Petrological properties of tephra beds based on the major and trace element composition of volcanic glass shards : a case study of the Osaka Group and the Takashima-oki drilling core sediments in Lake Biwa, Kinki district, Japan

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  • 火山ガラスの主成分・微量成分元素組成に基づくテフラ層の岩石学的特徴 : 大阪層群と琵琶湖高島沖コアに挟まるテフラ層の例
  • カザン ガラス ノ シュセイブン ・ ビリョウ セイブン ゲンソ ソセイ ニ モトズク テフラソウ ノ ガンセキガクテキ トクチョウ : オオサカソウグン ト ビワコ タカシマオキ コア ニ ハサマル テフラソウ ノ レイ

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Abstract

Tephra beds intercalated in the Plio-Pleistocene sediments of the Osaka Group and the Takashima-oki core sediments in Lake Biwa were classified into eighteen types based on the volcanic glass morphology, mafic mineral composition and grain component. These tephra beds were further classified into five groups according to the tephra types in combination with the major element composition of the volcanic glass shards, which was measured by a Scanning Electron Microscopy + Energy Dispersive X-ray Spectrometry (SEM-EDS). We also examined these tephras with trace element compositions of the volcanic glass shards using as Y-Sr/Y, La/Y-Fe/(Fe+Mg) mol% and MgO-Ba diagrams, for which the trace elements measured by an Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). Consequently, we classified the tephras into five groups; 1) Alkaline tephra group: characterized by high sodium and potassium contents which were derived from Ulreung-do Island; 2) High-K tephra group: characterized by relatively high MgO and Ba contents and are derived from the Aso caldera; 3) Medium-K tephra group: being composed of TACr (pumice type glass shard + amphibole dominant in heavy minerals+ vitric-crystal ash type) tephra type and Adakitic dacite tephra subtype; 4) Low-K tephra group: characterized by low La/Y ratio and low Ba content and were possibly derived from the Izu-Hakone regions; 5) High-silica and platy glass tephra group: characterized by high SiO_2 content with platy volcanic glass morphology. These petrological characteristics of the tephra types are useful for the first order identification of the sources.

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