東北地方を覆う古代の珪長質テフラ“十和田-大湯浮石”の同定

抄録

Recently, many widespread tephras have been discovered in Japan. However, researchers into local tephras have been puzzled by the fact that one tephra called by a local name may also be identified as a widespread tephra although some analytical data on the local one differ from those on the widespread one.
The felsic plural tephras, which erupted about 1, 000 years ago, are distributed in the Tohoku district. One that covers Aomori prefecture and the northern part of Iwate prefecture is named Towada-a. Others found in other areas are named Ooyu pumice, Akita-Yakeyama pumice, Isawa volcanic ash and light gray-colored volcanic ash in the various areas where they are found.
There are two theories about the origin of these tephras. One, that all of them erupted from Towada volcano, was advanced on the basis of the refractive indices of volcanic glass, orthopyroxene and plagioclase. The other theory is that these tephras originated from different volcanos, because the chemical composition of ferromagnetic minerals among these tephras especially the vanadium content, varied considerably.
We investigated the major and minor elements of these tephras and their content of volcanic glass, magnetic minerals and pyroxenes, and concluded that all the tephras erupted from Towada volcano and that Ooyu pumice, Akita-Yakeyama pumice, Isawa volcanic ash and light gray-colored volcanic ash were same tephra, but Towada-a was a different fall unit tephra from the others.
The V content of ferromagnetic minerals in tephra was higher in the smaller particles of titanomagnetite than the larger ones. The difference in V content of these minerals among tephras was due to the difference in the particle size; the farther it is from Towada volcano, the smaller was the particle size of the tephra. This result indicated that it is necessary to use the same-sized particles for the identification of tephra by the V-Zn belt of ferromagnetic minerals.