The nature of magnetic single domains in volcanic glasses of the KBS Tuff

Abstract

Magnetic and optical variations in volcanic glasses of the East African Rift have been investigated by means of experimental and analytical study. The results add to the formal understanding of magnetism in volcanic glasses and give us new perspectives on the nature, origin, and significance of fine‐grained magnetic minerals and on the magnetic petrology of glassy rocks. The samples that we have studied in detail, glass shards from the KBS Tuff of northern Kenya, contain a superparamagnetic precipitate of microcrystals, ∼2 to ∼10 nm in size. The precipitate is likely to have formed at elevated temperature, during or subsequent to eruption, about 1.9 m.y. ago. We have studied these microcrystals by means of transmission electron microscopy (TEM), ac and dc magnetic methods, electron spin resonance (ESR), Mössbauer spectroscopy, and optical spectroscopy. Our efforts have concentrated on determining the magnetic mineralogy and understanding the mechanism(s) responsible for the optical and magnetic behavior of these glasses. The particle size determination for the larger of these microcrystals has been achieved using TEM imaging and Mössbauer spectroscopy. The mineralogy is nonstoichiometric magnetite, based on Mössbauer, X ray diffraction, and electron diffraction results. The ratio of saturation rémanent magnetization to saturation magnetization for the larger microcrystals is very nearly that predicted by single‐domain theory for a uniaxial anisotropy, the origin of which may lie with stress or surface effects. We interpret magnetic and ESR results to infer the possible existence of microcrystals or amorphous clusters of Fe atoms, ∼1 nm or smaller, in KBS glasses that appear homogeneous in high‐resolution TEM images. Results from optical absorption experiments show that the color variations in KBS shards arise due to optical absorption by the precipitate, a result consistent with classical electromagnetic theory.