Precise Isotope Determination of Trace Amounts of Nd in Magnesium-rich Samples

Abstract

We developed a technique combining mass spectrometry with chemical separation to analyze Sm-Nd isotopic compositions of ultramafic rock samples, which have extremely high Mg and low Nd concentrations and are always difficult to analyze. In the case of ultramafic rocks, abundant Mg forms fluorides during decomposition by HF, and minor amounts of Sm and Nd are strongly partitioned into fluorides, which would be a significant reason of low recovery yields of these elements and obstructed to determine a reliable isotopic ratio of Nd and Sm. We introduced pre-decomposition by HCl prior to the conventional method by HF and HClO₄ that made it possible to elevate recovery yield to 85%. On the other hand, replicate measurements of Nd standard solution (La Jolla) by changing the ion current of Nd⁺ and the amount of Nd indicate that reliable isotopic ratios are measured under the condition of >0.04×10^-11 A of ¹⁴⁴Nd⁺ ion current and >2 ng of Nd. By applying our method, we analyzed ¹⁴³Nd/¹⁴⁴Nd ratio and Sm and Nd concentrations of PCC-1, which is an ultramafic standard rock sample of United States Geological Survey. The results of our analysis were 0.512226±0.000026, 0.00551±0.00041, and 0.0278±0.0011 ppm, respectively. These results are the first report of ¹⁴³Nd/¹⁴⁴Nd ratio for PCC-1.