Robust 24 ± 6 ka ⁴⁰Ar/³⁹Ar age of a low-potassium tholeiitic basalt in the Lassen region of NE California

⁴⁰Ar/³⁹Ar ages on the Hat Creek Basalt (HCB) and stratigraphically related lava flows show that latest Pleistocene tholeiitic basalt with very low K₂O can be dated reliably. The HCB underlies ∼ 15 ka glacial gravel and overlies four andesite and basaltic andesite lava flows that yield ⁴⁰Ar/³⁹Ar ages of 38 ± 7 ka (Cinder Butte; 1.65% K₂O), 46 ± 7 ka (Sugarloaf Peak; 1.85% K₂O), 67 ± 4 ka (Little Potato Butte; 1.42% K₂O) and 77 ± 11 ka (Potato Butte; 1.62% K₂O). Given these firm age brackets, we then dated the HCB directly. One sample (0.19% K₂O) clearly failed the criteria for plateau-age interpretation, but the inverse isochron age of 26 ± 6 ka is seductively appealing. A second sample (0.17% K₂O) yielded concordant plateau, integrated (total fusion), and inverse isochron ages of 26 ± 18, 30 ± 20 and 24 ± 6 ka, all within the time bracket determined by stratigraphic relations; the inverse isochron age of 24 ± 6 ka is preferred. As with all isotopically determined ages, confidence in the results is significantly enhanced when additional constraints imposed by other isotopic ages within a stratigraphic context are taken into account.