Magnetic properties of the Old Crow tephra: Identification of a complex iron titanium oxide mineralogy

France Lagroix, Subir K. Banerjee, Michael J Jackson

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The mineralogy and grain-size distribution of the Fe-Ti oxide population of the Old Crow tephra bed, outcropping at the Halfway House loess deposit in central Alaska, are characterized through multiple low- and high-temperature magnetization experiments. The characterization is facilitated by heavy liquid separation of the bulk sample into a low-density (<2850 kg/m3, LD) and high-density (≥2850 kg/m3, HD) fraction. Three phases of the magnetite-ulvöspinel solid solution series, Fe3-x TixO4, X = 0, 0.1, and 0.3 are identified along with one phase of the ilmenite-hematite solid solution series, Fe2-y TiyO3, y = 0.83. All four phases are present in both density separates, where coarser grains dominate the HD sample and finer more oxidized grains dominate in the LD sample. Low-temperature frequency dependence and field dependence of both the in-phase and quadrature components of magnetic susceptibility are found particularly useful in identifying the magnetic ordering temperature of titanohematite phases with y > 0.8 and may play an equally important role as magnetic indicator of titanomagnetite. Furthermore, we demonstrate the ability of low-temperature magnetism to locate a 1 mm thick tephra bed dispersed in loess over 10 cm depth, through the identification of very low concentrations of a titanohematite phase with y = 0.9. The potential for advancing regional correlation of sedimentary deposits through the identification of Fe-Ti oxides common to tephra beds by low-temperature magnetism is illustrated in this study.

Original languageEnglish (US)
JournalJournal of Geophysical Research: Solid Earth
Issue number1
StatePublished - Jan 10 2004


  • Frequency and amplitude dependence of AC susceptibility
  • Ilmenite-hematite and magnetite-ulvospinel solid solution series
  • Low-temperature magnetism
  • Stratigraphic correlation
  • Tephra

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