Rapid decomposition of geological samples by ammonium bifluoride (NH₄ HF₂ ) for combined Hf-Nd-Sr isotope analyses

Complete decomposition of silicate rock matrices is crucial in determining their isotopic compositions, but acid dissolution in a high-pressure steel-jacketed bomb, which has been the only powerful, effective technique thus far, is time-consuming and expensive. Rock dissolution using ammonium bifluoride (ABF), as described here, is a viable alternative.

Geological reference materials (GRMs) were digested using ABF in closed Teflon beakers at temperatures of 220/230°C in a convection oven and subsequently treated with HNO₃ . Hf-Sr-Nd were separated and purified using ion-exchange chemistry columns calibrated for 50-2 mg samples. The isotopic compositions of Sr-Nd were measured by Thermal Ionization Mass Spectrometry, while that of Hf by Multi-Collector Inductively Coupled Plasma Mass Spectrometry, both with normal 10¹¹  Ω and gain calibrated 10¹³  Ω amplifiers.

Total procedural blanks of our protocol are 0.5 ng for Sr, 0.2 ng for Nd and <25 pg for Hf. Test runs with GRMs, ranging in composition from basic to felsic and dissolved in ABF, yield accurate ⁸⁷ Sr/⁸⁶ Sr, ¹⁴³ Nd/¹⁴⁴ Nd and ¹⁷⁶ Hf/¹⁷⁷ Hf isotope ratios as compared with those obtained with the bomb dissolution technique. Reproducibilities were comparable, on the order of 10-20 ppm. Our technique allows combined Hf-Sr-Nd isotope analyses of low-mass (50-2 mg) samples.

The ABF digestion is an alternative technique to high-pressure bomb dissolution in matrix decomposition for accurate and reproducible Hf-Nd-Sr isotope analyses of geological samples within a reasonable time (3-4 days), with high sample throughput and low costs in geochemistry and environmental sciences.