A detailed 40Ar/39Ar step-heating geochronological study of pyroxene mineral separates from the Munro–Warden ultramafic sill in the Archean Abitibi Greenstone Belt of the Canadian Superior Province yields a precise primary age of 2703 ± 11 Ma (1σ). This is in excellent agreement with previous U–Pb zircon dates of 2700–2712 Ma for the enclosing felsic volcanics and a Sm–Nd age of 2622 ± 60 Ma for the sill. Electron microprobe analyses, coupled with the argon evolution patterns, demonstrate that deuteric amphibole is responsible for this primary age, whereas the volumetrically dominant diopside shows anomalously young ages. An isotope correlation plot reveals two components of contaminating argon with differing 40Ar/36Ar ratios in distinctly different sites. One component, trapped during the deuteric formation of amphibole, has an 40Ar/36Ar ratio that is significantly less (258 ± 3) than the present-day atmospheric value of 295.5. This may be an indication of the atmospheric ratio 2.7 Ga ago. The other component has a high 40Ar/36Ar ratio and was trapped during a later thermal event. The data demonstrate the potential of the 40Ar/39Arep-heating method for not only obtaining primary ages from low-potassium Archean ultramafic rocks, but also providing information on ancient atmospheric compositions.