Determination of Geomagnetic Palaeointensities in Vacuum

Abstract

Summary Specimens with low and intermediate Curie points from six Hawaiian historic basalt flows were used for the determination of the intensity of the historic geomagnetic field by the Thelliers’ method. The specimens were heated either in air or in a vacuum of lo-’ torr. The regional intensity of the Earth’s magnetic field at the time of extrusion of these rocks is known from direct observations. The palaeointensities determined in vacuum from four low and intermediate Curie point flows are correct within the uncertainty caused by the local field anomalies, whereas those determined in air from the same four flows are of lesser quality and accuracy. Unaltered submarine basalts have similarly low Curie points and thus may also be amenable to palaeointensity determination in vacuum. The behaviour of the remaining two flows, which had higher Curie points, was more erratic, and they yielded less accurate palaeointensities regardless of whether they were determined in air or vacuum. The Thelliers’ technique (Thellier & Thellier 1959) is one of the most widely used methods for the determination of palaeointensities of the Earth‘s magnetic field from rocks. It consists of comparing the thermal spectrum of the natural remanent magnetization (NRM) of an igneous rock with that of an artifically induced thermoremanent magnetization (TRM) in the same specimen. It is assumed that the NRM is a TRM acquired during the initial cooling of the rock and that its original thermal spectrum remained unchanged throughout the history of the rock. In practice these conditions are not easily met because of two reasons. First, the NRM acquired by the rock at the time of its formation may be different from that measured in the laboratory because of viscous decay or the superimposition on the NRM of secondary components of magnetization. In some cases, however, this problem may be overcome by thermal cleaning which is an integral part of this technique. Second, when rocks are heated they undergo alterations which affect the chemical composition, size, and shape of their magnetic mineral grains. The effect of these changes may be such that the NRM and TRM are produced in two magnetically different materials, thus rendering the determination of palaeointensity unreliable. Because oxidation is the change most likely to occur when heating is carried out in air, many authors have recommended the use of high Curie temperature rocks, which,