Nonspiked 40Ar and 36Ar quantification using a quadrupole mass spectrometer: A potential for K–Ar geochronology

Potassium–argon geochronology relies on the absolute quantification of radiogenic 40Ar (40Ar*) produced by the natural decay of 40K in K-bearing rocks and minerals. It implies the precise determination of the 40Ar and 36Ar contents to account for atmospheric contamination, which, for very young (<100 kA) and/or K-poor material (K < 0.2 wt.%), may represent over 99% of their argon budget. We have tested the applicability of a standard resolution quadrupole mass spectrometer (QMS) for the determination of 40Ar* in a set of K–Ar and 40Ar/39Ar international geochronological standards.The analytical apparatus is designed to perform in-line extraction, purification and measurement of 1012 to 1014 atoms of Ar from various K-bearing minerals. The sensitivity of the mass spectrometer is calibrated against the international K–Ar standard GL-O and determined with a 0.6% (1σ) uncertainty. The average 40Ar/36Ar ratio of repeated air-standards through a 1-month period is 262.5 ± 0.36 (1σ), and its daily standard deviation never exceeds 1‰.Geochronological standards MMhb-1 (520.4 ± 1.7 Ma, hornblende), LP-6 (127.8 ± 1.4 Ma, biotite), Taylor Creek Rhyolite (27.92 ± 0.04 Ma, TCR sanidine), Alder Creek Rhyolite (1.193 ± 0.001 Ma RAC-20 sanidine) and RMF96 (0.331 ± 0.002 Ma, sanidine) dated here at 529.7 ± 7.2, 130.1 ± 1.0, 29.13 ± 0.4, 1.189 ± 0.022 and 0.340 ± 0.006 Ma, respectively, are within or close to their published reference ages at the 1σ confidence level.Recent high-sensitivity and high-resolution quadrupole mass filters, equipped with appropriate and high stability electronic equipments, provide compact and fast analytical alternatives to expensive magnetic-sector-type mass spectrometers for applications such as K–Ar geochronology.