Analytical Procedures and Instrumental Techniques for Sr and Nd Isotope Analysis by TIMS in the Isotopic Geology Laboratory-UFRGS: An Overview

The Isotopic Geology Laboratory at Federal University of Rio Grande do Sul (LGI/UFRGS) has undertaken systematic analysis of the Sr and Nd isotopic compositions of a broad range of rocks by TIMS over the last decades. This work aims to provide a brief overview of the current chemical and analytical procedures employed in the lab for unspiked samples, including historical data of NIST SRM 987 and JNDi used as standards to monitor instrumental variations on our TRITON (Thermo Scientific). Chemical procedures are carried out in clean rooms, reagents are purified by sub-boiling distillation and Milli-Q water is used. Typically, approximately 100–200 mg of rock powder is accurately weighted for each sample in 15mL screwtop Savillex beakers. This is followed by the addition of a mixture of concentrated HF and HNO₃ heated on a hot plate for 2–3 days. The resulting solutions dry overnight and then dissolved in HCl, heated again for a day, and dried. Subsequently, samples are dissolved in 2–4 mL of 2.5 HCl and centrifuged. If residues are present, samples are treated with aqua regia until the solution becomes clear, then dried down, re-dissolved, and centrifuged again. Samples undergo purification through pre-purified ion-exchange resin columns. Initially, 1-2 mL of stock solution is introduced into a Teflon column packed with Biorad AG 50W-X8 cationic resin to separate Sr and REE. The Sr fraction is eluted with 2.5 M HCl, while the REE fraction is eluted with 6 M HCl. The Sr aliquot is collected in 15 mL screwtop Savillex beaker and dried down. The REE solution is dried down and dissolved in 0.18 M HCl. Then, the REE solution is introduced in a quartz column packed with Eichron Ln Spec anionic resin and the Nd fraction is eluted with 0.18 M HCl, collected in 15 mL screwtop Savillex beaker and dried down. Sr and Nd isotope measurements are conducted using a TRITON TIMS instrument equipped with nine Faraday detectors. Pre-degassed double filaments of rhenium ribbons (0.025 mm thick, 0.76 mm wide, and 99.99% pure from H. Cross Company) are used for Sr and Nd analyses. Purified Sr and Nd samples are dissolved in 2 µL of 0.25 M of H₃PO₄ and carefully loaded in the filaments. The ionization temperature for Sr measurements vary from 1250 to 1350 ◦C, while for Nd measurements range from 1650 to 1750 ◦C. Data acquisition is performed in static mode in multiple Faraday cup when the ⁸⁸Sr and ¹⁴⁴Nd ion beam intensity exceeds 1-3 V. Each run ideally consists of 10 blocks of 15 cycles each (8.3s integrating time) for Sr and 14 blocks of 15 cycles each (8.3s integrating time) for Nd. Data are corrected for mass fractionation by normalizing to ⁸⁶Sr/⁸⁸Sr ratio of 0.1194 and to ¹⁴⁶Nd/¹⁴⁴Nd ratio of 0.7219 using an exponential law. Historical measurements of NIST SRM 987 yielded an average ⁸⁷Sr/⁸⁶Sr ratio of 0.710254 ± 0.000017 (n = 81, 2SD) while measurements of JNDi yielded an average ¹⁴³Nd/¹⁴⁴Nd  ratio of 0.512107± 0.000016 (n = 142, 2SD), very consistent with those reported in the literature (e.g. Weiss et al. 2006). Blanks and reference materials, such as basalts from U.S. Geological Survey (USGS) BCR-2, BHVO-1 and BHVO-2, are routinely subjected to the same preparation steps and analytical procedures in order to assess the overall process. Our current results for ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios agree are in complete agreement with those reported by Weiss et al. (2006) ensuring reproducibility.   

Reference:

Weis, D., et al. (2006), High-precision isotopic characterization of USGS reference materials by TIMS and MC-ICP-MS, Geochem. Geophys. Geosyst., 7,Q08006, doi:10.1029/2006GC001283.