Abstract
The strength of the Earth’s early geomagnetic field is of importance for understanding the evolution of the Earth’s deep interior, surface environment and atmosphere. Palaeomagnetic and palaeointensity data from rocks formed near the boundary of the Proterozoic and Archaean eons, some 2.5 Gyr ago, show many hallmarks of the more recent geomagnetic field. Reversals are recorded1, palaeosecular variation data2 indicate a dipole-dominated morphology and available palaeointensity values are similar to those from younger rocks1,2,3. The picture before 2.8 Gyr ago is much less clear. Rocks of the Archaean Kaapvaal craton (South Africa) are among the best-preserved, but even they have experienced low-grade metamorphism4. The variable acquisition of later magnetizations by these rocks is therefore expected, precluding use of conventional palaeointensity methods. Silicate crystals from igneous rocks, however, can contain minute magnetic inclusions capable of preserving Archaean-age magnetizations. Here we use a CO2 laser heating approach and direct-current SQUID magnetometer measurements to obtain palaeodirections and intensities from single silicate crystals that host magnetite inclusions. We find 3.2-Gyr-old field strengths that are within 50 per cent of the present-day value, indicating that a viable magnetosphere sheltered the early Earth’s atmosphere from solar wind erosion.
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Acknowledgements
We thank P. Layer, A. Smirnov and P. Doubrovine for discussions. This research was supported by NSF.
Author Contributions J.A.T., M.K.W. and D.B. took part in field studies, and J.A.T., R.D.C. and D.B. in technique development, experiments and data analysis.
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This file contains Supplementary Methods, Supplementary Figures S1-S6 wit Legends, Supplementary Tables 1-3 and additional references (PDF 331 kb)
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Tarduno, J., Cottrell, R., Watkeys, M. et al. Geomagnetic field strength 3.2 billion years ago recorded by single silicate crystals. Nature 446, 657–660 (2007). https://doi.org/10.1038/nature05667
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DOI: https://doi.org/10.1038/nature05667
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