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Evidence for an Early Archean Geomagnetic Field: A paleomagnetic study of the Komati Formation, Barberton Greenstone Belt, South Africa
Author(s) -
Hale C. J.,
Dunlop D. J.
Publication year - 1984
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/gl011i002p00097
Subject(s) - archean , geology , paleomagnetism , greenstone belt , polar wander , remanence , apparent polar wander , banded iron formation , proterozoic , geochemistry , greenschist , geophysics , demagnetizing field , geomagnetic pole , thermoremanent magnetization , metamorphism , earth science , earth's magnetic field , paleontology , magnetization , magnetic field , tectonics , physics , quantum mechanics
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, α 95 =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.