Microwave paleointensities from dyke chilled margins: a way to obtain long-term variations in geodynamo intensity for the last three billion years

Abstract

Absolute paleointensity values of the geomagnetic field, measured using microwaves, are obtained from chilled margin samples of proterozoic dykes from the southern superior province of the Canadian Shield. Two age groups of dykes are studied; the 2446–2473 Ma Matachewan swarm, and 2076–2170 Ma dykes from the Biscotasing, Marathon and Fort Frances swarms. All samples selected for paleointensity study carry a dominant, primary magnetization component as determined from detailed paleomagnetic studies. Paleointensity results quoted below are based on individual sample determinations having a quality factor greater than 10. For the Matachewan dykes, the mean virtual dipole moment (VDM) is 2.53 ± 0.93 A m 2 × 10 22, based on 12 samples from 12 dykes. These results compare favourably with Thellier data from the same swarm by who obtained a mean VDM value, based on 25 samples from five dykes, of 2.80 ± 0.87 A m 2 × 10 22. For the Biscotasing, Marathon and Fort Frances swarms, the mean VDM, based on 12 samples from 12 dykes is 4.13 ± 1.82 A m 2 × 10 22, a value significantly greater than that obtained by Macouin et al. for the same swarms of 0.98 ± 0.10 A m 2 × 10 22, based on 25 samples from nine dykes. Possible reasons for this difference are discussed. The microwave method is shown to be an effective means for measuring paleointensity on dyke chilled margins and offers the opportunity that dyke swarms, which occur frequently throughout the last three billion years of geological history, can be used to track any long period variations in the geomagnetic field intensity that reflect stages in the evolution of the earth's core.