Volcanic products of submarine eruption on June 27

Basaltic fragments over fifty kilogram have been dredged from the sea bed of about 80m depth at the third submersible survey on August 29. Detail of the survey has been reported elsewhere.

These fragments have the high possibility of being volcanic ejecta by 6/27th eruption. We judged the volcanic products sampled at the sea bottom to be juvenile from the following reasons:

(a) Crater groups were discovered at the position just below the place where the discolored water area was found on June 27.

(b) The fresh crater landform is retained in spite of rapid ocean current at the sea bottom.

(d)More than 95% fragments observed at the sea bottom in the craters is basaltic and has the following features:

(d-1) It has a ribbon-like or ropy shape which shows spouting out as a liquid in the free space.

(d-2) It has many cracks which seem to be the water-cooling crush.

(d-3) It has a glassy vesiculated crust and a more or less crystalline interior.

(d-4) There is no mark caused by marine organism on the surface of the fragments.

A sample dredged from crater-A.

Sample description

Four crater landforms were found in the sea bed, and the samples were collected from each. The is no difference in the appearance of the samples among craters.

They are aphyric basalt, containing phenocrysts plagioclase ranging from approximately 0.5 to 2mm in diameter. The plagioclase is commonly zoned normally with glass inclusions in the core. The microphenocryst phases are plagioclase, clinopyroxene and magnetite.

Backscatter electron image

A: Rock-chip from Crater B. Long dimension approximately 2mm

B: ditto. Long dimension approximately 400兪

C: Rock-chip from Crater A. Long dimension approximately 2mm

D: ditto. Long dimension approximately 400兪

Water content in matrix glass

A FTIRchart of a sample recovered from crater B

Four glass chips from crater B were doubly polished to a thickness of approximately 50 um. Each glass chip was carefully positioned so that the infrared beam passed through brown glass avoiding crystals and vesicles. Transmission infrared spectra in the 1200-7000cm-1 range were collected. Square aperture with 55 x 55 micron is applied. The concentration of H2O was determined according to the Lambert-Beer's law from the intensity of 3550cm-1 band. The molar absorption coefficient for H2O is taken to be 6.1 m3/mol m, and density of anhydrous glass is assumed to be 2.75g/cm3.

Concentrations of water ranges form 0.20wt% to 0.30wt% and may reflect sample heterogeneity. These values are correspondent to dissolution quantity of water at the 100-200m water depth.

Sulfur and chlorine contents in glass inclusions

(by Y. Suzuki)

Sulfur and chlorine contents in glass inclusions in plagioclase phenocrysts wereanalysed by EPMA.

Glass inclusions can be classified into two based on their shapes, i.e. rounded and angular ones. Sulfur content in glass inclusions ranges from 0.1wt% to 0.3 wt% in SO2 basis. Sulfur content in undevitrified glass in groundmass is almost zero.

Mineral chemistry

Plagioclase microphenocrysts have a zonig structure, with An82-90 core and An60-70 rim.

Plagioclase microlites have almost identical in composition to the rim of microphenocrysts.

Clinopyroxene occures as microphenocrysts and microlites. Some of clinopyroxene microphenocrysts have dendritic texture. Significant variations in Ca content probably caused by quenching at the eruption.

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