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,**/  .  .  ,**/  ++  / Re-examination of the Magma Plumbing System beneath Usu Volcano,Hokkaido, Japan, during the +00- Eruption
Akiko MATSUMOTOῌ, Mitsuhiro NAKAGAWAῌ and Yugo NAKAMURAῌῌ
The +00- Usu eruption was the ﬁrst and largest of all its historic eruptive activities after ca. 1*** years’
dormancy. Our recent study divided the +00- eruption into three stages in following ascending order : Stage I,
small scale of plinian and phreatomagmatic eruptions ; Stage II, climactic plinian eruption (pumice fall as so-called
Us-b fall) ; and Stage III, vigorous phreatomagmatic eruptions. Previous petrological studies mainly focused on
the Stage II event.
The +00- juvenile materials are composed of three types, white (from all the stage), banded pumices (Stage Iand II) and gray essential lithic fragment (Stage I). Major phenocrystic minerals (i.e. plagioclase andorthopyroxene) show nearly bimodal compositional distributions, and can be divided into two types : Type A,iron-rich orthopyroxene (Mg#.0) and sodic plagioclase (An.,) ; and Type B, magnesian orthopyroxene (Mg#1*) and calcic plagioclase (An21). This observation suggests that the juvenile materials were mixing
products between maﬁc and felsic magmas. This observation is also consistent with linear trends in all the oxide
variation diagrams for whole-rock chemistry. Based on Wo (Ca/(CaMgFe)) content in orthopyroxene, Anand FeO* contents in plagioclase, however, the Type A phenocrysts can be further subdivided into two types :Type A+ (lower Wo, An and FeO* contents) and Type A, (higher Wo, An and FeO* contents). Moreover, theType A, phenocrysts are common in the juvenile materials of Stage I (gray essential lithic fragment) as well as in
the Stage-II. Based on phenocryst size, composition and whole-rock chemistry, it can be concluded that the TypeA, phenocrysts crystallized from the mixed magma between the maﬁc and felsic ones, and had grown for several
years before the +00- eruption.
Considering the eruption sequence and the types of erupted magma, the mixed magma was erupted during theinitial, weak eruption (Stage I), as well as the climactic, explosive eruption (Stage II). This indicates that themixed magma of Stage I event would not stagnate between maﬁc and felsic magmas, as is common in a normal
zoned magma chamber, but the top of the chamber. This could be explained by convective entrainment asfollows. The injection of the high temperature (+***) maﬁc magma into the lower temperature ( 2**)felsic magma could cause thermal convection to entrain the former into the latter. This entrainment would form
the mixed magma, which could rise to the top of the chamber. In e#ect, the upper part of the chamber would be
gravitationally stable until the eruption, because the mixed magma could be lighter than the felsic one. Our
petrological analysis concludes that the +00- eruption was derived from a compositionally reverse zoned chamberthat the mixed magma had existed above the normal zoned magma, and that maﬁc injection had occurred not justbefore the eruption.
Key words : Usu volcano, the +00- eruption, zoned magma chamber, eruption sequence!*0*ῌ*2+* "#$%&% +* '( 2 )*%+,-.-./.01234562.78
Department of Earth & Planetary Sciences, Graduate
School of Sciences, Hokkaido University, N+* W2,
Kita-ku, Sapporo *0*ῌ*2+*, Japan.
!*0*ῌ*2+* "#$%&% +* '( / )*%+,-.-./349:2.01/34;2.<=

Division of Geoscience, Graduate School of Environmental Earth Science, Hokkaido University, N+* W/,
Kita-ku, Sapporo *0*ῌ*2+*, Japan.
Corresponding author : Akiko Matsumoto
e-mail : am13*+*3@ep.sci.hokudai.ac.jp