title: Evolution of planetary cores and the Earth-Moon system from Nb/Ta systematics.
abstract: 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.
authors.affiliation: Institut für Mineralogie, Corrensstrasse 24, Universität Münster, 48149 Münster, Germany. muenker@nwz.uni-muenster.de
authors.forename: Carsten
authors.initials: C
authors.lastname: Münker
authors.forename: Jörg A
authors.initials: JA
authors.lastname: Pfänder
authors.forename: Stefan
authors.initials: S
authors.lastname: Weyer
authors.forename: Anette
authors.initials: A
authors.lastname: Büchl
authors.forename: Thorsten
authors.initials: T
authors.lastname: Kleine
authors.forename: Klaus
authors.initials: K
authors.lastname: Mezger
mesh_terms: D018534:Earth, Planet; D019419:Evolution, Planetary; D019015:Geologic Sediments / Q000737:chemistry; D006195:Hafnium / Q000032:analysis; D018535:Mars; D019110:Meteoroids; D018543:Minor Planets; D016081:Moon; D009556:Niobium / Q000032:analysis; D017640:Silicates; D013635:Tantalum / Q000032:analysis; D015040:Zirconium / Q000032:analysis
chemical_list: D017640:Silicates; D009556:Niobium; D013635:Tantalum; D015040:Zirconium; D006195:Hafnium