Potassium Isotope Geochemistry and Magmatic Processes

Morgan, L., Higgins, J., Davidheiser-Kroll, B., Lloyd, N., Faithfull, J. and Ellam, R. (2014) Potassium Isotope Geochemistry and Magmatic Processes. In: Goldschmidt2014, Sacramento, C.A., USA, 8-13 Jun 2014,

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Potassium stable isotopic (41K/39K) compositions are notoriously difficult to measure. TIMS measurements are hindered by variable fractionation patterns and too few isotopes to apply an internal spike method for instrumental mass fractionation corrections. Internal corrections via the 40K/39K ratio can provide precise values and are appropriate in some cases (e.g. identifying excess 41K [1]) but not others (e.g., determining fractionation effects and metrologically-traceable isotopic abundances). SIMS analyses have yielded results with 0.25‰ precisions [2]. Improved precision with the technique presented here identifies the first conclusive evidence for terrestrial K isotopic fractionation. We measured δ41K values ( 41K/39K relative to NIST SRM 999b) on NIST K standards with < 0.05‰ precisions (2σ) on the Thermo Scientific NEPTUNE Plus MC-ICP-MS. 39K and 41K were sufficiently resolved from the interfering 38ArH+ and 40ArH+ peaks in wet cold plasma and high-resolution mode. Measurements were made on narrow but flat, interference-free, plateaus (ca. 50 ppm by mass width for 41K). Although ICPMS does not yield accurate 41K/39K values due to significant instrumental mass fractionation (ca. 6%), this bias is sufficiently stable that relative 41K/39K values can be precisely determined via sample-standard bracketing. Measurement tolerances on matrix effects that are amplified by the cold plasma were tested; the use of clean samples and standards is critical. Most high temperature phases from a diverse range of whole rock and mineral samples yield δ41K values that span a range of 0.4‰. A few samples yield more extreme values, which indicate geological processes that may be informed by forthcoming work. In particular, there are indications for seawater recycling in subduction zones and fractionation during the formation of pegmatites. These and other potential future applications of stable K isotopic measurements will be discussed.

Item Type:Conference Proceedings
Additional Information:Keynote presentation
Glasgow Author(s) Enlighten ID:Morgan, Dr Leah and Ellam, Professor Rob and Faithfull, Dr John
Authors: Morgan, L., Higgins, J., Davidheiser-Kroll, B., Lloyd, N., Faithfull, J., and Ellam, R.
College/School:College of Science and Engineering > Scottish Universities Environmental Research Centre
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