“New 40Ar/39Ar dating of the Grande Ronde lavas, Columbia River Basalts, USA: Implications for duration of flood basalt eruption episodes” by – Discussion

Barry et al. (2010) presented 40Ar/39Ar ages to argue for a very limited duration (~ 0.4 m.y.) for the eruption of the Grande Ronde lavas. Critical examination of their data sets shows (1) a number of their plateau ages are statistically invalid and (2) many of their groundmass matrix samples were altered, and did not yield valid estimates of the time of crystallization. In general, their ages err on the young side, reflecting partial 40Ar * loss, due to alteration, and there is no compelling evidence therein, to argue for very limited duration for the Grande Ronde Basalt. The Imnaha, Grande Ronde and Wanapum Basalt erupted within ~ 0.7 m.y., starting at ~ 16.4 Ma. By contrast, the “ages” of Barry et al. (2010) span the interval ~ 16.6 to ~ 14.7 Ma and, in some instances, are out of stratigraphic sequence.