富士火山におけるマグマ供給系の進化 : 全岩化学組成の視点から(<特集>火山のモデル(II))

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Evolution of whole-rock chemistry of volcanic rocks in Fuji volcano is presented. Basaltic andesite is predominant during 66,000 to 80,000 y. B. P. ; they are poor in FeO^*, MgO, MnO, K_2O, TiO_2, P_2O_5, and abundant in Al_2O_3 and Na_2O, with slightly high K_2O/TiO_2, Rb/Y, and Zr/Y ratios. Basalts erupted during 22,000 to 66,000 y. B. P. show low FeO^*/MgO ratio (2.0 >), and are lacking in those with high K_2O, TiO_2, P_2O_5, Rb, Ba, and Zr, and low Al_2O_3 ; they are characterized by low K_2O/TiO_2, Rb/Y, and Zr/Y ratios. Eruption products during 10,000 to 22,000 y. B. P., especially in its later stage, are basalt with relatively high FeO^*/MgO, Rb/Y, and Zr/Y ratios, and K_2O, TiO_2, P_2O_5, Rb, Ba, and Zr contents, and low Al_2O_3 ; their chemistry resembles to those after 10,000 y. B. P.. Ejecta during 8,000 to 10,000 y. B. P. are basalt and high in FeO^*/MgO, Rb/Y, and Zr/Y ratios, and K_2O, TiO_2, P_2O_5, Rb, Ba, and Zr contents, and Rb/Y and Zr/Y ratios, and low in Al_2O_3. Volcanic rocks during 3,000 to 8,000 y. B. P. are basalt showing low K_20/TiO_2, Rb/Y, Ba/Y, and Zr/Y ratios, and K_2O, TiO_2, P_2O_5, Rb, Ba, Y, and Zr contents, and high FeO^*/MgO and Al_2O_3 ; they are rather similar to those erupted during 22,000 to 66,000 y. P. B. in chemistry. Erupted materials during 2,000 to 3,000 y. B. P. are basalt with low K_2O, TiO_2, P_2O_5, Rb, Ba, and Zr contents, and Rb/Y and Zr/Y ratios, and high Al_2O_3 and FeO^*/MgO ; their chemistry are akin to those during 3,000 to 8,000 y. B. P., but slightly enriched in incompatible elements. Eruption products after 2,000 y. B. P. are abundant in K_2O, TiO_2, P_2O_5, Rb, Ba, and Zr, and poor in Al_2O_3, with high FeO^*/MgO, Rb/Y, and Zr/Y ratios ; they are almost similar to basalts erupted during 8,000 to 10,000 y. B. P.. The Fuji volcano is classified into three magma-series on the view point of whole-rock Rb/Y and Zr/Y ratios : Early Ko-Fuji (66,000 to 80,000 y. B. P.), Late Ko-Fuji (10,000 to 66,000 y. B. P.), and Shin-Fuji (present to 10,000 y. B. P.). Basaltic magmas of each series may have probably been derived successively from chemically heterogeneous upper mantle. The high rate of change of source rock chemistry may reflect dynamic process in the upper mantle beneath the arc volcano.
特定非営利活動法人日本火山学会の論文
1991-07-15

富士火山におけるマグマ供給系の進化 : 全岩化学組成の視点から(<特集>火山のモデル(II))

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