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

Acapulcoites consistently yield some of the oldest Ar-40/Ar-39 ages reporte d for any meteorites, all indistinguishable from the age of 4510 +/- 22 Ma previously determined for whole-rock samples of Acapulco. These ages are si gnificantly younger, at face value, than the 4557 +/- 2 Ma Pb/Pb age report ed for phosphates, which dates cooling through similar to 550 degrees C. Me tallographic evidence for rapid cooling between 650 and 350 degrees C, rete ntion of Xe-129 in apatite, and the absence of shock features all suggest t hat the Ar-40/Ar-39 ages might be expected to be more consistent with the P b/Pb age. New Ar-40/Ar-39 data from plagioclase, determined to be the sole K-rich phase in Acapulco, yield a high-precision isochron age of 4507 +/- 1 8 Ma (2 sigma) based on currently accepted decay constants and standards bu t neglecting uncertainties in these quantities. These data allow unambiguou s association of the Ar-40/Ar-39 age with a closure temperature near 300 de grees C and preclude the possibility of artifacts due to Ar-39 recoil, low- temperature alteration products, or inherited pre-solar rains. The Ar-40/Ar -39 plagioclase age recalculated with decay constants believed to be more a ccurate than those conventionally used is 4554 Ma, weakening the basis of a previously inferred dramatic decrease in cooling rate shortly after closur e of the Pb/Pb system in phosphate. Systematic errors in Ar-40/Ar-39 dating , associated chiefly with K-40 decay constants and standards, must be redre ssed before the method can be applied to high-resolution inference of early Solar System evolution. Similar limitations affect ages based on other lon g-lived radionuclides such as Sm-147. (C) 2000 Elsevier Science B.V. All ri ghts reserved.