In concert with detailed remapping at 1:24,000 of the Three Sisters and their peripheral family of lesser volcanoes, we have initiated a comprehensive effort to calibrate the eruptive history of the cluster with ⁴⁰Ar/³⁹Ar ages. Step heating experiments on groundmass concentrates from lavas yield high precision (all errors ±1sigma) plateau ages and a single plagioclase separate yields a reasonable, but less precise result.

Our first experiments emphasize silicic lavas of Middle and South Sister and eruptive units mapped within the area of recent deformation on the west side of South Sister. In addition to the late Holocene rhyolites of Rock Mesa and the Devils Hill chain well characterized by W.E. Scott, there are at least 20 glacially modified rhyolitic and dacitic lava flows that have vented on or adjacent to the Middle and South Sister edifices, all or most of them in the late Pleistocene. Several rhyolite lavas issue from at least as high as 8600 ft on the south slope of andesite-capped 10,358-ft South Sister, from vents within 1.2 km of its crater rim. The agglutinated summit andesite (54-56% SiO₂, 1.0% K₂O) is so young that a satisfactory age has not yet been measured. From the NE toe of South Sister, a 100-m-thick rhyolitic lava (74% SiO₂, 3.1% K₂O) that extends 3 km eastward yields a plateau age of 35±2 ka. Plagioclase from Kaleetan Butte, a 2.5-km² rhyolite dome (73.8% SiO₂, 3.1% K₂O) 6 km south of the summit of South Sister, yields a plateau age of 62±20 ka.

At Middle Sister, two dacite flows yield indistinguishable ages. A 100-m-thick dacite coulee (64.4% SiO₂, 1.8% K₂O) that emerges from the south flank of the cone and caps a mesa west of Chambers Lakes gives a plateau age of 15.7±2.5 ka. The dacite of Renfrew Glacier (64% SiO₂, 1.7% K₂O), which extends 3 km NW from the summit and is as thick as 150 m near its terminus, yields an age of 17.3±2.9 ka. The venerable assumption that the Three Sisters chain youngs southward is being scrutinized.