MAGNETIC PROPERTIES OF ULTRAMAFIC ROCKS FROM RED-MOUNTAIN, CALIFORNIA

A representative ultramafic intrusion at Red Mountain southeast of San Francisco, California was sampled for the purpose of studying the remanent magnetism and susceptibility of ultramafic rocks as functions of serpentinization. Due to the dependence of grain density upon the degree of serpentinization, the former was taken as a measure of the latter. Grain densities were accurately determined from bulk densities by taking into account the porosity and amount of adsorbed water contained in these rocks. In general, the porosity was found to increase with serpentinization. The magnetic data suggest that the remanent magnetization is mainly a chemical remanent magnetization acquired during the process of serpentinization whereby iron atoms, released from the silicate structure of the paramagnetic olivine and pyroxene, are oxidized to form ferrimagnetic magnetite. The intensity of both remanent and induced magnetization increases exponentially with serpentinization. However, the remanent magnetization becomes highly unstable as the rock becomes intensely serpentinized. This is due to the growth of magnetite grains to multi-domain size and their oxidation, in some cases,to maghemite. The intensity of magnetization also depends on mode of occurrence and state of oxidation of the magnetite, and on original rock composition. Serpentinized dunites were found tobe less magnetic than equally serpentinized peridotites, because the olivine in the dunites has a lower iron content than the olivine and pyroxene in the peridotites. Therefore, it is expected that some serpentinized ultramafic bodies will have highly variable magnetic anomalies associated with them, while others of dunitic composition will have low magnetic anomalies. © 1969 Society of Exploration Geophysicists.