Potassium isotopic analysis of trace levels of K in basaltic and carbonate samples using an HF precipitation per-separation method and collison cell MC-ICP-MS

Abstract

Background

Stable potassium (K) isotopes have emerged as valuable chemical tracers for a diverse range of bio-, geo-, and cosmo-chemical processes since the advent of high-precision K isotopic analysis by MC-ICP-MS. Collision cell MC-ICP-MS offers the advantage of high signal intensity (>1000 V/ppm ³⁹K), enabling the analysis of low-K samples. However, analysis of low-K samples, such as some basaltic rocks and carbonate samples, presents significant challenges due to high matrix effects, low K concentrations, and trace-level interference from Ca polyatomic ions.

Results

We developed a HF precipitation method to preconcentrate K from major matrix elements, which is then combined with a conventional small-volume resin chromatographic procedure to further purify K in carbonates and basaltic samples. The HF acid effectively removes over 95 % of Ca and Mg from carbonates while ensuring complete K recovery. For basaltic samples, the HF method successfully eliminates Al, Ca, Mg, and portions of Fe and Na, thereby enhancing the feasibility of analyzing low-K silicate samples. Although K recovery from basaltic samples ranges from 83 % to 96 %, no significant K isotopic fractionation was observed during the HF precipitation process. The robustness of this method was validated using geostandards of basalts (BHVO-2, BIR-1), dolerite (DNC-1), and coral (JCp-1), and we firstly report δ⁴¹K value for dolostone GBW07114.

Significance

This presents an HF precipitation per-separation method preconcentrate K from the major matrix elements combined with chromatographic procedures to further purify K in low-K carbonates and basaltic samples. It also demonstrates great potential for analyzing low-K and/or high-Ca (–Mg) geological samples, including those from astronomical contexts, thereby broadening the scope of K isotopic investigations into K-depleted planetary reservoirs.