HIGH-PRECISION AND ACCURATE DETERMINATION OF LU AND HF CONCENTRATIONS AND HF ISOTOPIC COMPOSITIONS AT THE SUB-NANOGRAM LEVEL IN GEOLOGICAL SAMPLES USING MC-ICP-MS

With the rapid development of MC-ICP-MS in the mid-1990s, the ¹⁷⁶Lu-¹⁷⁶Hf radiogenic isotope system has become widespread application in the geochemistry, cosmochemistry and environmental sciences. It has become a routine analysis of Lu, Hf content for most sub-microgram levels geological samples (Hf con. > 0.1 mg g^-1). Nevertheless, it is becoming increasingly important for Hf isotopic compositions in some geological samples (e.g., peridotite, garnet, meteorite, lake sediment and etc.) with very low quantities of Lu and Hf contents (Hf con. < 0.1 mg g^-1). It is very difficult for such low Lu and/or Hf samples for following reasons: 1) the extremely low concentration of Lu and Hf in these materials makes it difficult to separate sufficient Lu and Hf from those samples for highly precise measurements; 2) Lu and Hf are strongly partitioned into fluorides (mainly MgF₂) produced by HF+HNO₃+HClO₄ decomposition of abundant MgO (>35%) in ultramafic rocks, resulting in low recovery yields for these elements; 3) these analyses are often compromised by problems that can arise during ion exchange chemistry, including high Lu and Yb blanks causing inaccurate interference corrections on 176Hf, insufficient Ti removal leading to reduced Hf transmission in the mass spectrometer.

The motivation of this work is to establish a simple, rapid and practical analytical protocol for Lu and Hf concentrations and Hf isotopic compositions of sub-nanogram level Lu and Hf in geological samples using MC-ICP-MS, without comprising the precision and accuracy. Samples were spiked with ¹⁷⁶Lu-¹⁸⁰Hf enriched tracer, followed by the complete digestion using the conventional HF, HNO₃ and HClO₄ acid protocol and then complete dissolution fluoride formed during the HF decomposition step using H₃BO₃. Mass spectrometric measurement was carried out on a Neptune Plus MC-ICP-MS with Aridus II desolvator. In order to demonstrate the robustness of present protocol, six rock reference materials (RMs) of UB-N, JP-1, NIM-D, MUH-1, HARZ01 and DTS-2b, encompassing various rock types with very low amounts of Lu and Hf (from nanogram to sub ng g^-1 level), were multiply analyzed with full column chemistry. The analytical results of the geological RMs are in good agreement with the published values. Combined with highly efficient and low-blank chemistry, this protocol greatly holds potential analysis in high-precision and accurate determination of Lu and Hf concentrations and Hf isotopic compositions at the sub-nanogram level in geological samples using MC-ICP-MS.