Abstract
Distinguishing the ancient eruption products of Daisen Volcano can be difficult: the Daisen tephras are not reliably differentiated on the basis of optical or chemical tests of hornblende or orthopyroxene. Major-element analyses of volcanic glass are affected by weathering. Here I explore a new approach, based on major-element analysis of pristine glass inclusions preserved in plagioclase phenocrysts.
Samples of three tephras (DKP, DSP, and DNP) at proximal locations near Daisen Volcano and samples of the DKP tephra from two distal locations were analyzed by energy-dispersive X-ray (EDX) techniques. Of the glass inclusions in the three Daisen tephras, only DKP is relatively homogeneous throughout each ash unit in the major-element composition (SiO2, Al2O3, FeO, CaO, and K2O) whereas both DNP and DSP differed significantly between their lower to middle parts and their upper parts. Although the DSP and DKP tephras have a slight degree of overlap, the two could be distinguished on the basis of their glass inclusions. The distal DKP tephras at Takano and Oda are shown to be correlated with the proximal DKP tephra at Daisen and distinct from the DSP tephra at Daisen.
Major-element analysis of plagioclase-hosted glass inclusions is a promising method for investigations of weathered tephras.
