Although it is widely recognised that remanent magnetism in ash-flow tuffs is carried by fine-grained Fe oxides, the origin, mineralogy, and significance of such magnetic carriers are not well understood. We have obtained transmission electron microscope images of distinctive Fe-oxide microcrystals in rhyolitic samples located 3.8, 7.6, and 18.5 m above the base of a 110-m-thick section of the Miocene Tiva Canyon Member of the Paintbrush Tuff. The Fe-oxide microcrystals are lath shaped and increase in size from ∼20 x 140 nm in the lowermost sample (near base of the member) to ∼120 x 800 nm in the uppermost sample (within the flow interior). Microcrystals in this size range are within or close to the range of single-domain grain size for magnetite. Electron diffraction and analytical X-ray data indicate that the microcrystals in the lower two samples are cubic Fe-oxides (magnetite/maghemite), with less than 10 mol% Ti end member, and that those in the uppermost sample are manganiferous hematite. Systematic variations in magnetic properties are consistent with the observed variations in size and mineralogy of the microcrystals. These micro-crystals are morphologically distinct from grains that we interpret to be fragments of phenocrysts. The morphology and spatial distribution of the microcrystals as well as their increase in grain size, from the rapidly cooled base of the ash-flow sheet into the flow interior, are consistent with an origin by nucleation and growth from volcanic glass at elevated temperature, subsequent to emplacement.

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