An in situ magnetic susceptibility survey of Unit 10 of the Eastern Layered Intrusion of the Isle of Rum, in a line perpendicular to the strike, was carried out as a guide to selecting sampling sites for subsequent laboratory magnetic studies. These laboratory studies indicate that the dominant magnetic phase is magnetite. An effective particle size of the magnetite was derived from the magnetic data: it was found that high concentrations (∼0·3%) of magnetite in the perioditites were achieved by the presence of fewer but larger particles. The regions of lower magnetite concentration (∼0·01%), which are mainly in the plagioclase-rich rocks, contain more abundant but smaller particles.
The variations in magnetic properties correlate with the abundance and inferred temperatures of formation of hydrothermal alteration minerals in the rocks. Petrographic observations indicate temperatures of alteration of 500–800°C in the olivine-rich peridodites in the lower part of the Unit, but of the order of 300°C in the plagioclase-rich rocks at the top of the Unit.
These relationships between magnetic mineralogy and hydrothermal alteration suggest that the magnetite was produced by olivine oxidation during hydrothermal alteration. It is proposed that variations in the magnetic properties of layered cumulate rocks may be used to map out variations in the temperature and intensity of hydrothermal fluid flow. The variations in the Unit 10 rocks studied are interpreted as indicating control of high-temperature hydrothermal fluid flow through them by contrast in permeability between brittle peridotites and quasiplastic plagioclase-rich rocks.