The magnetic mineralogy of fine-grained FeTi oxides, 1 um in size and smaller, in glassy and devitrified rhyolitic rocks and the geologic origin of these microcrystals are problems whose solution substantively influences any interpretation of paleomagnetic data of these rocks. These data, from ashflow tuffs, are routinely applied to structural-tectonic, strati- graphic, and paleopole studies. The PI will conduct a detailed magnetic, transmission electron microscopy (TEM) and Mossbauer spectroscopic study of ash-flow tuff units from Yucca Mountain, Nevada, in conjunction with a study of compositionally similar rhyolitic obsidian and phroclastic flows from N. California and Oregon, to address this problem. TEM and Mossbauer methods will distinguish amongst hematite, magnetite and maghemite. A tentative model for the geologic origin of fine-grained NRM carriers, which attributes primary remanence to Fe-Ti oxides that precipitated at elevated temperature in quenched glass, and phenocryst phases will be tested.
