Determination of Geomagnetic Palaeointensities in Vacuum

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 10⁻⁵ 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.