Identification of Tephra Layers by Inductively Coupled Plasma (ICP) Emission Spectrometry, and their Applications

Inductively coupled plasma emission spectrometry (ICP) is applied for the chemical analyses of volcanic glasses to improve the efficiency of identification of widespread tephra.The differences of specific gravity and magnetic property among glass and minerals are effective for purification.The ICP method is very useful for routine analyses. The values of chemical analysis by the ICP are close to the EPMA data. The reproducibility is exellent, while fluctuation coefficients of most elements of tephra are about 5 %. Good correlations are found between the sum of major elements except of SiO2 and the refractivity of glass, and between the Fe/Mg ratio and the refractivity of orthopyroxene.Unique tephra in middle Pleistocene can be classified into two different groups of tephra by their minor elements Y and MnO. Tephra in north eastern Hokkaido are mainly came from two sources, which can not be distinguished in the field. The contents of incompatible elements of two types of tephra are clearly different. The source regions of Pliocene to Pleistocene tephra in Tokai and Kinki area can be estimated by the regional characteristics of their chemical compositions. The ICP method is also successfully applied for identification of submarine tephra in eastern Japan Sea, and correlation of sediments of Kanto to Kyushu areas and discovery of source of some widespread tephra in early to middle Pleistocene.