Magnetic anisotropy of igneous rocks

Abstract

A torque-meter method of measuring the magnetic anisotropy of rocks has been used in a study of a 1050-foot bore core from a Tasmanian dolerite sill. Each cylindrical specimen was suspended in a uniform field of 10.6 kilo-oersteds perpendicular to its axis, and the torque T exerted on it was measured at 10° intervals of the angle θ between a mark on the specimen and the direction of the field. Fourier analyses of the torque curves T(θ) yielded terms in sin 2θ and sin 4θ, of which the first was due principally to shape alignment of the magnetic grains and the second to alignment of cubic crystallographic axes. The anisotropy of the bore core was found to be due to a slight but regular alignment of grain elongations with no regular alignment of crystallographic axes. Superimposed upon the pattern of anisotropy, statistical deviations from isotropy were observed in individual rock samples owing to the finite numbers of magnetic grains which they contained. The statistical effects were used to confirm the interpretation of the Fourier components of the torque curves and to estimate the mean elongation of the magnetic grains. If the criterion of acceptability of a rock for palaeomagnetic work is taken to be a maximum possible anisotropic deflection of remanent magnetism of 3°, then it is required that its anisotropy be less than 10 per cent. Equations are given, expressing this criterion in terms of susceptibility anisotropy as well as torque-meter measurements. The anisotropy of the bore core, as well as of a few specimens of basalt and porphyry, is much too small to have influenced the palaeomagnetic work which has been carried out on these rocks.