Magnetites from a new unidentified tephra source, Banff National Park, Alberta

Abstract

Electron microprobe examination of glass-encased magnetites present within surficial volcanic ash deposits located in Banff and Jasper National Parks revealed five distinct magnetite populations. Three of the magnetite populations represented the Mazama, St. Helens Y, and Bridge River volcanic units previously identificd in this area of the Canadian Kocky Mountains. The reinaining two magnetite groups are characterized by glass-encased magnetites which have titanium oxide concentrations of 11.59 and lQ.33%, values considerably higher than thosc characteristic of either Mazama, St. Helens Y, or Bridge River volcanic units. The high-titanium magnetites are of unknown provenance, and although the section provided no means for dating these volcanic groups, their distribution within the section suggests that they are older than Bridge River, and one group may predate Mazama.I.'examen B la microsonde electronique des grains de magnitite enrobks de ven-e qu'on retrouve dans les depbts de cendres volcaniques superficiels dans les parcs nationaux de Banff et de Jasper a revkle cinq populations distinctes de magnetite. Trois dcs populations de magnetite representent les unites volcaniques de Mazama, de St. Helens Y et de Bridge Kiver dkj2 reconnues dans cette partie des montagnes Kocheuses canadiennes. Les deux autres groupes de magnetite sont caracterises par des magnetites enrobees de verre qui ont des concentrations en oxyde de titane de 11.59 et de 10.33%, valeurs qui sont considerablement plus ilevees que celles des unites de Mazama, de St. Helens Yet de Bridge River. On ne connait pas la provenance de ces magnetites riches en titane et bien que la coupe ne permette pas de dater ces groupes volcaniques, leur distribution ii I'interieur de la coupe suggere yu'ils sont plus anciens que I'unite de Bridge ----

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

“…The use of magnetites to differentiate between tephras is not, however, without its problems. In a study of soil development within two upper subalpine soils within Banff and Jasper National Parks Brewster and Barnett (1979) reported the presence of glass-encased magnetites whose composition appeared to be at variance with published data on known volcanic ashes in the area (Westgate et al 1970). Brewster and Barnett reported two groups of glass-encased magnetites that had significantly higher concentrations of Ti02 than any tephraderived magnetites previously reported in either the American or Canadian Cordillera.…”

“…The analyses were calibrated against the following standard materials: Mg, Al: chromite (USNM 117075; Jarosewich et al 1979); Ti: kaersutite (Fleet and Barnett 1978); Mn: rhodonite (J. C. Rucklidge, Astimex Scientific, Toronto); and Fe: magnetite (UWO MT 633). The operating conditions and procedures used were identical to those used by Brewster and Barnett (1979). In this and subsequent samples one individual spot analysis per grain, on 19-36 grains, was performed.…”

“…Because of this recent attention has focussed on the use of the electron microprobe technique to determine the chemical composition of magnetites present within the various tephras to differentiate between these deposits and identify a specific source (Westgate et al 1970;Lerbekmo et al 1975;Westgate and Fulton 1975;Duford 1976;Duford and Osborn 1978;Westgate and Evans 1978;Brewster and Barnett 1979;Westgate and Gorton 198 1;Smith and Leeman 1982).…”

Mineralogy and Phase Chemistry of Mount St. Helens Tephra Sets W and Y as Keys to Their Identification

“…Westgate and his colleagues (Westgate and Fulton, 1975;Westgate et al, 1977;Westgate and Evans, 1978;Brewster and Barnett, 1979;Mathewes and Westgate, 1980) have recognized the effectiveness of iron-titanium oxide composition as a tool in distinguishing tephras in the plains of southwestern Canada. Their investigations include units which they believe to be the distal portions of tephras from MSH on the basis of several criteria.…”

“…Some layers (e.g., We, Wn) have thicknesses of 20 cm or more at distances of tens of kilometers from the volcano (Crandell and Mullineaux, 1978) and several layers have been traced as far as 900 km from the source (e.g., Westgate and Fulton, 1975;Smith et al, 1977;Westgate, 1977;Brewster and Barnett, 1979).…”