Argon Intercalibration Pipette System (APIS): Smoking from the Same Pipe

Abstract:

⁴⁰Ar/³⁹Ar age inter-calibration experiments, conducted as part of the US NSF sponsored EARTHTIME initiative, (http://www.earth-time.org), using two of the most commonly used ⁴⁰Ar/³⁹Ar mineral standards, Fish Canyon (FC, ~28.2 Ma) and Alder Creek (AC, ~1.2 Ma) sanidines, have revealed significant inter-laboratory inconsistencies. The reported ages for the AC sanidines range from 1.173 to 1.200 Ma (FC 28.02) (±~2%), ~4 times greater than the reported precision. These experiments have caused the ⁴⁰Ar/³⁹Ar community to scrutinize procedures and several informal lab intercalibrations have been conducted among different labs. This exercise is leading to better agreement, but discrepancies remain that need to be addressed.

In an effort to isolate the cause(s) of these inconsistencies, two Argon Inter-calibration Pipette System (APIS) were designed and constructed. Each consists of three gas canisters; one contains atmospheric Ar, while the other two contain artificial gas mixtures with ⁴⁰Ar/³⁹Ar ratios similar to those of FC and AC. Each canister has 4x10^-10 moles of ⁴⁰Ar, is equipped with 0.1, 0.2 and 0.4 cc pipettes, and can deliver gas volumes from 0.1-0.7 cc. All volumes were determined manometrically to 0.4% or better and then filled to uniform pressure with Ar standard gases. This experimental design eliminates sample heterogeneity, leaving only interlaboratory variations in gas purification, data reduction, and isotopic measurement as potential sources of interlaboratory calibration discrepancies.

APIS-1 was designated as a traveling unit that is brought to participating labs. APIS-2 is the reserve/master standard. Currently, APIS-1 is in its early stages in the voyage and has been to three labs (Rutgers, LDEO, and New Mexico Tech) as of this writing. The interlaboratory comparisons are ongoing, and will include ASU, BGC, Univ. of Wisconsin, and Oregon State University, plus additional laboratories of opportunity. A progress report will be presented at AGU.