Radiometric ages of basaltic achondrites and their relation to the early history of the solar system

Abstract

Pb Pb mineral isochrons were determined for three cumulate eucrites (Moama, Moore County, and Serra de Magé) and three noncumulate eucrites (Nuevo Laredo, Bouvante, and Stannern). Two noncumulate eucrites (Bereba and Cachari) show disturbed Pb isotope patterns. The mineral isochron ages of cumulate eucrites range from 4.40 Ga to 4.48 Ga. The latter is the age of Moore County (4.484 ± 0.019 Ga), which was also dated by the Sm Nd method at 4.456 ± 0.025 Ga. Nuevo Laredo and Bouvante give the same Pb-Pb age: 4.514 ± 0.015 Ga and 4.510 ± 0.004 Ga, respectively. The Pb isochron of Stannern corresponds to 4.128 ± 0.016 Ga, which is a reequilibration age. Both Cachari and Bereba show evidence of isotope resetting at about 4 Ga. A Pb-Pb multicotrelation approach is introduced and applied to the data. The younger ages of the cumulate eucrites preclude their formation by fractional crystallization from the melts that produced the noncumulate eucrites. The μ 1 of the source of the cumulate eucrites is ∼ 14, whereas that of the source of the noncumulate eucrites is -150. This large difference in μ 1 values suggests that eucrites were derived from separate planetesimals or from a single heterogeneous (layered?) parent body.