An improved method for TIMS high precision neodymium isotope analysis of very small aliquots (1-10 ng)

摘要

Technological and scientific developments have demonstrated both the attainability and the utility of very high precision (i.e. <10 ppm 2 sigma RSE) neodymium (Nd) isotopic measurements with thermal ionisation mass spectrometry (TIMS). However, such high precision has been limited to relatively large aliquots of Nd, on the order of several hundred nanograms. Many investigations of geological, cosmochemical and environmental materials would benefit from precise Nd isotopic measurements but do not always permit such large samples to be obtained, instead yielding only a few nanograms of Nd. Analyzing Nd isotopes as the oxide (NdO+) is a well known technique, frequently involving an oxygen bleed valve and/or silica gel. This paper presents a method that foregoes the bleed valve and load samples with a tantalum oxide (Ta2O5) phosphoric acid slurry which provides the oxygen source. Using an in-house Nd isotopic standard solution, 4 ng loads easily yield stable 2.0-2.5 V beams resulting in internal precisions as low as 8 ppm (2 sigma RSE). Long term (9 months, six analysts) external precision is 20 ppm 2 sigma RSD (n = 83), and within barrel precision is 13 ppm 2 sigma RSD (n = 19). As a further test of this method, a natural rock sample was dissolved and the Nd separated using TRU-spec and HIBA column chemistry. Within barrel external precision is 21 ppm (2 sigma RSD) on 4 ng loads (n = 18). This precision represents a significant advance over previous NdO+ analyses of small samples using an oxygen bleed valve or silica gel with phosphoric acid. The Ta2O5 loading method is of particular utility in refining Sm/Nd garnet geochronology, permitting the analysis of small (10 s of milligrams) garnet samples where Nd concentrations are very low (<1 ppm). Garnet Nd-143/Nd-144 from several field areas was measured with the TA(2)O(5) method and full column chemistry yielding internal run precisions of 8-42 ppm 2 sigma with garnet Nd load sizes ranging from 2.3-12 ng Nd. These garnet analyses yielded age precisions of 0.4 to 4.6 million years (2 sigma) depending on the Sm-147/Nd-144 ratio. Given successful column chemistry, and sufficiently low blanks, this method allows for high precision (13-21 ppm 2 sigma RSD) analysis of similar to 4 ng (or potentially smaller) aliquots of Nd separated from natural samples.