Characteristics of volcanic products from summit eruption on August 18

Although more than 60% of volcanic products of the August 18 eruption consists of vesiculated basaltic fragments (ashes, lapilli and bombs) which have same mineral assemblage regardless of their size and vesicularity, it is still doubtful whether they are juvenile or not. These vesiculated basaltic fragments were analyzed by EPMA to obtain the clue of their origin.

Plagioclase is always present as phenocrysts, microphenocrysts and microlites, in all the samples analyzed. A normal zoning pattern occurs in most phenocryst plagioclase. Some of them have brown glass inclusions. Clinopyroxene occurs as microphenocrysts and microlites in most samples, and is very rarely observed as phenocryst. Olivine occurs as phenocrysts and microphenocrysts, but not presents as microlites. Magnetite occurs as microlites, and microphenocryst magnetite is exceptional and destabilized.

Typical mineral assemblage of volcanic products of the August 18 eruption.
phenocrysts and microphenocrysts= ol, pl, cpx
matrix crystals= pl, cpx, mt
Cross polars, long dimension approximately 3mm.

Backscatter electron image of a bomb(~30cm in diameter) sampled at Sonn'ei-Bokujyo.
Long dimension approximately 2mm.

Backscatter electron image of lapillus (~3cm in diameter) sampled at Oyama-Rindo.
Long dimension approximately 0.35mm

Backscatter electron image of a ash (~0.5mm in diameter) sampled in the front of TEPCO-office at Kamitsuki.
Long dimension approximately 0. 5mm

Mineral compositions of 8/18 basaltic fragments are compared with those of vesiculated basaltic ashes produced by phreatic eruptions in July and the early August. Mineral compositions of volcanic products from June 27 submarine eruption are also plotted.

Compositions of plagioclases

Both microphenocrysts and rims of phenocrysts of 8/18 plagioclase have composition range An68-75, which is slightly higher than those of volcanic products by phreatic eruptions in July and the early August.

There is no difference in the mineral composition among the 8/18 eruption products (ashes, lapilli and bombs).

Compositions of pyroxenes

Compositions of olivines

As to pyroxenes and olivines, the 8/18 products have the highest Mg#.

Compositions of magnetites

In a Al2O3/TiO2 diagram, the 8/18 products have variations similar to those of the other volcanic products group but displaced to lower TiO2.

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