Equilibration and ages - Rb-Sr studies of breccias 14321 and 15265

摘要: We have previously demonstrated that the Rb-Sr system of Fra Mauro breccia 14321 did not completely equilibrate at time assembly. Basalt and microbreccia clasts, with approximately same internal isochron ages (about 4.0 b.y.) distinctly different initial ratios. This result preclude possibility individual clast were reset by short range diffusion in a hot ejecta blanket. now demonstrate equilibration between basalt adjacent occur about 4 b.y. on 1 mm scale. The clasts therefore probably date pre-assembly (possibly pre-Imbrium) magmatic impact events. also report presence mare relatively KREEP-rich 15265, indicating formation such breccias after basin-forming