Letter

Hadean age for a post-magma-ocean zircon confirmed by atom-probe tomography

  • Nature Geoscience volume 7, pages 219223 (2014)
  • doi:10.1038/ngeo2075
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Abstract

The only physical evidence from the earliest phases of Earth’s evolution comes from zircons, ancient mineral grains that can be dated using the U–Th–Pb geochronometer1. Oxygen isotope ratios from such zircons have been used to infer when the hydrosphere and conditions habitable to life were established2,3. Chemical homogenization of Earth’s crust and the existence of a magma ocean have not been dated directly, but must have occurred earlier4. However, the accuracy of the U–Pb zircon ages can plausibly be biased by poorly understood processes of intracrystalline Pb mobility5,6,7. Here we use atom-probe tomography8 to identify and map individual atoms in the oldest concordant grain from Earth, a 4.4-Gyr-old Hadean zircon with a high-temperature overgrowth that formed about 1 Gyr after the mineral’s core. Isolated nanoclusters, measuring about 10 nm and spaced 10–50 nm apart, are enriched in incompatible elements including radiogenic Pb with unusually high 207Pb/206Pb ratios. We demonstrate that the length scales of these clusters make U–Pb age biasing impossible, and that they formed during the later reheating event. Our tomography data thereby confirm that any mixing event of the silicate Earth must have occurred before 4.4 Gyr ago, consistent with magma ocean formation by an early moon-forming impact4 about 4.5 Gyr ago.

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Acknowledgements

This research was supported by NSF-EAR0838058, DOE-93ER14389 and the NASA Astrobiology Institute. T.C., D.R., D.F.L., D.J.L. and P.C. thank their colleagues at CAMECA in Madison, Wisconsin, for their contribution to these efforts. WiscSIMS is partly supported by NSF-EAR1053466.

Author information

Affiliations

  1. WiscSIMS, NASA Astrobiology Institute, Department of Geoscience, University of Wisconsin, Madison, Wisconsin 53706, USA

    • John W. Valley
    • , Aaron J. Cavosie
    • , Takayuki Ushikubo
    •  & Michael J. Spicuzza
  2. University of Puerto Rico, Mayaguez, Puerto Rico 00681, USA

    • Aaron J. Cavosie
  3. CAMECA, Madison, Wisconsin 53711, USA

    • David A. Reinhard
    • , Daniel F. Lawrence
    • , David J. Larson
    • , Peter H. Clifton
    •  & Thomas F. Kelly
  4. Curtin University, GPO Box U1987, Perth 6845, Western Australia, Australia

    • Simon A. Wilde
  5. University of Western Ontario, London, Ontario N6A 5B7, Canada

    • Desmond E. Moser

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Contributions

J.V. initiated this project, selected samples, assisted in data reduction and interpretation, and wrote most of the paper. A.C. and S.W. dated the 4.4-Gyr-old zircon by SIMS, and assisted in interpretation and rewriting. TU made SIMS analysis of trace elements and SEM images, and assisted in interpretation. D.R. performed data analysis of APT data, and assisted in interpretation. D.F.L. prepared samples by FIB. D.J.L., P.C. and T.K. performed atom-probe analysis and assisted in interpretation. D.M. conducted EBSD analysis and SEM imaging of the zircon. M.S. assisted in analysis and interpretation. All authors reviewed and approved this paper.

Corresponding author

Correspondence to John W. Valley.

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