1. Department of Earth and Environmental Sciences,
2. Department of Physics and Astronomy, University of Rochester, Rochester, New York 14627, USA
3. School of Geological Sciences, University of KwaZulu-Natal, Durban 4041, South Africa

Correspondence to: John A. Tarduno^1,2 Correspondence and requests for materials should be addressed to J.A.T. (Email: john@earth.rochester.edu).

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 rocks^{1, 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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