Comparative microwave versus thermal Thellier palaeointensity study of the Lesotho Basalt, southern Africa
Abstract
The microwave variant of the Thellier palaeointensity method has been compared with the conventional thermal Thellier method using eight lava flows from 180 Ma Lesotho Basalt. Two control flows, for which the conventional Thellier method previously yielded technically acceptable palaeointensity estimates, have shown a major difference when studied with the microwave technique. For one flow, the two techniques yield concordant palaeointensity estimates at the individual sample level, resulting in statistically undistinguishable flow-mean palaeointensity values. For another control flow, however, mean palaeointensity obtained with the microwave technique is about three times less than that obtained with the conventional thermal technique. This discrepancy is tentatively ascribed to a greater degree of alteration occurring during thermal Thellier experiments. Out of six flows for which palaeointensity estimates could not be obtained with the conventional Thellier method, five yielded acceptable palaeointensities using the microwave technique. Thus, alteration appears to be strongly reduced in microwave experiments. This suggests that changes in the magnetic properties of ferrimagnetic fraction of a rock sample during a conventional Thellier experiment originate mainly from changing the mechanical environment of ferrimagnetic grains, most likely internal as well as external stresses. Taking into account new palaeointensity determinations with microwave technique, mean virtual dipole moment (VDM) for the Lesotho Basalt is found to be (4.0 ± 2.0)\cdot 1022 Am2 (N = 16), lending further support for the predominantly low dipole field in the Early Jurassic.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....4764K