CHEMICAL COMPOSITION OF FERROMAGNETIC MINERALS IN VOLCANIC ASH LAYERS WITH SPECIAL REFERENCE TO THEIR ORIGINS

Abstract

In order to establish the laboratory method to identify volcanic ash layers or to determine their origins, the chemical composition (Fe, Ti, Mn, V, Zn, etc) of ferromagnetic minerals was studied. The ash layers used for this investigation were erupted from Mt. Meakan, Mt. Kamui-nupuri, Mt. Tarumae, Mt. Eniwa, Mt. Zao, Mt. Asama, Mt. Futatsudake and Mt. Daisen. The results obtained are as follows; 1) Compared with Fe and Ti, Mn, V and Zn in ferromagnetic minerals were found to be highly different in the concentration depending on their origins. 2) Determining the concentrations of various elements in the main minerals separated from volcanic ashes, it was shown that the concentrations of Fe, Ti, V and Zn were the greateat in ferromagnetic minerals, but that of Mn was the greatest in pyroxenes. 3) In order to lessen the influence of impurities on the relationship between the chemical composition of ferromagnetic minerals and their origins, the weight ratios of V/Fe, Zn/Ti, Mn/Ti, Zn/V, Zn/Fe, etc were examined. It was indicated that the combination of V/Fe-Zn/Ti among the various combinations of the ratios was closely related with the origins and was least influenced by the impurities. On the other hand, the ratios including Mn were considerably changed by the contamination of pyroxenes. 4) Using Tarumae-a ash layer, recent one erupted from Mt. Tarumae, the chemical composition of ferromagnetic minerals was investigated in relation to the depth of the layer and the distance from the origin. It was found that there was no difference throughout the layer except the bottom and at any distance from the origin. From the results mentioned above, it can be concluded that the analysis of the chemical composition of ferromagnetic minerals is highly useful for the identification of volcanic ash layers and for the determination of their origins if it is used together with the stratigraphic results.