The magnetic balance and its application to studying the magnetic mineralogy of igneous rocks.

Abstract

A magnetic balance operating between -196°C and 800°C with magnetic fields up to 1.2MAm-1 is described. The typical noise level of the instrument corresponds to a sample dipole moment of 1.6×10-7 Am2 (the moment of 1.7μg of magnetite magnetized to saturation at room temperature or about 200μg of typical basaltic rock). Small samples may be used which leads to consistency between the calibration figure for both ferrimagnetic and paramagnetic minerals coexisting in the same sample and also allows rapid temperature change. An important feature is a programmable temperature-time regime. Above room temperature heating rates between 2°/min and 100°/min can be selected, and the influence that heating rate can have on the form of the thermomagnetic curve is demonstrated.The magnetic balance operating in high field is the principal instrument for Curie point temperature determination. However, the balance has much wider application than this one task. The balance is a powerful tool in magnetochemistry or in monitoring temperature-induced structural transformations. We present here results which, for the first time, show the temperature dependence of the inversion process by which the cation-deficient magnetic minerals in submarine basalts transform to other magnetic phases. It may be that inversion proceeds at a slow rate under submarine conditions. Such inversion, which is accompanied by a change in saturation magnetization and grain size, will inevitably affect the intensity of remanent magnetization and magnetic susceptibility of the submarine crust.