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[MTT28-P07] The regional characterization of volcanic glass shards for the forensic science by trace element analysis
4 samples (Samples A, B, C and D) collected in the Kanto Region were used in the present study. Sampling points are shown in Fig. 1. Samples A and B are stream sediments collected from the Arakawa River and they were chosen from 3024 stream sediments for the forensic soil sediment database. They are indistinguishable using the heavy element and heavy mineral compositions. Samples C and D are soils. Sample C contains a volcanic ash from Mt. Asama while Sample D contains ashes from Mt. Fuji and Mt. Hakone. After mesh controls, heavy minerals in these samples were removed by heavy liquid separation (SG ~2.4). Residues were embedded into resin and the surface was polished. 50 particles of volcanic glass were identified for each samples using a polarizing microscope and a micro Raman spectrometer for trace element analysis by LA-ICP-MS. Twelve elements were selected to analyze based on the previous study.
Here we examined the characterization using two elements, 89Y and 90Zr. It has been pointed out that the tephra deposits of the Kanto Region can be separated into two groups bordering the northern latitude of 36 degrees (see Fig.1): the ash from Mt. Asama for north side, the ashes from Mt. Fuji and Mt. Hakone for south side. Figure 2 shows a plot of intensities of 89Y and 90Zr (normalized to the intensity of 29Si) of volcanic glasses in Samples C and D. As shown in Fig. 2, there are different compositional tendency between the volcanic glasses in Samples C (Group N) and D (Group S). We confirmed the significant difference between these two groups using an analysis of variance. Because the crystallization differentiation of the magma in Mt. Asama was promoted compared to those in Mt. Fuji and Mt. Hakone, it is considered that the volcanic glasses in Sample C were rich in Y and Zr as incompatible elements. We built a linear discriminant to discriminate two groups of volcanic glasses using the normalized intensities of 89Y and 90Zr of volcanic glasses in Samples C and D and applied it for volcanic glasses in Samples A and B. As the result, 85.4% of the volcanic glasses in Sample A were classified into Group N while 75.9% of those in Sample B were Group S. Therefore, we could successfully distinguish 2 stream sediments having same heavy element and heavy mineral composition using the trace elemental difference of volcanic glass shards measured by LA-ICP-MS.