An Archean Geomagnetic Reversal in the Kaap Valley Pluton, South Africa
The Kaap Valley pluton in South Africa is a tonalite intrusion associated with the Archean Barberton Greenstone Belt. Antipodal paleomagnetic directions determined from the central and marginal parts of the pluton record a geomagnetic reversal that occurred as the pluton cooled. The age of the reversal is constrained by an 40Ar/39Ar plateau age from hornblende at 3214 +/- 4 million years, making it the oldest known reversal. The data presented here suggest that Earth has had a reversing, perhaps dipolar, magnetic field since at least 3.2 billion years ago.
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P. W. Layer, Geophysical Institute, University of Alaska, Fairbanks, AK 99775, USA. A. Kroner, Institut fur Geowissenschaften, Johannes Gutenburg-Universitat, 55099 Mainz, Germany. M. McWilliams, Department of Geophysics, Stanford University, Stanford, CA 94305, USA.
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McWilliams
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