Simultaneous measurement of neodymium stable and radiogenic isotopes from a single aliquot using a double spike

Abstract

Neodymium (Nd) stable isotopes have the potential to provide new constraints on a diverse range of geological processes from planetary formation and magmatic differentiation to weathering and ocean circulation. In this contribution, we present a technique for the high-precision measurement of Nd stable isotope ratios by thermal ionisation mass spectrometry (TIMS). We use a ¹⁴⁵Nd–¹⁵⁰Nd double spike (DS), composed of 28% ¹⁴⁵Nd and 67% ¹⁵⁰Nd, to correct for mass dependent fractionation resulting from sample preparation and mass spectrometry. Isotope ratios are expressed as δ¹⁴⁶Nd which is the per mil deviation in the measured ¹⁴⁶Nd/¹⁴⁴Nd ratio relative to reference material JNdi-1. Repeated analyses show that an internal precision (n = 400 cycles; 2 se) of ≤0.005‰ on δ¹⁴⁶Nd can be achieved in agreement with theoretical predictions. An interlaboratory comparison of the primary standard JNdi-1 over a three-year period shows that resolvable offsets in δ¹⁴⁶Nd are induced within and between mass spectrometers as the result of systematic biases in the efficiency of the Faraday collectors. Following normalisation of the data to JNdi-1 = 0‰ the long-term reproducibility of δ¹⁴⁶Nd on a range of international reference materials processed through chemical separation is better than ±0.015‰ (2 sd). In addition to stable isotope compositions this method allows for the simultaneous measurement of ¹⁴³Nd/¹⁴⁴Nd with a precision of ≤11 ppm. Over a wide range of ¹⁴³Nd/¹⁴⁴Nd (0.5118–0.5132) the DS analyses here agree within analytical uncertainty with published values using conventional techniques. To allow the accurate age correction of these radiogenic isotope compositions in ancient geological samples (>2 Ga) here for the first time we have combined a DS and an elemental tracer spike. A ¹⁴⁹Sm tracer spike was calibrated and used to obtain isotope dilution Sm concentrations and ultimately parent-daughter ratios. Measured Sm–Nd ratios are within uncertainty of previous estimates for a range of reference materials, with the addition of the Sm spike having no resolvable effect on δ¹⁴⁶Nd values. This technique allows δ¹⁴⁶Nd, ¹⁴³Nd/¹⁴⁴Nd and Sm/Nd to be obtained simultaneously, therefore allowing constrains to be placed on both the source or age of a material and the processes involved in its formation.