Palaeomagnetism and geochronology of the Proterozoic dolerite dyke from southwest Greenland: indication of low palaeointensity

Abstract

SUMMARY Intensity of the geomagnetic field during the Archaean can potentially be used to study an evolution of the dynamo activity in the Earth's core. In order to investigate this issue, we present new palaeomagnetic and geochronological results from the dolerite dyke, which have been intruded into the Archaean Gneisses of Nuuk area, southwest Greenland. 40Ar/39Ar dating of the pyroxene grains from dolerite yield an age of 2585 ± 21 Ma. The high temperature component, which has been recognized as a characteristic direction, is identified in 24 dolerite samples. Comparison of the palaeomagnetic directions from dolerite dykes with those from host gneisses suggests a primary origin for this component. Magnetic mineralogical and grain size investigations revealed a pseudo-single domain magnetite as a dominant career of magnetization. Thellier palaeointensity determinations of 14 dolerite specimens yield a mean field value of 13.9 ± 2.5 μT. Strength of the virtual dipole moment (2.30 ± 0.42 × 1022 Am2) obtained from this study is about one quarter of the present Earth's filed value. A simulation based numerical model of the Thellier experiments suggests that the presence of multidomain grains can play a role in enhancing the strength of palaeointensity. Even after taking in to account the effects of multidomain grains, an intensity value (13.9 μT) from this study suggests that the strength of geomagnetic field at about 2.6 Ga was much lower than that of the present time.