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

Abstract : [1] 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/m 3 , LD) and high-density (!2850 kg/m 3 , HD) fraction. Three phases of the magnetite-ulvöspinel solid solution series, Fe 3Àx Ti x O 4 , x = 0, 0.1, and 0.3 are identified along with one phase of the ilmenite-hematite solid solution series, Fe 2Ày Ti y O 3 , 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. INDEX TERMS: 1540 Geomagnetism and Paleomagnetism: Rock and mineral magnetism; 1512 Geomagnetism and Paleomagnetism: Environmental magnetism; 1519 Geomagnetism and Paleomagnetism: Magnetic mineralogy and petrology; 8404 Volcanology: Ash deposits; 5109 Physical Properties of Rocks: Magnetic and electrical properties; KEYWORDS: low-temperature magnetism, frequency and amplitude dependence of AC susceptibility, ilmenite-hematite and magnetite-ulvospinel solid solution series, tephra, stratigraphic correlation
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France Lagroix, Subir K. Banerjee, Mike. Jackson. Magnetic properties of the Old Crow tephra: Identification of a complex iron titanium oxide mineralogy. Journal of Geophysical Research : Solid Earth, American Geophysical Union, 2004, ⟨10.1029/2003JB002678⟩. ⟨insu-01311139⟩

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