Development of Argon Isotope Reference Standards for the U.S. Geological Survey
A P. Miiller
The comparison of high-precision ages of geological materials measured by laboratories engaged in geochronological studies has been limited by the lack of high-accuracy mineral standards (monitors) for which reported ages have differed by as much as 2 %. In order to address this problem, the U.S. Geological Survey is planning to calibrate the conventional 40K/ 40Ar age of a new preparation of an international hornblende standard labeled MMhb-2. The 40K concentration in MMhb-2 has already been measured by the Analytical Chemistry Division at NIST with an uncertainty of 0.2 %. The 40Ar concentration will be measured by the USGS using the argon isotope reference standards described in this paper that were recently developed by NIST. The isotope standards were constructed in the form of pipette/reservoir systems and calibrated by gas expansion techniques to deliver small high-precision aliquots of high-purity argon. Two of the pipette systems will deliver aliquots of 38Ar with an imprecision of 0.025 % and initial molar quantities of 1.567 x 10-10 moles and 2.312 x 10-10 moles with uncertainties of 0.058 % and 0.054 %, respectively. Three other pipette systems will deliver aliquots (nominally 4 x 10-10 moles) of 40Ar:36Ar artificial mixtures with similar precision and accuracy and with molar ratios of 0.9974 + 0.06 %, 29.69 + 0.06 %, and 285.7 + 0.08 %. These isotope reference standards will enable the USGS to measure the 40Ar concentration in MMhb-2 with an uncertainty of ~ 0.1 %. In the process of these measurements, the USGS will measure and re-assign the isotopic composition of atmospheric Ar and calculate a new value for its atomic weight with greater accuracy. Upon completion of the USGS calibrations, the MMhb-2 mineral standard will be certified by NIST for its K and Ar concentrations and distributed as a Standard Reference Material (SRM). The new SRM and the NIST-calibrated transportable pipette systems have the potential for dramatically improving the accuracy of interlaboratory calibrations and thereby the measured ages of geological materials, perhaps by as much as a factor of ten.
argon isotope standards, K-Ar dating, potassium-argon dating, transportable pipette systems
Development of Argon Isotope Reference Standards for the U.S. Geological Survey, Journal of Research (NIST JRES), National Institute of Standards and Technology, Gaithersburg, MD
(Accessed June 4, 2023)