Single grain (U-Th)/He ages from phosphates in Acapulco meteorite and implications for thermal history

The cooling history of the Acapulco meteorite for > 400degreesC is well established using various chronometers suggesting extremely fast cooling ( > 1000degreesC/Ma). In contrast, the thermal history for low temperatures ( < 400 degrees C) is poorly understood because of large uncertainties in the chronometers applicable to this temperature range. To better constrain the cooling history for the low-temperature range, we applied (U-Th)/He dating techniques to individual phosphate grains. One whitlockite and 11 apatite grains yielded (U-Th)/He ages ranging from 1272 22 (1(7, analytical error only) Ma to 4584 +/- 51 Ma, with tight clustering at similar to 4.55 Ga. The weighted mean of the five oldest ages (4538 +/- 32 Ma, 1 sigma uncertainty including systematic error) is suggested to be the minimum age representing primary cooling of the Acapulco body passing through similar to 120 degrees C. Although it is impossible to precisely quantify the effects of energetic alpha particle ejection from the outermost similar to 20 mu m of the phosphates, petrographic evidence suggests that most dated samples are fragments likely derived from the interior of larger grains, thus greatly reducing this source of error. Indeed the five oldest samples cannot have suffered substantial ejection since the uncorrected ages are identical with the crystallization age of the Acapulco meteorite. The new (U-Th)/He data suggest rapid cooling of Acapulco down to similar to 120 degrees C. This evidence suggests that the younger 40Ar/39Ar age (4507 9 (1(7) Ma) obtained from Acapulco plagioclase, which should reflect cooling through similar to 300 degrees C, is spuriously young due to systematic errors (i.e., decay constants and/or standard data) in the 40Ar/39Ar method, as suggested by comparison between high-precision 40Ar/39Ar and U/Pb ages for terrestrial volcanic rocks. The scattered He age distribution < 4.0 Ga implies very heterogeneous thermal disturbances after the primary cooling of the body. (C) 2003 Elsevier Science B.V. All rights reserved.