Accurate measurement of K/Ca in low‐[K] carbonate samples using laser‐ablation sector‐field inductively coupled plasma mass spectrometry

Abstract

RationalePotassium (K) is a major component of several silicate minerals and seawater, and, therefore, constraining past changes in the potassium cycle is a promising way of tracing large‐scale geological processes on Earth. However, [K] measurement using inductively coupled plasma mass spectrometry (ICP‐MS) is challenging due to an ArH+ interference, which may be of a similar magnitude to the K+ ion beam in samples with <0.1% m/m [K].MethodsIn this work, we investigated the effect of the ArH+ interference on K/Ca data quality by comparing results from laser‐ablation (LA)‐ICP‐MS measured in medium and high mass resolution modes and validating our LA results via solution ICP‐optical emission spectroscopy (OES) and solution ICP‐MS measurements. To do so, we used a wide range of geological reference materials, with a particular focus on marine carbonates, which are potential archives of past changes in the K cycle but are typically characterised by [K] < 200 μg/g. In addition, we examine the degree to which trace‐element data quality is driven by downhole fractionation during LA‐ICP‐MS measurements.ResultsOur results show that medium mass resolution (MR) mode is sufficiently capable of minimising the effect of the ArH+ interference on K+. However, the rate of downhole fractionation for Na and K varies between different samples as a result of their differing bulk composition, resulting in matrix‐specific inaccuracy. We show how this can be accounted for via downhole fractionation corrections, resulting in an accuracy of better than 1% and a long‐term reproducibility (intermediate precision) of <6% (relative standard deviation) in JCp‐1NP using LA‐ICP‐MS in MR mode.ConclusionOur [K] measurement protocol is demonstrably precise and accurate and applicable to a wide range of materials. The measurement of K/Ca in relatively low‐[K] marine carbonates is presented here as a key example of a new application opened up by these advances.