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Science 4 July 2003:
Vol. 301. no. 5629, pp. 84 - 87
DOI: 10.1126/science.1084662
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Reports

Evolution of Planetary Cores and the Earth-Moon System from Nb/Ta Systematics

Carsten Münker,1* Jörg A. Pfänder,1 Stefan Weyer,1{ddagger} Anette Büchl,1,2 Thorsten Kleine,1 Klaus Mezger1

It has been assumed that Nb and Ta are not fractionated during differentiation processes on terrestrial planets and that both elements are lithophile. High-precision measurements of Nb/Ta and Zr/Hf reveal that Nb is moderately siderophile at high pressures. Nb/Ta values in the bulk silicate Earth (14.0 ± 0.3) and the Moon (17.0 ± 0.8) are below the chondritic ratio of 19.9 ± 0.6, in contrast to Mars and asteroids. The lunar Nb/Ta constrains the mass fraction of impactor material in the Moon to less than 65%. Moreover, the Moon-forming impact can be linked in time with the final core-mantle equilibration on Earth 4.533 billion years ago.

1 Institut für Mineralogie, Corrensstrasse 24, Universität Münster, 48149 Münster, Germany. 2 Max-Planck-Institut für Chemie, Postfach 3060, 55020 Mainz, Germany.



{ddagger} Present address: Institut für Mineralogie, Universität, Franfurt, Senckenbergenlage 28, 60325 Frankfurt, Germany.

* To whom correspondence should be addressed. E-mail: muenker{at}nwz.uni-muenster.de

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