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

Abstract

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 3− x 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 2− y 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.