Nb-92-Zr-92 and the early differentiation history of planetary bodies

The niobium-92-zirconium-92 (Nb-92-Zr-92) extinct radioactive decay system (half-life of about 36 million years) can place new time constraints on ear ly differentiation processes in the silicate portion of planets and meteori tes. Zirconium isotope data show that Earth and the oldest Lunar rust have the same relative abundances of Zr-92 as chondrites. Zr-92 deficits in calc ium-aluminum-rich inclusions from the Allende meteorite constrain the minim um value for the initial Nb-92/Nb-93 ratio of the solar system to 0.001. Th e absence of Zr-92 anomalies in terrestrial and lunar samples indicates tha t large silicate reservoirs on Earth and the moon (such as a magma ocean re sidue, a depleted mantle, or a crust) formed more than 50 million years aft er the oldest meteorites formed.