Analytical method for chemical compositions of volcanic glasses in Tono Geoscience Center; Quantitative measurements of Major Elements by using EPMA and Minor Elements by LA-ICP-MS

Kagami, Saya; Yokoyama, Tatsunori; Umeda, Koji*

JAEA-Testing 2021-001, 49 Pages, 2021/08

JAEA-Testing-2021-001.pdf:3.86MB

To make a contribution to safety assessment for geological disposal of high level radioactive and/or TRU waste, we need to assess long-term stability of geological environment and predict long-term changes of geotectonic events that will occur in the future, especially for Quaternary period ( 2.6 million years ago-present). In the most case, we investigate chronological data of geological events by radiometric dating. When some geological samples have no objects to which radiometric dating method can be applied (e.g., zircon, biotite, wood fragments and plant residues), we can use tephrochronology, which is geological dating method using each layer of tephra (erupted volcanic ash), for dating of geological layers. This chronological method is essential in Japan, where volcanism is very active. Tephra is usually characterized by petrographic characteristics and/or chemical composition (mainly major elements) of volcanic glasses and/or minerals in tephra. In Tono Geoscience Center (Japan Atomic Energy Agency), we develop an analytical technique of chemical composition including trace elements of volcanic glasses for detailed tephra identification. In this paper, we report a sample preparation procedure and analytical methods of chemical compositions of individual volcanic glass shards by using an electron probe microanalyzer and a laser ablation-inductively coupled plasma-mass spectrometer.

Zircon U-Pb dating and fission track age of a Pliocene Ohta Tephra in the Tokai Group

Ueki, Tadamasa; Niwa, Masakazu; Yonaga, Yusuke; Iwano, Hideki*; Danhara, Toru*

no journal, , 

no abstracts in English

An Analytical technique of trace elements using LA-ICP-MS for tephra identification

Kagami, Saya; Yokoyama, Tatsunori; Umeda, Koji*

no journal, , 

Tephrochronology is one of the geochronological techniques for Quaternary and generally used as well as radiocarbon dating. A catalogue of wide-spread tephras provides fundamental criteria for the establishment of a regional chronostratigraphy. Tephras have been identified by petrographic characteristics, types of volcanic glass shards in tephra, and major elements compositions of them. Major element analyses of volcanic glass shards are usually carried out by EPMA. Recently, it has been reported that the some tephras which were quite similar to others in major element compositions were distinguishable by trace element compositions. Although trace element data of volcanic glass shards are important for tephra identification, the data are still lack in a catalogue of wide-spread tephras. To measure trace element abundances of the glass shards, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is a powerful method because individual glass shards can be analyzed. In this study, we performed trace element analysis of tephras using LA-ICP-MS at Tono Geoscience Center, JAEA. We analyzed the concentration of 41 elements with the internal standard element of Si and two calibration standards of NIST SRM 612 and 614. Three tephra samples were prepared: Aso-4, Aira-Tn (AT), and Akogi (identified to Znp-Ohta) tephras. While a fused glass bead of Aso-4 was prepared, the others were mounted on epoxy resin. In Aso-4 and AT tephra samples, the concentrations of trace elements (Rb, Sr, Y, Zr, Ba, La, and Ce), by which tephras are especially identified, were consistent with those in the previous studies (RSD:10%). We obtained the pattern of trace element compositions of Akogi tephras, which was similar to that of Znp-Ohta tephras. This analytical method will contribute to establish the detailed catalogue of tephras.

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

Kagami, Saya; Yokoyama, Tatsunori; Umeda, Koji*; Yasue, Kenichi*; Niwa, Masakazu; Furusawa, Akira*; Tamura, Itoko*

no journal, , 

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. When tephras similar in the characteristics are discriminated, trace element compositions are valid for identification of tephra. We focus on the enrichment of 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. In this study, 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 in multiple sites for evaluation of secondary effect during the deposition.