High-precision potassium isotope analysis using the Nu Sapphire collision cell (CC)-MC-ICP-MS

This study presents high-precision analyses of stable potassium (K) isotope ratio using the recently-developed, collision-cell multi-collector inductively coupled plasma mass spectrometry (CC-MC-ICP-MS, Nu Sapphire). The accuracy of our analyses is confirmed by measuring well-characterized geostandards (including rocks and seawater). Our results are consistent with literature values and a precision of 0.04 parts per thousand (2SD) has been achieved based on multiple measurements of BCR-2 geostandard over a six-month period. We also evaluate factors that may lead to artificial isotope fractionations, including the mismatches in K concentration and acid molarity between samples and bracketing standards, as well as potential matrices. As the K adsorption capacity of AGW50-X8 (200-400 mesh) is reduced with an increasing amount of matrix elements, less than 150 mu g K was loaded during the column chemistry. To evaluate the potential use of K isotopes as an archive of paleo seawater composition, delta K-41 values of an international seawater standard (IAPSO), a Mn-nodule (NOD-P-1), and two iron formation standards (FeR-2 and FeR-4) are reported. The delta K-41 value of IAPSO is consistent with other seawater samples reported previously, further substantiating a homogeneous K isotopic distribution in modern global oceans. The K isotopes in Mn-nodule (NOD-P-1: -0.121 +/- 0.013 parts per thousand) and iron formation samples (FeR-2: -0.538 +/- 0.009 parts per thousand; FeR-4: -0.401 +/- 0.008 parts per thousand) seem to be an effective tracer of their formation genesis and compositional changes of ancient seawater. Our results suggest that high-precision measurements of stable K isotopes can be routinely obtained and open up a large variety of geological applications, such as continental weathering, hydrothermal circulation and alteration of oceanic crust.