Detailed Comparison of Petrographic Characterization of DNP, DSP, and DKP Tephras in the Lower-Middle Members of Daisen Tephra Formations, Southwestern Japan.

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  • Furusawa Akira
    FURUSAWA Geological Survey
  • Umeda Koji
    Japan Nuclear Cycle Development Institute Tono Geoscience Center Present address: Research and Development Bureau, Ministry of Education, Culture, Sports, Science and Technology

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  • 新期大山テフラＤＮＰ，ＤＳＰ，ＤＫＰの岩石記載的特徴の再検討
  • シン キ ダイセン テフラ DNP DSP DKP ノ ガンセキ キサイテキ トクチョウ ノ サイケントウ

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We examined the petrographic characteristics of pumice clasts from DNP, DSP, and DKP tephras that erupted from Daisen Volcano located in southwest Japan. Samples were collected from each of the tephras at plural horizons separated by 10cm in thickness. We examined the variations in the refractive index of volcanic glass, orthopyroxene, and hornblende phenocrysts in order to provide clear petrographic parameters for identification of DNP and DKP widespread tephras.<br>The range and mode of refractive indices of volcanic glass, orthopyroxene, and hornblende contained in DNP, DSP, DKP tephras vary in the samples, suggesting compositional heterogeneity even in a single plinian pumice fall layer. Refractive index variation of volcanic glass may be caused by differences in the hydration sheets due to weathering. Refractive indices of hornblende and orthopyroxene partly overlap between DKP and DNP, which makes it difficult to do tephra correlation. However, the range and mode of refractive indices differ from each other and are consistent with values previously reported. Based on the detailed examinations described above, we recommend the use of range and mode values of refractive indices of hornblende and orthopyroxene for correlation and identification of DKP and DNP. Because the two tephras are stratigraphieally close to each other, misidentification may occur, particularly in sediments in which these tephras are mixed with other detrital fragments.

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