In situ Nd isotopic analysis of geological materials by laser ablationMC-ICP-MS

Abstract

Neodymium (Nd) isotopes have widespread applications in the earth sciences, both in geochronology and in provenance/tracer studies. The isotopic analyses are conventionally done by chemical separation of pure Nd from the sample matrix. This process is time consuming and loses potentially vast amounts of information held at small spatial scales. Here, we describe a laser ablation protocol for the measurement of Nd isotopes in geological materials with relatively high Nd concentrations (≥100 ppm; e.g., apatite, titanite, and ferromanganese nodules). The procedure allows, for the first time, the exploitation of information held in Nd isotopes at scales of less than 0.1 mm. The principal analytical issues to be overcome are the interferences from isobaric Sm isotopes, the correction for which must be very accurate in order to maintain a high accuracy for the Nd isotopic ratios. We show that careful attention to mass discrimination effects along with an iterative correction procedure allows a correction that results in an accuracy on the ¹⁴³Nd/¹⁴⁴Nd isotope ratio of the order of 0.5 epsilon units (0.005%) for Sm/Nd ratios as high as 1.2. The total analysis time is short (∼2–5 min), enabling a large number of analyses (>30) in each analytical session. Spatial resolution is governed by the Nd concentration and the required accuracy and for apatites and titanites it is typically ∼90 μm. Analytical precision is degraded by around a factor of 2–3 relative to the best TIMS and solution MC-ICP-MS data. Crucially, however, the rapid sample throughput and the extremely high spatial resolution by far outweigh this reduction in precision. A high resolution study of an Atlantic ferromanganese crust illustrates the usefulness of the described approach.