A numerical approach toward the classification of magnetites from tephra in southern Alberta

Abstract

High-titanium glass-encased magnetites associated with intermixed tephras in upper subalpine soils in Banff National Park are shown to be characteristic of Mazama tephra and of regional significance. Similar magnetites occur in a Mazama layer above a paleosol at Fish Creek, Alberta and within a sample of Mazama ash (UA 396) from Windy Point, Alberta. Detailed sampling and analysis of the Fish Creek tephra indicate that the magnetites do not represent a separate eruptive event or an older ashfall. Cluster analysis of glass-encased magnetites from reference samples of Mazama, St. Helens Y, and Bridge River ashes revealed composite grouping of Mazama and Bridge River magnetites and separate and strong groupings of high-TiO2 magnetites. Discriminant analysis identifies MgO, TiO2, and Al2O3 as the most significant chemical discriminants between the various ashes. Discriminant function analysis identifies the high-titanium magnetites as Mazama in type and successfully classifies 96% of the tephra-derived magnetites in two soils developed from intermixed ashes. Numerical analysis confirms the utility of compositional variations in tephra-derived magnetites in providing a time-stratigraphic framework in the analysis of volcanic ash soils in western Canada.