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

• DOI: 10.1126/science.1084662
• Publication Date: 2003-07-03T19:45:48Z
• Authors: Carsten Münker; Jörg A. Pfänder; Stefan Weyer; Anette Büchl; Thorsten Kleine; Klaus Mezger
• Keywords: Geologic Sediments; Earth, Planet; Niobium; Silicates; Mars; Meteoroids; Tantalum; 01 natural sciences; Minor Planets; 13. Climate action; Zirconium; Moon; Evolution, Planetary; Hafnium; 0105 earth and related environmental sciences

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.