Comment on '~(40)Ar/~(39)Ar age of plagioclase from Acapulco meteorite and the problem of systematic errors in cosmochronology' by Paul R. Renne

摘要

Applying the ~(40)Ar/~(39)Ar technique, Renne [1] dated plagioclase from the meteorite Acapulco to 4507 ± 9 (1σ) Ma. He confirmed age results obtained on whole rock samples [2,3] and provided support for the conclusion by Pellas et al. [2] that the actually dated phase of whole rock samples is plagioclase. Previously, the difference between the Pb-Pb age of Acapulco phosphates of 4557 ± 2 Ma [4] and the plagioclase ~(40)Ar/~(39)Ar age was interpreted as cooling time between Pb-Pb closure (at ～720 K [5]) and Ar closure (at ～560 K [2]), and used to infer a cooling rate of 3.7 K/Ma [2]. This cooling rate was intermediate between high cooling rates obtained by petrographic or metallographic methods (applicable to high temperature cooling regimes; [2,3] and references therein) and very low cooling rates of 1.7 ± 0.5 K/Ma derived from ~(244)Pu fission track densities in various mineral detectors that monitor cooling at low temperatures, between 550 and 360 K [2]. However, Renne [1] concluded that the results of the absolute U-Pb and K-Ar chronometers could not be used to infer an age difference and a significant cooling time interval, due to the uncertainties of the respective decay constants. Particularly, Renne [1] noted that a value for the K decay constant (0.580 and 4.884 * 10~(-10)/yr for electron capture and β~- decay of ~(40)K) different from common use (0.581 and 4.962 * 10~(-10)/yr [6]) would result in better accordance of the Pb-Pb and ~(40)Ar~(39)Ar age, consistent with a monotonous rapid cooling indicated by various methods. This would conflict with the results of the ~(244)Pu fission track method [2], unless a change to slow cooling occurred after cooling below Ar isotopic closure.