SOURCING AND IDENTIFYING ANDESITIC TEPHRAS USING MAJOR OXIDE TITANOMAGNETITE AND HORNBLENDE CHEMISTRY, EGMONT VOLCANO AND TONGARIRO VOLCANIC CENTER, NEW-ZEALAND

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 source s were used for the study. Titanomagnetite chemistry provided the best discrimination between these two sources with classification efficien cies of 99% for sample averages and 95% for individual analyses. The d ifference between sources for hornblende chemistry was less marked, bu t classification efficiencies of 100% for sample averages and 87% for individual analyses were achieved. Using the same methods a preliminar y discrimination of individual Egmont volcano-sourced tephras was atte mpted. 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 i mprovement on previous methods for chemically distinguishing distal te phra from the two sources as well as potentially identifying individua l tephras from a particular source. A major advantage over previous di scrimination techniques is that individual analyses can be classified with a known probability of group membership (with groups such as volc ano source or an individual tephra unit). Tephras in a depositional en vironment where mixing is common such as within soil, loess and marine sequences, can be sourced or identified more easily with classificati on of individual grains.