Preliminary palaeomagnetic results of an Archaean dolerite dyke of west Greenland: Geomagnetic field intensity at 2.8 Ga

Abstract

The geomagnetic field intensity during Archaean times is evaluated from a palaeomagnetic and chronological study of a dolerite dyke intruded into the 3000 Ma Nuuk Gneisses at Nuuk (64.2° N, 51.7° W), west Greenland. Plagioclase from the dolente dyke yields a mean K-Ar age of 2752 Ma. Palaeomagnetic directions after thermal demagnetization of the dyke and the gneiss reveal a positive baked-contact test, indicating that the high-temperature-component magnetization of the dyke is primary. Thellier experiments on 12 dyke specimens yield a palaeointensity value of 13.5 ± 4.4 μT. The virtual dipole moment at ca. 2.8 Ga is 1.9 ± 0.6 × 1022 Am2, which is about one-quarter of the present value. The present study and other available data imply that the Earth's magnetic field at 2.7-2.8 Ga was characterized by a weak dipole moment and that a fairly strong geomagnetic field similar to the present intensity followed the weak field after ca. 2.6 Ga.