Collaborative Research: Development of 40Ar/39Ar Intercalibration Pipettes

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Potassium (K), one of the more abundant elements in the Earth?s crust is found in many rock forming minerals. One of the isotopes of K, 40K is radioactive and decays to 40Ar. This scheme forms the physical basis for one of the most common geochronologic techniques applied to rocks and minerals. In recent years, a special technique requiring neutron bombardment of samples to produce 39Ar from 39K has become more favored than the traditional ?K-Ar? method, referred to as the 40Ar/39Ar method. In fact, the 40Ar/39Ar method of radioisotopic dating has the broadest range of applicability of any geochronometer, being useful from early solar system formation (~4.56 billion years ago) to volcanic materials as young as 2,000 years old. These studies cover topics such as volcanic and seismic hazards, timing and extent of mineralization events, global climate change and variability, paleomagnetic reversal timescale, and dating the fossil record, including human evolution. Thus any improvement in the accuracy and precision 40Ar/39Ar method will improve our understanding of the timing and rates of geologic processes.Despite exemplary precision, recent experiments conducted via the EARTHTIME initiative have revealed significant interlaboratory inconsistencies such that ages determined by different labs may vary by us much as 4 %, approximately an order of magnitude larger than typical reported measurement precision. Four possible causes for these discrepancies have been identified; 1) differences in the data reduction protocols employed by the participating laboratories; 2) unaccounted nonlinearity of the mass spectrometer source/detector systems used to collect the data; 3) unrecognized heterogeneity in the mineral standards or an experimental artifact arising from variations in neutron irradiation dosage in some of the sample packages; and 4) incomplete degassing or isotopic fractionation of Ar from the analyzed sanidines in various laboratories. This research project addresses the issue of interlaboratory consistency by developing an argon pipette system that will travel between the participating 40Ar/39Ar labs within the United States. The pipette system will deliver gas samples with exactly the same isotopic composition(s) and similar gas volume(s), thus controlling the variables and issues associated with heterogeneity of natural samples. In addition, variable gas sample volumes from the pipette system will also be measured to assess the response linearity of noble gas mass spectrometers. The ultimate goal of this study is to intercalibrate the various NSF supported 40Ar/39Ar laboratories and allow for the direct comparison of the ages produced from the participating labs, at the level of 0.1% or better. The long-term use of the pipette system will maintain the calibration between the 40Ar/39Ar laboratories. Undergraduate and graduate students will be traveling with the intercalibration pipette assisting with the measurements. This will provide the students the opportunity to see, work, and interact with personnel from the different 40Ar/39Ar laboratories and serve as a unique training opportunity for the next generation of geochronologist/isotope geochemists.