[sup.92]Nb-[sup.92]Zr and the Early Differentiation History of Planetary Bodies

The niobium-92-zirconium-92 ([sup.92]Nb-[sup.92]Zr) extinct radioactive decay system (half-life of about 36 million years) can place new time constraints on early differentiation processes in the silicate portion of planets and meteorites. Zirconium isotope data show that Earth and the oldest lunar crust have the same relative abundances of [sup.92]Zr as chondrites. [sup.92]Zr deficits in calcium-aluminum-rich inclusions from the Allende meteorite constrain the minimum value for the initial [sup.92]Nb/[sup.93]Nb ratio of the solar system to 0.001. The absence of [sup.92]Zr anomalies in terrestrial and lunar samples indicates that large silicate reservoirs on Earth and the moon (such as a magma ocean residue, a depleted mantle, or a crust) formed more than 50 million years after the oldest meteorites formed.