Determination of Rare Earth Element Isotopic Compositions Using Sample-Standard Bracketing and Double-Spike Approaches

Abstract

Rare earth elements (REEs) have been found to have numerous uses to trace geological and cosmochemical processes through analyses of elemental patterns, radioactive decay, nucleosynthetic anomalies, and cosmogenic effects. Stable isotopic fractionation is one aspect of REE geochemistry that has been seldom studied, with most publications focusing on the development of analytical methodologies for individual REEs, and most applications concerning terrestrial igneous rocks. In this study, we present a method to systematically analyze stable isotopic fractionations of 8 REEs, including Ce, Nd, Sm, Eu, Gd, Dy, Er, and Yb, using sample-standard bracketing (SSB) and double-spike (DS) approaches. All REEs are separated and purified using a fluoropolymer pneumatic liquid chromatography (FPLC) system. We introduce procedures for identifying and correcting some isobaric interferences in double-spike data reduction. Several geostandards, including igneous rocks and sediments, are analyzed using SSB and DS methods. The results indicate that REE isotopic fractionation in igneous processes is limited, except for Eu. Other REEs can still be isotopically fractionated by low-temperature processes and kinetic effects at a high temperature.

Sequential separation of cerium (Ce) and neodymium (Nd) in geological samples for high-precision analysis of stable Ce isotopes, and stable and radiogenic Nd isotopes by MC-ICP-MS

“…, ∼1.4 to 2 mL) required for previous separation methods based on DGA or LN resins. 50–53 Residual α-HIBA is effectively removed by eluting the column with 4 M HNO 3 , and Ce or Nd is quantitatively recovered in 6 mL of 0.1 M HCl (details in Section 3.3.4). After purification, the Ce or Nd cut is dried and re-dissolved in 2% (v/v) HNO 3 for isotopic analysis.…”

“…Several chromatographic methods have been established to separate Ce or Nd alone from geological samples for stable isotope analysis in the past few years (summarized in ESI Table S1†), but only three of them have been demonstrated to be able to sequentially separate Ce and Nd from the same sample for stable Ce and Nd isotope analysis 50–53 (Table 1). One method is to first separate bulk REEs as a group using 2 mL Bio-Rad AG 50W-X8 resin, and then sequentially separates Ce and Nd using tandemly arranged columns, with the first column filled with 0.3 mL TRU Spec resin and the second one filled with ∼1.25 mL LN Spec resin.…”

“…, 70 cm) chromatographic column filled with ∼1.4 mL LN resin. 50,53 Prepacked TODGA columns (2 mL resin) were used to separate REEs as a group before separation of individual REEs by the FPLC system. This FPLC system is advantageous in terms of its automation and demonstrated capability to sequentially separate 8 rare earth elements, including Ce and Nd, for stable isotope analysis.…”

“…, ∼80 mL HCl) that can potentially increase REE blanks. Also, the existing elution protocol on the FPLC system showed relatively low Nd yield (40–70%) and, consequently, caused somewhat biased stable Nd isotope results for some geological reference materials via the simple sample–standard bracketing method, 50,53 possibly due to incomplete sample dissolution associated with the low-molarity acid used for FPLC separation. As a result, the more complicated double spike technique is preferred for Nd separation.…”

“…As a result, the more complicated double spike technique is preferred for Nd separation. 53 Although all three methods have proven effective in sequential Ce and Nd separation from geological samples in support of high-precision stable isotope measurements of the two elements, they all use relatively large amount (∼1.4–2 mL) of expensive specific resins ( i.e. , LN or DGA).…”