Abstract
Geochemical models describing the behaviour of niobium during Earth’s growth rely on the general paradigm that niobium was delivered by Earth’s asteroidal building blocks at chondritic abundances. This paradigm is based on the observation that niobium is traditionally regarded as a refractory and strongly lithophile element, and thus stored in the silicate portions of Earth and differentiated asteroids. However, Earth’s silicate mantle is instead selectively depleted in niobium, in marked contrast to the silicate mantles of many asteroids and smaller planets that apparently lack any significant depletion in niobium. Here we present results of high-precision measurements for niobium and other lithophile elements in representative meteorites from various small differentiated asteroids. Our data, along with the results of low-pressure experiments, show that in more reduced asteroids—such as Earth’s first building blocks—niobium is moderately chalcophile and more so than its geochemical twin tantalum by an order of magnitude. Accordingly, niobium can be sequestered into the cores of more reduced asteroids during differentiation via the segregation of sulfide melts in a carbon-saturated environment. We suggest that the niobium deficit in Earth’s silicate mantle may be explained by the Earth’s silicate mantle preferentially accreting the silicate portions of reduced asteroidal building blocks.
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Change history
31 October 2017
In the version of this Article originally published, the meteorite group IAB was incorrectly defined in the 'High field strength elements in differentiated meteorites' section. This error has been corrected in the online version of the Article.
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Acknowledgements
We appreciate funding by DFG grant Mu 1406/6 and ERC grant 669666 ‘Infant Earth’. K. Mezger is thanked for access to the MC-ICPMS facility at ZLG Münster from 2004 to 2008. G. Mallman is thanked for help during LA-ICPMS measurements at ANU Canberra. We thank R. Schumacher, Mineralogical Museum Bonn, Germany, A. Bischoff, Universität Münster, and J. Zipfel, Senckenberg Museum Frankfurt, Germany, for providing meteorite samples.
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C.M. performed most of the analytical work and manuscript writing. R.O.C.F. performed all experimental work and contributed to manuscript writing. T.S. performed part of the analytical work.
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Münker, C., Fonseca, R. & Schulz, T. Silicate Earth’s missing niobium may have been sequestered into asteroidal cores. Nature Geosci 10, 822–826 (2017). https://doi.org/10.1038/ngeo3048
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DOI: https://doi.org/10.1038/ngeo3048
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