Elemental Composition of the 1783 Eruption Products from Asama Volcano

Abstract

Chemical analysis for major and minor elements was performed on the pumice fall deposit, essential blocks from the two pyroclastic flow deposits and the lava flow, erupted in succession during the 1783 (Temmei) volcanic activity on Mt. Asama. The pumice samples representing a vertical column exhibit no remarkable trend of chemical variation with time. The analytical results for the lava flow show small spatial variation in K, Mg, Sr and some other elements. Further inspection of the data for all the samples indicates that Fe, Na, Ti, Sr, Cu, Co and Ni tend to increase with time throughout the whole eruptive sequence. A plot of Sr/(SiO₂ + K₂O) vs. (Fe₂O^*₃+K₂O)/(SiO₂+K₂O) illustrates that the erupting magma became progressively more mafic and more enriched in Sr during the activity. This type of plot, combined with the spatial distribution of certain elements superimposed on the distribution pattern of the lava flow, reveals that, during the lava eruption, the composition of erupting magma still shifted to slightly mafic.