Niobium in Planetary Cores: Consequences for the Interpretation of Terrestrial Nb Systematics

Details Niobium in Planetary Cores: Consequences for the Interpretation of Terrestrial Nb Systematics Niobium in Planetary Cores: Consequences for the Interpretation of Terrestrial Nb Systematics

Dec 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.v71c..03j&link_type=abstract

American Geophysical Union, Fall Meeting 2002, abstract #V71C-03

Physics

1010 Chemical Evolution, 1015 Composition Of The Core, 1025 Composition Of The Mantle, 1065 Trace Elements (3670)

Scientific paper

In order to search for a hidden container for the 'missing' Nb in the Earth's mantle, we measured Nb, Ta, V and Cr in magmatic iron meteorites thought to be fragments of metallic cores of asteroid-sized bodies. We found high Nb/Ta (> 75) with Nb concentrations (0.007-0.04 ppm) that are correlated with V and Cr, thus supporting the hypothesis of Wade and Wood (2001) that Nb entered the Earth's core as a weakly siderophile element. From our data of terrestrial samples, SNC meteorites and eucrites, we estimate Nb/Ta ratios of the bulk silicate phases of Earth (14+/-2), Mars (16+/-1) and Vesta (17), all of which are lower than the chondritic value of 17.5. Assuming that these lower-than-chondritic ratios are caused by partitioning of Nb into the cores, and that the precursors of the planetary bodies have a chondritic composition, we calculate the mass fractions of the Nb budget of 20%, 9% and 2% for the cores of Earth, Mars and Vesta. These values correspond to Nb concentrations of 0.3 ppm, 0.2 ppm, 0.03 ppm, respectively. Metal/silicate partition coefficients of Nb, calculated from our estimates of the bulk silicate phases and from our Nb data of the iron meteorites increase strongly with increasing pressure consistent with the experimental data of Wade and Wood (2001). These results have important consequences for the interpretation of terrestrial Nb systematics. The lower primitive-mantle Nb abundance of 0.55 ppm means that primitive-mantle ratios such as Nb/U will be lowered by 20% (Nb/U=24 instead of 30). This, in turn, changes conventional mass balance estimates of the major terrestrial silicate reservoirs (Hofmann et al., 1986). Using new estimates for continental crust U and Nb (Rudnick and Fountain, 1995) in conjunction with the classical three-reservoir Silicate Earth model, we now obtain a primitive reservoir of about 30% consistent with recent mantle models assuming a primitive reservoir of relatively low volume located in the deep mantle (Kellog et al., 1999). If the non-depleted reservoir is enriched rather than primitive, the results are also consistent with the model of Tolstikhin and Hofmann (this volume) invoking an enriched reservoir at the base of the mantle.

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