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‰(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 than150 μg K was loaded during the column chemistry. To evaluate the potential use of K isotopes as an archive of paleo seawater composition, δ41K 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 δ41K 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‰) and iron formation samples(FeR-2: –0.538±0.009‰; FeR-4: –0.401±0.008‰) 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.