The tungsten isotopic composition of the Earth’s mantle before the terminal bombardment

Abstract

Many precious, ‘iron-loving’ metals, such as gold, are surprisingly abundant in the accessible parts of the Earth, given the efficiency with which core formation should have removed them to the planet’s deep interior1. One explanation of their over-abundance is a ‘late veneer’—a flux of meteorites added to the Earth after core formation as a ‘terminal’ bombardment that culminated in the cratering of the Moon2. Some 3.8 billion-year-old rocks from Isua, Greenland, are derived from sources that retain an isotopic memory of events pre-dating this cataclysmic meteorite shower3,4. These Isua samples thus provide a window on the composition of the Earth before such a late veneer and allow a direct test of its importance in modifying the composition of the planet. Using high-precision (less than 6 parts per million, 2 standard deviations) tungsten isotope analyses of these rocks, here we show that they have a isotopic tungsten ratio 182W/184W that is significantly higher (about 13 parts per million) than modern terrestrial samples. This finding is in good agreement with the expected influence of a late veneer. We also show that alternative interpretations, such as partial remixing of a deep-mantle reservoir formed in the Hadean eon5,6 (more than four billion years ago) or core–mantle interaction7, do not explain the W isotope data well. The decrease in mantle 182W/184W occurs during the Archean eon (about four to three billion years ago), potentially on the same timescale as a notable decrease in 142Nd/144Nd (refs 3 and 6). We speculate that both observations can be explained if late meteorite bombardment triggered the onset of the current style of mantle convection.

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Figure 1: ε 182 W measurements of Isua and post-Archean samples.
Figure 2: Model estimates of ε 182 W in modern mantle.

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Acknowledgements

We thank the IODP, C. Storey (University of Portsmouth) and M. D. Norman (ANU Canberra) for providing sample material. We thank C. Coath for assistance with mass spectrometric analyses and T. Kleine and F. Moynier for comments. We acknowledge funding from NERC (NE/DO12805/1, NE/H011927/1), STFC (ST/F002734/1), and DFG (WI 3579/1-1).

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Samples from Isua were collected by S.M. Analytical development and sample analyses were carried out by M.W. Modelling and manuscript preparation was carried out by T.E. and M.W. All authors contributed to discussing the results and implications.

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Correspondence to Matthias Willbold.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Information

This file contains Supplementary Text and Data, Supplementary Figures 1-4 with legends and Supplementary Tables 1-5 (see separate excel files for Supplementary Tables 6 and 7). (PDF 1270 kb)

Supplementary Table 6

This table shows the results of Monte-Carlo simulations using shallow mantle partition coefficients for hidden reservoir formation. (XLS 1042 kb)

Supplementary Table 7

This table shows the results of Monte-Carlo simulations using deep mantle partition coefficients for hidden reservoir formation. (XLS 1289 kb)

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Willbold, M., Elliott, T. & Moorbath, S. The tungsten isotopic composition of the Earth’s mantle before the terminal bombardment. Nature 477, 195–198 (2011). https://doi.org/10.1038/nature10399

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