Intensity of the Earth's magnetic field in late Archean obtained from diabase dikes of the Slave Province, Canada

Abstract

The geomagnetic field intensity in Late Archean times is evaluated from Archean diabase dikes intruded into the Yellowknife greenstone belt of the Slave Province, N.W.T., Canada. The dike set used is referred to as the 8a dikes, and their age is bracketed between 2620 and 2642 Ma by crosscutting relationships between the 8a dikes and adjacent granitic rocks dated by U–Pb method. Paleomagnetic directional data after thermal demagnetization (ThD) and results of some field tests suggest that the characteristic remanent magnetizations (ChRM) of the dikes have been acquired at ca. 2.6 Ga. Several rock magnetic studies indicate that the dike samples are suitable for paleointensity determinations. Thelliers' method with consistency checks was used for the paleointensity determinations, and two of the 8a dikes yielded the mean paleointensities of 30.9±1.1 and 43.9±1.2 μT, respectively. The corresponding virtual dipole moments (VDM) of (6.3±0.2)×10 22 A m 2 and (9.0±0.2)×10 22 A m 2 are similar to the present day value, suggesting that, in the Earth's core, the dynamo process of comparable activity to that of the present day has already existed at ca. 2.6 Ga.