Simultaneously obtaining stable and radiogenic Nd isotope ratios through a single DGA column using double spike TIMS

Abstract

In this study, we present a high-precision technique for measuring Nd stable isotopes by thermal ionization mass spectrometry (TIMS). A single and efficient Nd separation procedure is adopted with above 99% recovery using the DGA resin (Eichrom, USA). A ¹⁴⁵Nd–¹⁵⁰Nd double spike (DS) was used to correct mass-dependent fractionation during column chemistry and mass spectrometry. Repeated analyses of the JNdi-1 standard yielded a total average δ^146/144Nd of 0.001 ± 0.016‰ (2SD, N = 12), and a δ^148/144Nd of −0.005 ± 0.045‰ (2SD, N = 12), respectively. The pure Nd standard, La Jolla, produced a δ^146/144Nd of −0.210 ± 0.006‰ (2SE). The δ^146/144Nd of twelve geological reference materials, including COQ-1, DNC-1a, AGV-1, AGV-2, BHVO-2, BCR-2, G-2, RGM-1, GSP-2, NOD-A-1, NOD-P-1, and SDC-1, were also reported. Among them, the δ^146/144Nd of COQ-1 was first reported. Simultaneously, the radiogenic ¹⁴³Nd/¹⁴⁴Nd of reference materials could be calculated using a double spike deconvolution with a precision of ≤5 ppm (2SE), in agreement with published values within an analytical uncertainty measured by conventional methods. Nd isotope fractionation during column chromatography was investigated. The known JNdi-1 was eluted with DGA resin using HCl as a solute and was sequentially collected into 8 fractions. δ^146/144Nd systematically increased from −0.110 ± 0.011‰ (2SE) of the first Nd cut to +0.334 ± 0.011‰ (2SE) of the tailing cut following the mass-dependent fractionation law, suggesting significant Nd isotope fractionation during column chemistry. This phenomenon contradicts previous observations that light isotopes are preferentially retained onto the resin. The technique established in this study can obtain stable and radiogenic Nd isotopic compositions simultaneously from a single aliquot using a double spike by TIMS, providing at least two-dimensional constrains to the source of a material.