Combined Stable and Radiogenic Nd Isotope Characterisation of Sedimentary and Iron Formation Reference Materials by Double Spike MC-ICP-MS

Abstract

This study presents combined stable (¹⁴⁶Nd/¹⁴⁴Nd, ¹⁴⁸Nd/¹⁴⁴Nd) and radiogenic (¹⁴³Nd/¹⁴⁴Nd) Nd isotope ratios obtained by double spike (DS) MC-ICP-MS for twenty-five geological reference materials (RMs), focusing particularly on the initial characterisation of sedimentary, phosphate and iron formation (IF) RMs. Pure Nd solutions and previously characterised igneous RMs of basaltic to rhyolitic composition were utilised to validate the DS MC-ICP-MS method. The (combined) expanded uncertainty (95% c.i.) on JNdi-1 normalised δ^146/144Nd measurement results was improved by ca. 4 ppm when data collection was increased from 50 to 100 cycles, which translates into a shift from ±0.016 to ±0.012‰. Intermediate measurement precision (over nearly 3 years) of δ^146/144Nd determined from Nd solutions (n = 4) and igneous, sedimentary and iron formation RMs with multiple digestions (n = 17) is on average ±0.016‰ 2s. Our obtained results agree well with published stable δ^146/144Nd and radiogenic ¹⁴³Nd/¹⁴⁴Nd values for all igneous RMs analysed herein, attesting to the accuracy of the DS MC-ICP-MS approach. Sedimentary RMs are overall isotopically light with δ^146/144Nd signatures between -0.057 and -0.006‰, overlapping closely with igneous RMs (-0.038 to +0.010‰ δ¹⁴⁶Nd) and the bulk silicate Earth. The two phosphate RMs with δ^146/144Nd of +0.040 and +0.090‰, are significantly heavier in comparison. In contrast, global IF RMs exhibit a large range in δ^146/144Nd from -0.057 to +0.261‰ and include the heaviest (so far) recorded Nd stable isotope signature in a terrestrial geological material. Correlations of heavy δ^146/144Nd with seawater proxies (e.g., Y/Ho) and light δ^146/144Nd with tracers for continental contamination (e.g., Zr) suggest a strong potential for the application of stable Nd isotopes in sediments and IF for palaeo-reconstruction purposes.