Detailed identification by trace-element composition of volcanic glass shards for tephrochronology: Example of Znp-Ohta tephra

Abstract

Tephrochronology is important to understand geotectonic events for the last few Myr. Tephra is usually characterized by petrographic characteristics and chemical compositions (mainly major elements) of volcanic glass shards. Trace element composition is useful in identifying tephras with similar characteristics. In this study, we aimed to enhance the database of trace elements compositions in widespread tephras, which is not reported enough. We used EPMA and LA-ICP-MS for major and trace element concentration of volcanic glass shards in Znp-Ohta tephra and Tng tephra, of which major elements similar to Znp-Ohta tephra. It was clear that Znp-Ohta tephra had zirconium concentration [Zr] = 44-58 ppm and Zr/Pb = 2.5-3.4, while Tng tephra had [Zr] = 70-81 ppm and Zr/Pb = 3.8-4.6. Znp-Ohta and Tng tephras were discriminated not only by characteristic type, Ba/La, and La/Y of glass shards (Tamura and Yamazaki, 2004) but also by Zr-Pb correlation. In the future, we compare the trace element concentrations of Znp-Ohta tephra at multiple sites for evaluation of secondary effects during or after the deposition.