An Archean Geomagnetic Reversal in the Kaap Valley Pluton, South Africa

Science  16 Aug 1996:
Vol. 273, Issue 5277, pp. 943-946
DOI: 10.1126/science.273.5277.943


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.