The origin of air-like noble gases in MORB and OIB

There has been an ongoing debate for almost two decades on the age and duration of Deccan trap volcanism. Some of the best available ages have been determined using the 40Ar/39Ar method, but differences in sample location, laboratory procedure, type of analyzed material, and ages of monitors may have been responsible for much of the apparent spread in the data. We have collected and analyzed samples from close to the bottom and top of the main lava pile in the Western Ghats near Bombay. We confirm previous findings that alteration and Ar recoil strongly affect these rocks, particularly measurements made on whole rocks. When mineral separates (mostly plagioclase) are analyzed, the spread is much reduced. We find a mean age of 65.4±0.7 Ma for five flows near the base of the section (Jahwar and Igatpuri formations) and 65.2±0.4 Ma for a dyke crosscutting the Poladpur formation, below the topmost Mahabaleshwar formation. This implies that at least 1800 m out of the 2500 m composite trap section were erupted in a short interval, less than 1 Ma long and close to 65.5 Ma, i.e. close to the Cretaceous¯Tertiary boundary. We find that older ages (near 67 Ma) found by several authors are essentially based on whole rock samples, which display disturbed spectra (when this can be checked, i.e. when the spectra are published).