40Ar/39Ar and U-Th-Pb dating of separated clasts from the Abee E4 chondrite
Determinations of 40 Ar/ 39 Ar and U-Th-Pb are reported for three clasts from the Abee (E4) enstatite chondrite, which has been the object of extensive consortium investigations. The clasts give 40 Ar/ 39 Ar plateau ages and/or maximum ages of 4.5 Gy, whereas two of the clasts give average ages of 4.4 Gy. Within the range of 4.4–4.5 Gy these data do not resolve any possible age differences among the three clasts. 206 Pb measured in these clasts is only ∼1.5–2.5% radiogenic, which leads to relatively large uncertainties in the Pb isochron age and in the 207 Pb/ 206 Pb model ages. The Pb data indicate that the initial 207 Pb/ 206 Pb was no more than 0.08±0.07% higher than this ratio in Can˜on Diablo troilite. The U-Th-Pb data are consistent with the interpretation that initial formation of these clasts occurred 4.58 Gy ago and that the clasts have since remained closed systems, but are contaminated with terrestrial Pb. The 40 Ar/ 39 Ar ages could be gas retention ages after clast formation or impact degassing ages. The thermal history of Abee deduced from Ar data appears consistent with that deduced from magnetic data, and suggests that various Abee components experienced separate histories until brecciation no later than 4.4 Gy ago, and experienced no appreciable subsequent heating.