IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

1W_1E-P15

Date/Time: July 21 Poster

The structure of iron-silica rich chimney in shallow marine hydrothermal environment at
Iwo-Jima Island, Kikai caldera, southern Japan
Takashi Kuratomi1 , Shoichi Kiyokawa1 , Minoru Ikehara2 , Syusaku Goto3 , Fumihiko Ikegami1 , Yuto
Minowa11Kyushu university, Japan, 2 Kochi core center, Japan, 3 GSJ, Japan
E-mail: t.kuratomi.0808@gmail.com

Satsuma Iwo-Jima is a volcanic island in the northwestern rim of Kikai caldera. Two post-caldera volcanoes
exist, so one has volcanic and hydrothermal activity at the present. Nagahama bay where the orange-brownish
discoloration is taken place is locatd in the southwestern island. The bay is half-closed environment and separated
into two parts of East and West-site. The origin of the discoloration is ferrous rich hot spring (pH=5.5, 55-60 degree
Celsius) (Shikaura and Tazaki, 2001). Kiyokawa et al. (2012) reported the fast deposition of about 1 m per tenyears at West-site which is under control of tide, rain and wind. East-site is distinct because of the presence ofchimney mounds, but yet studied in detail.
Samples were the massive chimney (20-30cm) and the floating particle divided from the sea water. We observed
the structure of chimney with X-ray CT scan, thin section and FE-SEM, as well as EDS for composition analysis.
The massive chimney is classified into two parts: black high-density hard layer and brownish low-density soft layer.
The result of X-ray CT scan shows that the chimney is constructed by the aggregation of convex structures (3-4cm).
Low-density layer has many pipes (radius of 1mm). Petrographic observations indicate that both high- and low-density layers have the filament-like form, and the form at the low-density layer is vertical to outer rim. In thelow-density layer, the number of particles attaching to the filament-like form increases toward the high-densitylayer. FE-SEM observation shows that the filament-like form at the high-density layer is consists of aggregation ofbacillus-like structure that looks the chain of particles (about 2µm). At low-density layer, bacteria-like form attachedparticles (<1µm) existed and this form is classed into 3 types (helix, ribbon-like, twisted). The floating particles were
aggregation of fine particles (<0.5µm) has no bacteria-related form. All particles consist of Fe, Si and O, and thoseparticles are chemically homogeneous by EDS.
According to the observation results above, we present a hypothesis of growth process of a chimney-complex
mound in Nagahama bay. The chimney is constructed by aggregation of convex structure with many pipes probably
work as the hydrothermal vent. All grains are consist of Fe, Si and O. Bacteria-like form resembles Gallionella spp.
as iron-oxidizing bacteria. The bacteria is known as neutrophilic bacteria and prefers to living at redox interface(Weber et al., 2012). The activity of bacteria around hard rim makes high density layer. The chimney is likely to be
influenced by activity of microbe.
Shikaura.T and Tazaki.K (2001), clay mineral 40,4,229-241.
Kiyokawa S., Ninomiya T., Nagata T., Oguri K., Ito T., Ikehara M. and Yamaguchi K. (2012), Island Arc 21. 66-78.
Karrie A. Weber, T.L. Spanbauer, D. Wacey, M.R. Kilburn, D.B. Loope, and R.M. Kettler (2012), Jornal of Geology
40. 8. 747-750©IAVCEI 2013 Scientific Assembly. All rights reserved.

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