U. We applied corrections for background, tailing of the 232Th, 235U and 238U beams on 230Th and 234U, 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 230Th/238U, 234U/238U, and 230Th/232Th 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 (230Th/238U) = 1.000 ± 0.006 and (234U/238U) = 0.994 ± 0.014, and (230Th/238U) = 1.493 ± 0.032 and (234U/238U) = 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.