Physics
Scientific paper
Apr 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja....12002m&link_type=abstract
EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #12002
Physics
Scientific paper
It has long been assumed that Nb and Ta are both lithophile elements that are hosted by the silicate mantle of differentiated solid planets and thus occur with a chondritic ratio of ca 17.6. Recent experimental studies, however, predict that Nb can have moderately siderophile properties at higher pressures (above 20 GPa). In order to assess the mass budget of Nb in terrestrial planets and asteroids we performed high precision measurements of Nb/Ta and Zr/Hf in a variety of extraterrestrial and terrestrial materials using MC-ICPMS and a mixed 180Ta--180Hf--94Zr tracer. Measurements of six carbonaceous chondrites (CI, CM, CK) now define the chondritic Nb/Ta and Zr/Hf ratios at 20.5±0.7 and 34.1±0.5, respectively. In order to determine bulk silicate Nb/Ta in differentiated planets, the spread in Nb/Ta and Zr/Hf that is caused by internal silicate differentiation is corrected to the known chondritic Zr/Hf (both lithophile). When corrected to the chondritic Zr/Hf, samples from Mars (20.1±0.9) overlap with the chondritic value, while samples from Vesta display slightly lower Nb/Ta (18.1±0.8). In contrast, the silicate Earth and the Moon have Nb/Ta of 14.0±0.3 and 17.0±0.8, clearly below the chondritic value. The observation that Nb/Ta in Archean greenstones (Isua, Baberton) overlap with those in present day MORB and OIB suggests, that the Nb-Ta inventory of Earth's depleted mantle has been little modified by subduction processes through the last 4 Gy. This model is supported by recently reported high precision Nb/Ta measurements in arc rocks that show little variation in Nb/Ta and overlap with MORB values. Hence, there is no evidence for a long lived silicate reservoir on Earth that has superchondritic Nb/Ta. Rather, the Nb depletion in the silicate Earth is a pre-Archean feature that was established by metal segregation during formation of Earth's core. On Earth, metal silicate segregation and formation of the metal core occurred at higher pressures (ca. 25 GPa) than on Mars and Vesta, thus explaining the more pronounced Nb depletion. The difference in Nb/Ta between Earth and the Moon can put new constraints on the origin of the Earth-Moon system that is explained by a giant impact. If the impactor was Mars-sized or smaller, the amount of impactor material in the Moon is constrained by Nb/Ta systematics to more than 50%. Most physical models, however, postulate a smaller contribution of impactor material. We therefore suggest that the elevated Nb/Ta of the Moon reflects the Nb/Ta of the proto Earth, where metal-silicate segregation occurred at lower pressures. After the impact event, metal-silicate segregation continued at higher pressures in a deep magma ocean that was triggered by the impact. The Moon is therefore older than 4.533 Ga.
Büchl Anette
Mezger Klaus
Münker Carsten
Pfänder Jörg A.
Weyer Stefan
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