Sourcing and identifying andesitic tephras using major oxide titanomagnetite and hornblende chemistry, Egmont volcano and Tongariro Volcanic Centre, New Zealand

Abstract

Canonical discriminant function analysis was employed to discriminate between electron microprobe-determined titanomagnetite and hornblende analyses from Egmont volcano and Tongariro Volcanic Centre. Data sets of 436 titanomagnetite and 206 hornblende analyses from the two sources were used for the study. Titanomagnetite chemistry provided the best discrimination between these two sources with classification efficiencies of 99% for sample averages and 95% for individual analyses. The difference between sources for hornblende chemistry was less marked, but classification efficiencies of 100% for sample averages and 87% for individual analyses were achieved. Using the same methods a preliminary discrimination of individual Egmont volcano-sourced tephras was attempted. Titanomagnetite chemistry enabled the discrimination of several individual tephras or at least pairs of tephra units, but hornblende chemistry provided little discrimination. This technique provides an improvement on previous methods for chemically distinguishing distal tephra from the two sources as well as potentially identifying individual tephras from a particular source. A major advantage over previous discrimination techniques is that individual analyses can be classified with a known probability of group membership (with groups such as volcano source or an individual tephra unit). Tephras in a depositional environment where mixing is common such as within soil, loess and marine sequences, can be sourced or identified more easily with classification of individual grains.