Discussion of a paper by McKerrow, Lambert and Chamberlain on the Ordovician, Silurian and Devonian time scales

Abstract

In their paper on the Ordovician, Silurian and Devonian time scales McKerrow, Lambert and Chamberlain conclude that Rb Sr whole-rock isochron ages for acid volcanics always resetting events significantly later than extrusion. In contrast we cite here evidence that Rb Sr whole-rock isochrons for acid volcanics may yield ages as reliable as those determined by any other method and argue that, although any radiometric clock may be reset, this should not be assumed to have occurred without proof. McKerrow, Lambert and Chamberlain do not present any new data and their conclusions appear to stem from quoting errors associated with fission track ages at the 1σ level and assigning incorrect errors to Rb Sr whole-rock isochron ages as a result of a failure to use modern least squares regression techniques. We make the point that most of the available data for the calibration of the Ordovician, Silurian and Devonian time scales are still rather imprecise and that it would be more useful for geochronologists to devote their efforts to acquiring better data rather than to continue to rediscuss old results many of which are undoubtedly not up to modern analytical standards. We conclude that, provided the errors associated with radiometric ages are assigned correctly and at the 2σ level, then a minor revision of the time scale of Gale, Beckinsale and Wadge increasing most time scale points by about 5–10 Ma would reconcile almost all the calibration data presently available.