Abstract
3.5 Ga old peridotitic komatiites of the Komati formation, Barberton greenstone belt, South Africa have a characteristic NRM (natural remanent magnetization) that is stable during alternating field and thermal demagnetization. The thermal nature of this remanence is demonstrated by successful Thellier‐type paleo‐intensity determinations on some samples in the temperature range 350 to 575°C. The characteristic NRM is an almost total thermal overprint acquired during unroofing and cooling following pervasive greenschist metamorphism at 3470±20 Ma. The mean direction of magnetization after cleaning is D=326° I=81° (k=60, α95=8°, N= 7 sites), with a paleopole at 11°S, 21°E (δp=δm=15°). The paleopole falls in southern Africa; ancestral southern Africa was therefore in polar latitudes around 3.5 Ga, the same position it occupied in the late Archean and early Proterozoic. Paleofield intensities are about one‐half present‐day intensities for polar latitudes. A vigorous earth's magnetic field was already in existence in the early Archean, implying that the core had a suitable size, composition and convective flow to support dynamo action.
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