Fachbereich 14 - Geowissenschaften
Institut für Planetologie
Analytische Planetologie

The 40Ar/39Ar age of plagioclase from Acapulco meteorite and
the problem ofsystematic errors in cosmochronology

Renne [1] dated plagioclase from the meteorite Acapulco, applying the ⁴⁰Ar/³⁹Ar technique, to 4507±9 Ma. He confirmed age results obtained on whole rock samples [2] and 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 and the plagioclase ⁴⁰Ar/³⁹Ar age was interpreted as cooling time between Pb-Pb closure (at 720 K) and Ar-closure (at 560 K), and used to infer a cooling rate of 3.7 K/Ma (Pellas et al. [2]). Renne [1] argued now 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, he noted that a value for the K-decay constant (0.580 and 4.884 x 10^-10/year for electron capture and β^- decay of ⁴⁰K) different from common use (0.581 and 4.962 x 10^-10/year) would result in better accordance of the Pb-Pb and ⁴⁰Ar/³⁹Ar age, consistent with a monotonous rapid cooling indicated by various methods. This would conflict with the results of the ²⁴⁴Pu fission track method [2], unless a change to slow cooling rates occurred after cooling below Ar isotopic closure. We argue that with the present chronological data (a) the suggested different value of the K-decay constant [1] is improbable, and (b) rather low cooling rates of 6 - 8 K/Ma between 720 and 550 K are to be inferred for Acapulco. However, for a positive test of the K decay constant a precisely dated meteorite is needed for which fast accretion as well as fast cooling is established both in the high and low temperature regime. This must be a meteorite with a Pb-Pb age close to Allende CAIs and high metallographic, petrographic and ²⁴⁴Pu fission track cooling rates.

References

[1] P.R. Renne (2000) ⁴⁰Ar/³⁹Ar age of plagioclase from Acapulco meteorite and the problem of systematic errors in cosmochronology, Earth Planet. Sci. Lett. 175 13-26.

[2] P. Pellas, C. Fieni, M. Trieloff and E.K. Jessberger (1997) The cooling history of the Acapulco meteorite as recorded by the ²⁴⁴Pu and ⁴⁰Ar-³⁹Ar chronometers. Geochim. Cosmochim. Acta 61, 3477-3501.

Drittmittelgeber:

Deutsche Forschungsgemeinschaft

Beteiligter Wissenschaftler:

Elmar K. Jessberger et al.

Veröffentlichungen:

Trieloff M., Jessberger E. K., and Fiéni C. (2001): Comment on "⁴⁰Ar/³⁹Ar age of plagioclase from Acapulco meteorite and the problem of systematic errors in cosmochronology" by Paul R. Renne. Earth Planet. Sci. Lett. 190, 267-269.