92 Nb- 92 Zr and the Early Differentiation History of Planetary Bodies

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Abstract
The niobium-92–zirconium-92 (92Nb-92Zr) 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 92Zr as chondrites. 92Zr deficits in calcium-aluminum–rich inclusions from the Allende meteorite constrain the minimum value for the initial 92Nb/93Nb ratio of the solar system to 0.001. The absence of 92Zr 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.