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Potassium Isotopic Compositions of NIST Potassium Standards and 40Ar/39Ar Mineral StandardsKnowledge of the isotopic ratios of standards, spikes, and reference materials is fundamental to the accuracy of many geochronological methods. For example, the 238U/235U ratio relevant to U-Pb geochronology was recently re-determined [1] and shown to differ significantly from the previously accepted value employed during age determinations. These underlying values are fundamental to accurate age calculations in many isotopic systems, and uncertainty in these values can represent a significant (and often unrecognized) portion of the uncertainty budget for determined ages. The potassium isotopic composition of mineral standards, or neutron flux monitors, is a critical, but often overlooked component in the calculation of K-Ar and 40Ar/39Ar ages. It is currently assumed that all terrestrial materials have abundances indistinguishable from that of NIST SRM 985 [2]; this is apparently a reasonable assumption at the 0.25per mille level (1σ) [3]. The 40Ar/39Ar method further relies on the assumption that standards and samples (including primary and secondary standards) have indistinguishable 40K/39K values. We will present data establishing the potassium isotopic compositions of NIST isotopic K SRM 985, elemental K SRM 999b, and 40Ar/39Ar biotite mineral standard GA1550 (sample MD-2). Stable isotopic compositions (41K/39K) were measured by the peak shoulder method with high resolution MC-ICP-MS (Thermo Scientific NEPTUNE Plus), using the accepted value of NIST isotopic SRM 985 [2] for fractionation [4] corrections [5]. 40K abundances were measured by TIMS (Thermo Scientific TRITON), using 41K/39K values from ICP-MS measurements (or, for SRM 985, values from [2]) for internal fractionation corrections. Collectively these data represent an important step towards a metrologically traceable calibration of 40K concentrations in primary 40Ar/39Ar mineral standards and improve uncertainties by ca. an order of magnitude in the potassium isotopic compositions of standards.
Document ID
20140002419
Acquisition Source
Johnson Space Center
Document Type
Conference Paper
Authors
Morgan, Leah (Scottish Universities Environmental Research Centre East Kilbride, United Kingdom)
Tappa, Mike (Aerodyne Industries Houston, TX, United States)
Ellam, Rob (Scottish Universities Environmental Research Centre East Kilbride, United Kingdom)
Mark, Darren (Scottish Universities Environmental Research Centre East Kilbride, United Kingdom)
Higgins, John (Princeton Univ. Princeton, NJ, United States)
Simon, Justin I. (NASA Johnson Space Center Houston, TX, United States)
Date Acquired
March 28, 2014
Publication Date
December 9, 2013
Subject Category
Inorganic, Organic And Physical Chemistry
Geosciences (General)
Report/Patent Number
JSC-CN-29421
Meeting Information
Meeting: American Geophysical Union Annual Meeting
Location: San Francisco, CA
Country: United States
Start Date: December 9, 2013
End Date: December 13, 2013
Sponsors: American Geophysical Union
Distribution Limits
Public
Copyright
Public Use Permitted.

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