Low paleointensities recorded in 1 to 2.4 Ga Proterozoic dykes, Superior Province, Canada

Paleointensity studies have been conducted on six mafic dyke swarms from the Superior Province in Canada with U–Pb ages between 1141 Ma and 2473 Ma (Buchan et al., Tectonophysics 319 (2000) 167–198). The mean direction of the characteristic magnetization for each dyke swarm coincides with results of earlier studies in which the primary origin of the magnetization was established on the basis of contact and secular variation tests. This primary component was isolated over a very narrow range of high unblocking temperatures (usually 550–580°C), indicating that it is carried by magnetite or low titanium titanomagnetite. Magnetic mineralogy and grain size experiments suggest that the magnetization is dominated by pseudo-single domain or single domain grains. Taken together, the characteristics of this component suggest that it is thermal in origin. Paleointensity experiments were conducted with a specially designed oven, using a revised approach of the Thellier–Coe method. Eighty-five successful determinations of paleointensity were obtained for 20 sites from six dyke swarms, doubling the number of existing well-constrained Precambrian data. The paleofield estimates for the dyke swarms vary between 5 and 11.2 μT, yielding virtual dipole moments between 0.85±0.1 and 2.8±0.87×1022 Am2. The overall convergence between these new results and the rest of the database strongly reinforces the existence of a weaker geomagnetic field during the 1000–2400 Ma period than has been recorded between 0.3 and 400 Ma. However, additional determinations are required, particularly for the 400–2000 Ma interval, in order to establish whether a long-term evolution in the time-averaged field intensity can be linked to the onset of the growth of the Earth’s inner core.