High-precision 41K/39K measurements by MC-ICP-MS indicate terrestrial variability of δ41K

Morgan, L. E., Santiago Ramos, D. P., Davidheiser-Kroll, B., Faithfull, J. , Lloyd, N. S., Ellam, R. M. and Higgins, J. A. (2018) High-precision 41K/39K measurements by MC-ICP-MS indicate terrestrial variability of δ41K. Journal of Analytical Atomic Spectrometry, 33(2), pp. 175-186. (doi: 10.1039/C7JA00257B)

Abstract

Potassium is a major component in continental crust, the fourth-most abundant cation in seawater, and a key element in biological processes. Until recently, difficulties with existing analytical techniques hindered our ability to identify natural isotopic variability of potassium isotopes in terrestrial materials. However, measurement precision has greatly improved, and a range of K isotopic compositions has now been demonstrated in natural samples. In this study, we present a new technique for high-precision measurement of K isotopic ratios using high-resolution, cold plasma multi-collector mass spectrometry. We apply this technique to demonstrate natural variability in the ratio of 41K to 39K in a diverse group of geological and biological samples, including silicate and evaporite minerals, seawater, and plant and animal tissues. The total range in 41K/39K ratios is ca. 2.6‰, with a long-term external reproducibility of 0.17‰ (2σ, N=108). Seawater and seawater-derived evaporite minerals are systematically enriched in 41K compared to silicate minerals by ca. 0.6‰, a result consistent with recent findings (1, 2). Although our average bulk- silicate Earth value (-0.54‰) is indistinguishable from previously published values, we find systematic δ41K variability in some high-temperature sample suites, particularly those with evidence for the presence of fluids. The δ41K values of biological samples span a range of ca. 1.2‰ between terrestrial mammals, plants, and marine organisms. Implications of terrestrial K isotope variability for the atomic weight of K and K-based geochronology are discussed. Our results indicate that high-precision measurements of stable K isotopes, made using commercially available mass spectrometers, can provide unique insights into the chemistry of potassium in geological and biological systems.

• Item Type: Articles
• Keywords: isotope geochemistry, mass spectrometry, earth science, igneous petrology
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Morgan, Dr Leah and Ellam, Professor Rob and Faithfull, Dr John
• Authors: Morgan, L. E., Santiago Ramos, D. P., Davidheiser-Kroll, B., Faithfull, J., Lloyd, N. S., Ellam, R. M., and Higgins, J. A.
• Subjects: Q Science > QD Chemistry; Q Science > QE Geology
• College/School: College of Science and Engineering > Scottish Universities Environmental Research Centre; University Services > Library and Collection Services > Museum and Art Gallery
• Journal Name: Journal of Analytical Atomic Spectrometry
• Journal Abbr.: J. Anal. At. Spectrom
• Publisher: Royal Society of Chemistry
• ISSN: 0267-9477
• ISSN (Online): 1364-5544
• Published Online: 02 November 2017
• Copyright Holders: Copyright © 2018 The Royal Society of Chemistry
• First Published: First published in Journal of Analytical Atomic Spectrometry 33(2):175-186
• Publisher Policy: Reproduced in accordance with the copyright policy of the publisher