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,*+* / 0 0 +1 123 ,*+* / ++ 0 ,. 12Conduit Magma Process of Miyakejima Volcano Inferred from Basaltic Juvenile Materials
in the Products of the Small Ash Eruptions during the Continuous Degassing
Nobuo GESHI῍ and Hiroshi SHINOHARA῍
The Miyakejima volcano repeats small ash eruptions from the degassing vent opening inside the A.D. ,***
caldera. The juvenile glassy fragments in the tephra indicate that the basaltic magmas with primitive composition
are ascending in the open conduit and degassing at the shallow depth.
We examined the representative +, tephra samples erupted between October ,**+ and July ,*+*, and foundthe juvenile glassy fragments with basaltic character. Their fresh surfaces and free from alteration or weathering
indicate that these glassy grains are juvenile materials. The glassy fragments occupy less than + vol.ῌ of thetephra, that mainly consists of the fragments of lavas and pyroclastics derived from the previous ediﬁce. The
juvenile fragments consist of volcanic glass with pale-brownish color and euhedral crystals of plagioclase,clinopyroxene and olivine up to several tens micrometers across. Larger crystals of plagioclase and olivines morethan +** micrometers are also contained as microphenocryst.
The range of the Fo contents of the olivine microphenocrysts in the glassy fragments is 1-῍2-, which is higher
than that of the phenocryst olivine in the basaltic bombs erupted during the August +2, ,*** eruption and similar
to that of the olivine phenocrysts derived from the primitive basaltic magma, indicating that the continuous
degassing since the A.D. ,*** eruption has been driven by the basaltic magmas with primitive character. The low
sulfur and chlorine contents in the groundmass glass indicate the degassing under low pressure.
Key words : Miyakejima, degassing, juvenile material, volcanic glass, tephra, magma+ῌ ῐ ῎ ῑ ῏456768#59:;<=>"?@ABCDEFG

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Geological Survey of Japan, AIST, AIST No.1, +῍+῍+

59*QAXµTh ,*** / 2 0¯

Higashi, Tsukuba, Ibaraki -*/῍2/01
Corresponding author : Nobuo Geshi
e-mail : geshi-nob@aist.go.jp

°

242

῍ῐΐ῔ῌῒ῎῏ῑFig. +. A : The topographic map of the Miyakejima Volcano. Contour interval is +** m. B ; The relief mapof the A.D. ,*** caldera. Point “C” shows the site from which the photograph Figure +C was taken.
Contour interval is ,* m. C : Inside of the main crater taken from the top of the southern rim of thecaldera. Arrows indicate representative altered parts. Taken on May ,0, ,**3. The relief maps A and
B were created from the DEM data (+* m mesh) published by Geographical Survey Institute with
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244

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246





Fig. -. A : A vesiculated glassy fragment in the May 3ῌ++, ,**3 tephra. B : Microscope photograph of a
glassy fragment with an euhedral olivine microphenocryst in the April +, ,**3 tephra. C : A scanningelectron microscope (SEM) image of a vesiculated glassy fragment in the April +, ,**3 tephra. D : A
SEM image of a glassy fragment with smooth surface in the April +, ,**3 tephra. E : A backscatteredelectron image (BEI) of a glassy fragment in the December ,, ,**. tephra.

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Fig. /. Sulfur contents in the groundmass glass of theglassy fragments plotted against their chlorine
contents. Broken line shows the compositional
range of the glass inclusions of the basaltic bombsof the August +2, ,*** eruption (after Saito et al.,,**/). Solid line shows the compositional rangeof the interstitial glass of the +- and +2 August,,*** eruptions.

Fig. .. The SiO,-MgO (A) and SiO,-K, O (B) diagramsof the groundmass glasses. The compositional
ranges of the juvenile materials of the ,*** eruptions are also shown (broken lines : andesitic juvenile
materials of the +.῍+/ July eruptions ; solid lines :basaltic juvenile materials of the +- and +2 August
eruptions.). Whole-rock and groundmass compositions of the August +2 eruption are after Geshi etal. (,**,b). OY in B shows the range of thewhole-rock compositions of lavas erupted during
the Oyama Stage and pre-Oyama stages (Niihori etal., ,**-).

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248

Fig. 0. A+ : BEI image of the olivine crystal with high Fo content in the glassy fragment of the April +, ,**3
tephra. This crystal is surrounded by the fracture surface consisting of the grain surface and the crystal
face contacting with the groundmass glass is observed only in the upper-left part of the grain (indicated byarrow). The white circles show the representative analysis points and the numbers indicate the Focontents in each point. A, : BEI image of the olivine crystal with low Fo content in the glassy fragmentof the April +, ,**3 tephra. This grain is surrounded by the glassy groundmass and show the euhedral
outline. B : The histogram of the compositions of the olivine crystals in the glassy fragment of the April+, ,**3 tephra. The olivine compositions in the August ,*** tephra (Geshi et al., ,**,b) and the lavas in
the Shinmyo stage (+0ῌ,*th centuries : Niihori et al., ,**-) are also shown. The compositional ranges ofthe olivine phenocrysts in the Augst +2, ,*** bombs presented by Amma-Miyasaka et al. (,**/) and Saito
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GJTU;+F] >G ¡F]>¢rῌ ῑ ῐ ῍Aiuppa, A., Moretti, R., Federico, C., Giudice, G., Liuzzo,
M., Papale, P., Shinohara, H., and Valenza, M. (,**1)
Forecasting Etna eruptions by real-time observation of
volcanic gas composition. Geology, -/, +++/ῌ+++2.
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the +.03ῌ+32- A.D. of Miyake-jima Volcano, IzuMariana arc : Inferences from temporal variations ofmineral compositions of crystal-clots. J. Petrol., .., ,++-ῌ
,+-2.
Amma-Miyasaka, M., Nakagawa, M. and Nakada, S.
(,**/) Magma plumbing system of the ,*** eruption of
Miyakejima Volcano, Japan. Bull. Volcanol., 01, ,/.ῌ,01.
Geshi, N., (,**3) Asymmetric growth of collapsed caldera
by oblique subsidence during the ,*** eruption of
Miyakejima, Japan. Earth Planet. Sci. Lett., ,2*, +.3ῌ+/2.
Geshi N., Oikawa T (,**2) Phreatomagmatic eruptions
associated with the caldera collapse during the Miyakejima ,*** eruption, Japan. J. Volcanol. Geotherm. Res.,+10, ./1ῌ.02.
Geshi N., Shimano T., Chiba T., Nakada S. (,**,a) Caldera collapse during the ,*** eruption of Miyakejima volcano, Japan. Bull. Volcanol., 0., //ῌ02.
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$ % &"'()*+) (,**.) ,-./01234567$89:CDEFG H IJ! +*/ 2-ῌ3/.
Kazahaya K., Shinohara, H., Saito, G., (,**,) Degassing
process of Satsuma-Iwojima volcano, Japan : Supply ofvolatile components from a deep magma chamber. Earth
Planets Space, /., -,1ῌ--/.
Kazahaya, K., Shinohara, H., Uto, K., Odai, M., Nakahori,Y., Mori, H., Iino, H., Miyashita, M., Hirabayashi, J.
(,**.) Gigantic SO, emission from Miyakejima volcano,Japan, caused by caldera collapse. Geology, -,, .,/ῌ.,2.
mn/ (,**/) 01op2 3 - 4 /,.ῌ/-0.
Kumagai, H., Ohminato, T., Nakano, M., Ooi, M., Kudo,
A., Inoue, H. and Oikawa, J. (,**+) Very-long-period
seismic signals and caldera formation at Miyake Island,
Japan. Science, ,3-, 021ῌ03*.
Kuritani, T., Yokoyama, T., Kobayashi, K. and Nakamura,E. (,**-) Shift and rotation of composition trends bymagma mixing : +32- eruption at Miyake-jima Volcano,Japan. J. Petrol., ... +23/ῌ+3+0.
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Miyabuchi, Y., Ikebe, S., and Watanabe, K. (,**2) Geological constraints on the ,**-ῌ,**/ ash emissions from the
Nakadake crater lake, Aso Volcano, Japan. J. Volcanol.
Geotherm. Res., +12, +03ῌ+2-.
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Saito. G., Uto, K., Kazahaya, K., Shinohara, H., Kawanabe,Y., Satoh, H. (,**/) Petrological characteristics andvolatile content of magma from the ,*** eruption of
Miyakejima volcano. Bull. Volcanol., 01, ,02ῌ,2*.
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plumbing system of the ,*** eruption of Miyakejima
volcano, Japan, deduced from volatile and major component contents of olivine-hosted melt inclusions, J.
Geophys. Res., ++/, B++,*,,
Shinohara, H., Kazahaya, K., Saito, G., Matsushima, N.
and Kawanabe, Y. (,**,) Degassing activity from Iwodake rhyolitic cone, Satsuma-Iwojima volcano, Japan :Formation of a new degassing vent, +33*ῌ+333. Earth
Planets Space, /., +1/ῌ+2/.
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