AGS Monthly Meeting

Speaker Ajoy Baksi

Topic: The timing and duration of flood basalt traps, India and the Columbia River Basalt Group, USA.

Speaker bio: Ajoy Baksi obtained a M.S. (1967) and Ph.D. (1970), from the University of Toronto (Canada) in (Geo)Physics, following degrees in Physics and Exploration Geophysics in India. Worked in Canada, India and Australia, before moving to Louisiana State University in 1982. Research was focused on geochronology (argon dating methods) and geochemistry; retired in 2008 and moved to Atlanta in 2019. In “Emeritus state” have continued to publish occasionally. Most recently three papers in 2022, on the Deccan and Rajmahal Traps (India), Columbia River Basalts (USA).

Abstract: Interest in flood basalt volcanism (encompassing > 200,000 km3 of rocks) has increased over the past 60 years, including the possible effect on the environment, of their eruption. This effort looks at the latest research on geochronological aspects of their timing and duration of formation. Until the last decade, the argon dating methods were the the only tools used for this purpose. Initial K-Ar work in the 1970s, on whole-rock basalts, showed that bulk of the Columbia River Basalt (CRB) was formed in < 2 m.y. at ~16 Ma. The next two decades produced scattered results for the Deccan and Rajmahal Traps (India), the CRB (USA), the Siberian Traps (Russia), the Parana (South America) and Karoo and Etendeka (South Africa). Following improvements on technical fronts, 40Ar/39Ar dating of both whole-rock samples and plagioclase separates, showed that the bulk of the Siberian Traps and Parana were (each) formed in ~ 1 m.y. around 250 Ma and 135 Ma, respectively. This effort summaries work on the Deccan and the CRB over the past decade. The latest 40Ar/39Ar results show that each of these was formed in ~ 700 ka around 66 Ma and 16.4 Ma, respectively. The advent of U-Pb dating on single zircons, taken from layers intercalated with the flows in these provinces, has reinforced these conclusions. The obstacles facing such efforts are examined, as well as the lacunae of the 40Ar/39Ar and U-Pb dating methods. These techniques produce ages with precision < ±0.5 m.y. over the Phanerozoic; their accuracy must be carefully judged on both geological and physical/mathematical grounds.