Response to Baksi, A., 2012, ‘New 40Ar/39Ar dating of the Grande Ronde lavas, Columbia River Basalts, USA: Implications for duration of flood basalt eruption episodes’ by Barry et al., 2010—Discussion’

Baksi (2012) claims that a number of radiometric ages presented in Barry et al. (2010) are statistically invalid and that many of the samples were altered, thus not able to date the eruptions, and that there is no compelling evidence for a limited duration for the eruption of the Grande Ronde Basalts (GR Basalts), USA. However, Baksi (2012), has simply reiterated some of the assessment of the data presented in the paper and has used an ‘Alteration Index’, in order to assess the data. The ‘Alteration Index’ commonly shows that basaltic groundmass is altered and from this Baksi (2012) deduces that the ages are incorrect. In contrast, much work on young volcanics has shown that phenocrysts are often sources of incorrect ages. In any assessment of data, we maintain that whilst alteration is an important consideration, the best approach should include not just alteration, but also experimental reproducibility, stratigraphic order, comparison with other analyses, and analytical techniques, and that the application of an ‘Alteration Index’ is not a straightforward guide to the validity of an age.