New 40Ar/39Ar ages from the Grande Ronde and Wanapum Basalt, Columbia River Basalt Group (CRBG): Compilation of all ages and relationship to the geomagnetic polarity time scale for ~17–15 Ma

• Published in: Journal of Earth System Science 2022-09, Vol.131 (3), p.158, Article 158
• Main Author: Baksi, Ajoy K
• Format: Article
• Language: English
• Subjects: Acid leaching; Argon; Basalt; Dating; Decades; Deccan Traps and other Flood Basalt Provinces – Recent Research Trends; Earth and Environmental Science; Earth Sciences; Feldspars; Geomagnetism; Lava; Lava flows; Leaching; Polarity; Radiometric dating; Rivers; Single crystals; Space Exploration and Astronautics; Space Sciences (including Extraterrestrial Physics; Time; Volcanic activity
• ISSN: 0973-774X; 0253-4126; 0973-774X
• DOI: 10.1007/s12040-022-01892-3

New 40 Ar/ 39 Ar ages for the Grande Ronde and Wanapum Basalts, on samples carefully selected with respect to the previously known magnetostratigraphic units, are presented. These confirm earlier reports (both 40 Ar/ 39 Ar and U–Pb) of ages that indicate the bulk of the Columbia River Basalt Group, northwestern USA, began erupting ~16.6 Ma, and continued for ~700 Ka. These ages and magnetic units are correlated with the geomagnetic polarity time scale for ~17–15 Ma. Other reports based on 40 Ar/ 39 Ar dating, suggesting a much longer period of volcanic activity for the Picture Gorge and Steens Basalt, are shown to be incorrect; these errors arise from improper interpretation of what constitute (statistically) valid plateau/isochron ages, and from the dating of altered material – leading to incorrect ages. Single crystal total fusion argon ages for material recovered from tuffs intercalated with lava flows, are shown to be prone to error; the total fusion technique cannot unequivocally detect cases of partial loss of 40 Ar* and/or the presence of excess 40 Ar. The benefits of acid leaching of whole-rock basalts (with HNO 3 or HCl) and feldspars (with dilute HF) are emphasized. The importance of screening all available argon ages by the above methods is reiterated and shown to be critical to the dating of flood basalt material. The ages and magnetostratigraphic units are compared to various versions of the geomagnetic polarity time scale for ~17–15 Ma. An attempt is made to bring argon and U–Pb ages for the CRBG into agreement; this suggests that a minor (~0.5%) adjustment to younger values, needs to be made to the argon ages.