Chemical characterization of volcanic glass shards by energy dispersive X-ray spectrometry with EDAX Genesis APEX2 and JEOL JSM-6390

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A chemical characterization of individual volcanic glass shard by electron-beam (E-beam) methods including SEM-EDS is effective for tephra fingerprinting. We installed a new SEM-EDS system composed of EDAX Genesis APEX2 and JEOL JSM-6390. This paper discusses reproducibility, accuracy, and precision of this system based on the analytical data sets of three representative Japanese widespread tephras and four provided samples by INTAV intercomparison. 710 analyses on homogeneous natural glass shards extracted from a representative Japanese widespread tephra (Aira-Tn: AT) under the same analytical conditions demonstrate reproducibility of measurements and absence of machine drift. For testing inter-laboratory reproducibility, three representative Japanese widespread tephras (AT, K-Ah, and Toya) and four natural glasses (rhyolitic Lipari obsidian, phonolitic Sheep Track tephra, basaltic Laki 1783 A.D. tephra, rhyolitic Old Crow tephra) were selected. Results of comparison prove that the new system mostly enabled us to determine the major oxide concentrations comparable with other laboratories. However, in case of lower oxide concentrations less than ca. 0.3 wt% such as TiO2, MnO, and MgO, similarity and difference in concentrations are not significant because these values are less than the detection limit of SEM-EDS analysis. Also, results of comparison suggest that attention should be paid in determination of glass chemistry with higher content of CaO and Na2O concentrations for basaltic and phonolitic tephras such as Laki 1783 A.D. and Sheep Track tephras.

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