Alan Stephenson

On the standardization of measurements of the anisotropy of magnetic susceptibility

Abstract The magnetic anisotropy of several artificially constructed samples has been measured with different types of instruments in several laboratories. Susceptibility bridge determinations have given consistent results, but the magnitude of the anisotropy determined by the Digico anisotropy delineator is incorrect. For future measurements with this instrument it is necessary to make either a simple calibration change or to make a minor change in the associated computer program. A set of equations for correcting the old published data is given.

Gyroremanent magnetization in a weakly anisotropic rock sample

Abstract Gyroremanent magnetization (GRM) has been detected in a rock sample which, from initial susceptibility measurements, is only weakly anisotropic ( ⋍ 1% ). In accordance with theory the GRM is independent of the frequency of the alternating field for frequencies of 50 and 274 Hz.

Further investigations into lunar palaeointensity determinations

Abstract The Thellier double-heating method, involving the heating of samples to successively higher temperatures and cooling first in zero magnetic field and then in a non-zero magnetic field has been carried out on five lunar samples. It is thought that thermal alteration (oxidation) of the magnetic carriers has been largely avoided but a possible physical interaction or chemical alteration at about 300° C has made it impossible to determine any reliable palaeointensity data from the samples used. Experiments with synthetic samples support Pearce et al.s suggestion that the interaction is between iron and troilite.

Paleointensity determinations of lunar samples

Abstract Evidence for the existence of an ancient lunar field comes from the NRM of lunar rocks and from measurements of local lunar surface magnetic fields. Even when the Fe particles present in the rocks possess enough magnetic stability to preserve a record of the original magnetizing field, there are considerable difficulties in estimating the strength of the field, primarily because of the chemical changes which take place on heating in the laboratory. These difficulties are discussed together with a paleointensity method involving heating in a more indirect way. A tentative interpretation of the results of paleointensity-age measurements is given.

The ages of the lunar maria and the filling of the mare basins

Abstract The age discrepancy between lunar highlands and mare is examined in terms of a hydrostatic head model incorporating an impact crater being fed continuously from depth with basaltic magma. It is shown that if the age difference is attributed to the filling time of the impact basin, the dimensions of the volcanic feeder conduits are of the order of a few metres.

The behaviour of ulvöspinel samples containing magnetized inclusions

Abstract Composite samples containing either magnetized iron or a ferrite magnet embedded in ulvospinel have been cooled through the ulvospinel Curie point. The observed changes in magnetic moment which occur are shown to be a result of TRM acquisition in the ulvospinel rather than an effect of high permeability. Nevertheless by defining an effective permeability of about (1 + TRM/ H ), magnetostatic theory relevant to magnetized inclusions in a permeable medium can be applied to explain the main features observed in this system.

Palaeomagnetism and planetology

Abstract The discovery of remanent magnetism in lunar samples has led to much research into its origin. There is now considerable evidence that the magnetism was acquired in a lunar magnetic field of internal origin, existing between 4000 and 3200 My ago. Investigations have been carried out into the intensity of the ancient lunar field and its variation with time in an attempt to provide evidence for the origin of the field in a dynamo process in a molten lunar core. In the absence of firm evidence for such a core from other sources, lunar palaeomagnetism may hold the key to this possible aspect of early lunar history. An early lunar core is of exceptional interest in our ideas about lunar history and structure, and may indeed have implications for other bodies in the Solar System.