Accurate Measurement of Rare Earth Elements by ICP-MS after Ion-Exchange Separation: Application to Ultra-Depleted Samples

This study reports precise and accurate data for rare earth elements (REE) measured on eight geological reference materials, five enriched in REE (BE-N, BHVO-2, BR, BR-24 and RGM-1) and three very depleted in REE (BIR-1, UB-N and DTS-2). Data were acquired by quadrupole ICP-MS after isolation of the REE using an ion-exchange chromatography procedure. All the measured REE abundances are similar within ! 5% (10% for the most REE-depleted sample DTS-2) to the high quality measurements previously published in the literature. We also show that using an internal Tm spike, the reproducibility of the data is improved to ~1%. Applying this technique to the analysis of ultra-depleted rock samples (sub ng.g-1), we show that significant improvements are obtained relative to the routine trace element measurement method. The chondrite-normalized patterns are smooth instead of displaying irregularities. Although the classical method gives excellent results on REE-rich samples, we believe that our technique improves the precision and accuracy of measurements for highly REE-depleted rocks. Introduction For decades, most geochemical studies use trace element data to identify geological processes such as melting of mantle sources or fractional crystallization of magmas (Henderson, 1984). In particular, the accurate determination of Rare Earth Element (REE) concentrations provides important information for the understanding of geological processes. Because of their high charge (trivalent cations except for Ce4+ under oxidizing conditions and Eu2+ under reducing conditions) and large radii, REE are incompatible elements during most mantle melting processes (White, 2003), and they are usually more abundant in melt-derived rocks than in residual ultramafic rocks. The extreme REE-depletion currently observed in the latter type of samples (sub ng.g-1) explains why it is so difficult to measure them accurately (e.g. Nakamura and Chang, 2007). Quadrupole inductively coupled plasma-mass spectrometer (ICP-MS) is one of the most commonly used technique to quickly and precisely determine the REE concentrations,