VARIATIONS IN MAGNETIC-PROPERTIES OF UNIT 10, EASTERN LAYERED INTRUSION, ISLE-OF-RUM, SCOTLAND - IMPLICATIONS FOR PATTERNS OF HIGH-TEMPERATURE HYDROTHERMAL ALTERATION

An in situ magnetic susceptibility survey of Unit 10 of the Eastern La yered Intrusion of the Isle of Rum, in a line perpendicular to the str ike, was carried out as a guide to selecting sampling sites for subseq uent laboratory magnetic studies. These laboratory studies indicate th at the dominant magnetic phase is magnetite. An effective particle siz e of the magnetite was derived from the magnetic data: it was found th at high concentrations (similar to 0.3%) of magnetite in the perioditi tes were achieved by the presence of fewer but larger particles. The r egions of lower magnetite concentration (similar to 0.01%), which are mainly in the plagioclase-rich rocks, contain more abundant but smalle r particles. The variations in magnetic properties correlate with the abundance and inferred temperatures of formation of hydrothermal alter ation minerals in the rocks. Petrographic observations indicate temper atures of alteration of 500-800 degrees C in the olivine-rich peridodi tes in the lower part of the Unit, but of the order of 300 degrees C i n the plagioclase-rich rocks at the top of the Unit. These relationshi ps between magnetic mineralogy and hydrothermal alteration suggest tha t 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 tem perature and intensity of hydrothermal fluid flow. The variations in t he Unit 10 rocks studied are interpreted as indicating control of high -temperature hydrothermal fluid flow through them by contrast in perme ability between brittle peridotites and quasiplastic plagioclase-rich rocks.