PROGRESS ON INTER-LABORATORY CALIBRATION OF ARGON ISOTOPE MEASUREMENTS

Small biases between chronometers and laboratories are becoming significant as we achieve ever-increasing levels of precision, and it is important to develop strategies to improve our ability to compare results from different laboratories. Here we report progress on an inter-laboratory calibration experiment for the ⁴⁰Ar/³⁹Ar community that we hope will lead to improvements, analogous to those in the U-Pb community (Condon et al., 2010, GCA). This portable system is composed of three 2.7 L canisters each equipped with three pipettes of 0.1, 0.2 and 0.4 cc volumes. With these pipettes it is possible to combine them to provide 0.1, 0.2, 0.3 (0.1+0.2), 0.4, 0.5 (0.1+0.4), 0.6 (0.2+0.4), and 0.7 (0.1+0.2+0.4) cc. The configuration allows a simple test for interlab biases and for volume/pressure dependent mass fractionation on the measured ratios for a gas with a single argon isotope composition. In preliminary experiments at Lamont (VG5400) and Rutgers (MAP 215-50) one canister was filled with air and another with unirradiated biotite. We found a consistent 6.3% offset between the Lamont and Rutgers systems. Using the air standard to correct the composition of the biotite, the Rutgers ⁴⁰Ar/³⁶Ar of the biotite gas is 575.31 and Lamont’s is 575.02.

A potentially added benefit of the system is the possibility to mix gas from any one of the three canisters in proportions of these increments. If canister 1 has a ⁴⁰Ar*/³⁹Ar of 1.73 and canister 2 has a ⁴⁰Ar*/³⁹Ar of 40.98 (~ Alder Creek and Fish Canyon in the same irradiation), there are 69 combinations of known ratio and gas volume. Of these there are seven combinations of the possible mixes of pipettes that will yield a ⁴⁰Ar*/³⁹Ar of 3.32, also with a factor of seven range in gas load. There are two cases of three combinations with the same constant intermediate ratios and four cases of two, with a factor of three and two range in gas amount respectively. The third canister will contain atmospheric argon, allowing designing the most appropriate ⁴⁰Ar/³⁶Ar for the samples being measured. Such a system would allow characterizing much of the range of values that might be encountered with natural samples. A productive follow-up to the intercalibration would be to have each EARTHTIME lab equipped with one of these systems permanently. We will give the latest update on the experiments in our presentation.