Long Term Variations in Geomagnetic Intensity From Layered Intrusions

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Gee 0087551 Records of geomagnetic intensity that span time scales appropriate to planetary evolution -- hundreds of millions to billions of years - are important sources of information about the early development of the Earth and of its geodynamo. The paleomagnetism of layered intrusive rocks provides this type of record. Preliminary data suggest that many rocks from layered intrusions retain primary thermoremanence carried by single-domain magnetite -- an ideal situation for paleointensity studies. The slow cooling of large intrusive complexes is likely to average short-term variations in the direction and intensity of the geomagnetic field. Providing that care is taken to account for the effects of slow cooling and remanence anisotropy, layered intrusive rocks will likely yield high-quality determinations of paleointensity. The investigators propose three related sets of experiments on samples from layered intrusions: (1) a more complete study of paleointensity variations using their collection of oriented specimens from the Archean Stillwater Complex (Montana, USA) and drill cores from the Stillwater Mining Company; (2) paleointensity experiments on samples from approximately 15 other layered intrusions; and (3) tests to determine the spatial and temporal scales over which large, slowly-cooled plutons average geomagnetic field variations.