Differentiated meteorites including basaltic and iron meteorites preserve a record of formation and differentiation of young terrestrial planets. Dating of these meteorites makes it possible to understand the timescales of the earliest stages of planetary evolution. The U-Pb isotopic chronometer is capable of precise dating of basaltic/gabbroic meteorites (achondrites), but it cannot be applied to meteorites in which U-rich phases are absent, like iron meteorites. Instead chronometry using short-lived (now-extinct) radionuclides such as182Hf and 92Nb potentially allows chronological ordering of iron meteorites and achondrites. The challenge in building one timescale with several chronometers is making sure their readings are consistent. In particular, the application of short-lived isotopic chronometers requires that the short-lived radionuclides were uniformly distributed in the meteorite-forming region. This can be evaluated if there are reliable multiple anchors for the early Solar System timescale, i.e., meteorites for which absolute ages can be precisely defined and in which the initial abundances of short-lived radionuclides can be additionally determined. Yet few reliable time anchors are currently available.
We have conducted U-Pb dating of various achondrites to understand the timescale of planetary crust evolution and to establish new reliable time anchors. We found that the studied achondrites have a wide range in U-Pb age from 4563 Ma to ca. 4440 Ma and that the eucrite Agoult and the ungrouped achondrites Ibitira and Northwest Africa 6704 can potentially serve as the time anchors. We have further investigated the Nb-Zr isotope systematics of Agoult, Ibitira and the basaltic angrite Northwest Africa 4590 whose U-Pb age was previously determined. The results allow us to define the Solar System initial92Nb abundance and to evaluate its uniformity. We will discuss the prospects and limitations of the Nb-Zr chronometer in the early Solar System chronology.