20ῌ῍ῌ ῌ    +-  -  ,+   +.  /  +- Geochemical Interpretation on Hydrothermal FormationMechanism of the Thermal Water in the IwodaniRegion and the Vicinity of the Hayashida Regionof the Kirishima Volcanic AreaShun-ichi FUJITA and Hayao SAKAMOTOGraduate School of Science and Engineering, Kagoshima UniversityAbstractThe Kirishima volcanic region, lying on the northern extension of the Ryukyu arc, islocated in Kagoshima Prefecture, south Kyushu, Japan. There are many thermal manifestations related to volcanic activity in this area. We deﬁne chemical features ofhydrothermal system of the Iwodani region and the vicinity of the Hayashida region inthe Kirishima volcanic area on the basis of Giggenbach’s chemical geothermometry,mineral equilibrium calculation, etc. The water samples for chemical analysis werecollected from some hot spring in the vicinity of the Hayashida region and mud pool inthe Iwodani fumarolic area. Almost all the collected water samples were acid-SO. typewater, generated by steam heating, and the Giggenbach’s Na-K-Mg and K/Mg-K/Cadiagram indicated immaturity of the water. It was considered that water samples weresecondary thermal water formed by mixture of shallow groundwater and the steam,which separated from two-phase geothermal ﬂuid formed underground. Therefore it isbelieved that there is hardly any mixture of the thermal water in the Iwodani region thatderives from deep underground, but it is believed that there is a little mixture of thethermal water in the vicinity of the Hayashida region that derives from deep underground. The reservoir temperature of the ﬂuid was roughly estimated to be about /-ῌ11 (the Iwodani region), 2-ῌ31 (the vicinity of the Hayashida region) by K/Mggeothermometry. These acid-SO. type waters lie on the Giggenbach’s rock-dissolutionline. The result showed that thermal water has been formed by water-rock interaction.
Key words : Kirishima volcano, Hydrothermal system, Chemical geothermometry,V /, ¼ ,**, /k0ῌ2 v6I %VW\%»Water-Rock interaction :    O]^21  ῍+     +3/1 ῌ  +303  !"#$%& '()ῌ*+ +3/1 + +33. ,-%./0#$ %1
  2%3  -* 45()6789:;<=>)?@ ABCDEFGC:HC ID>)(# + +33.$ J9 HC%1 GC%1KL%ABCDEF  +*  MJ@ NO P8)(#:;<=>)(#$ Q9/8) NOR=>#% GC%1%ST U%/VWD>XWQ9Y%F0#$ J9 Z= +330 % ![\"%]^ _#: #%` +* km $a bcb%@/&O"#:d'D>)(#$e(I  f)%*+g89/,"# Ohba et al. +331 ;ῌhi,**+$ J9 j%T)ῌ-.) !"#/k0I /k0lmn1opq/rD>)(#s D= T)ῌ-.) !"#2 ῌ34ῌt5(I u` 2 v6I:"#x/yU,"# = +33* ; Ohba et al. +331 ;w= ,***$ /k0I%%73z$a ?(){|/wx{l}~ _8)8QU9&)?@ %9&:;89<= Mud Pool /&O"#$ > 2 v6IF{|/wx{l}~ _8)8Q(9&)(,(%F /k0I%_ ,<==?=>#,($@]^F /k0 ῌ 2

v6I=?=>#%AWS(9BGiggenbach, +322 ( j%=C%VW\ '();Q9$, /k0ῌ2 v6I=?=>#%D>E = ,***Q9%FF"#$ /k0I=Q9l}~ Gz()4Q9l}~HFIQ9FD 9$¡>E Na¢, K¢ £¤¤E Ca,¢, Mg,¢ ¥¦¤¤¡E SiO, §¨kE Cl© ª«¬­b®c¯ IC EJ9°«±w² ῌ E _#¤¤E SO.,©
ª«¬­b®c¯ IC EJ9 JE(9$--+ /k0ῌ2 v6I=?=>#:/k0I=?=>#l}~%¡Table +"$ J9 ³ :ª«:/wª«%´µ Fig. +"$ /k0I=?=>#%³ :ª«Kf)%*+g89e(I=?=>#_@Y# L($ Q9/8) 7>=%  ³ :ª«K%¶·)¸(f)%MNO,¹P,($ Ingebritsen and Sorey +322 Yano and Ishido +33/  f)%T) VWD>#³:ª«K%L(%ºW '() QRS Cap rock %TU/f)%MNO,¹j22Table + Chemical data of thermal waters collected from the Iwodani area and around the Hayashidaarea, and of condensate collected from the Iwodani area of the Kirishima volcanic region. The data ofthermal water were cited from Fujita et al. (,***).
LocationTemp. Na
KCa, Mg,Cl;SO.,; SiO, K-Mg**mg l;+ mg l;+ mg l;+ mg l;+ mg l;+ mg l;+ mg l;+DatepH+,.
/0
12*32/+*/,/32/+*/,/32/+*/,/32/+*/,/32/+*/,/32/+*/,/32/+*/,/32/+,/+*,.3-.*-.+-.,-.*,.2,.1.0.,/2.,12.,/0.+0+.,2/.00.+32.02.1/2.+1+..*+..3+,.03.*++,.2*.,0/..1/.*3++.*++.11.+1-.3,+*.,*.*+0.+31.++2.**+*.++/.2,.3,0.,0-.13-.01../0/.3++.//.*,1.3*..*0-.2,.0,,..-.1/,.++,.+,*./+,.+,,3-.0-,/.+.
00+/0+0*.,+..
+02,-0,,,,,.
,/*,/30-.00,.11.110.*/1./-.03..

Hayashida +
,
.
/01
2
3
+*++32/+*/,/32/+*/,/32/+*/,/32/+*/,/32/+*/,/32/+*/,/32/+*/,/32/+*/,/32/+*/,/32/+*/,/32/+*/,/-.,-.,-.+-.,-.,-.,-.,-.-..
-.*-.+0*.3/1.//1.00/.2/2.2.2.2.2.,0+.2/2.2/-.1/+.+-+.,-*.,-*.2-/.+.*.1,,.1,-.2-3.1-1..
,1.0,/..

,+.2,*.0,,.*+1..
,*.0+1.1+0.3,+.+,*.+2.+2.++,.2+-.*+,.++-.,+0.*3.0.
+*.,+-.2+,.+*.2+*.*0.3+0.1+0.331./2.,2..2//.*.
1.,10./+/.3.
/..,+/.2+/.,,*.3+/.3,-.0++.*++.*,*.0+2.,+,.0+-.2,+0,++,.2,/,3++1/+0.
,-.
,*3,-1,,*+-0+-+.*+.0+1+*3++*+3++1.
+,2+--23.022./23.12,.22/.222.221.+22.+22./21.*21.3Myoban+,.
/32/+*/,/32/+*/,/32/+*/,/32/+*/,/32/+*/,/-.,-.,-.+-.,-..

01.+0-.*/1.,02.1-/./-/.+,3.2-*.,-,.++3.2,,.,,/.+-*.+,1.*+..*+*.,3./2+,.++*.*++.21.,0.1,1.110.3.
../.

,-.0,/.0+2./+3.3++..

,+1,*2,-3,-,+/,+01+/*+01+01+,*23./3-.030.03/.,2-.1Enoo+,-32/+*/,/32/+*/,/32/+*/,/-.-.,-.-/-.,/*.2.1.2,/.,...
,0.++1.+,,.,,*.1+*./3.2.
+*.2/..*/.*.
/.0/+1.++0.0+1.1+2*+1*+3.

++1++/++320./3..,3*.3IwodaniNote : * condensate of fumarolic gas, **geothermometric temperature&'()*+,-.+, /+0!

"#$%1+2+,

 3456789 ,,3:00+ mg l;+ 0 <=1+2+,'( 345678>1+2+,?@A9BCDEF0 GHIJῌK%LEMNO9DE-.+, P"0 #$% !0 QRS9TFUVWXYZ[\*]^_`-.+, /+01+2+,abcdefgh456789 *./+ mg l;+ BCT i,9>jkl%m'(1+no "pbq-.+, ir0 1+no "pbsn9 ht*uvM 34569"+/,E*0ht9UVq9UVWX3h/,wx 3456hy-.+,1+2+,abcdef 3456789 0*., mg l;+ gh45678z{_|}FE&~01+2+,q"» /, ¼ \,**,] '³´µῌ => n²`¶·¸+i¹º  

3 4 05 67''(89(:2=>  

 +,-.+/012 '( !"#$%&'()*< 23;5?@ ABCD& > EFῌGHIJKLM&(:2; N > EFῌGHI05 OP05? Q+0R () ; N=> NQ+ ++.*S,0./ mg lT+ U? 0R&; '  VH079NJK:2=> WXYZ[;Na-K-Mg ῎῍-ῌ,Giggenbach \+322]  ^ Na_, K_, Mg,_  - ` ab῎cdefghN+ij05? I0klm nopqrsoHtfuhm(N NaῌK +itv( KῌMg +itvfwxyz{ Na-K-Mg w`|f}~N& ; 'cde0m 0 n00lm %b  %b b    U ;' j0;ῌalbite_K_K-feldspar_Na_

῍*.2 K-mica_*., chlorite_/.. silica_,K_,.2 K-feldspar_+.0 H, O_Mg,_h($ ῌ ῍ j`?j \Full equilibrium] f; N N 4.50 abm0 fm@0¡& ¢` \Immature water] !£0+322]; ῌ=>rs¤"N \Fig. ,]; ῌ => < )\Giggenbach,w`f Na-K-Mg w`|04.'(D& ¢`!£0; <  ¤"¥(¦*jfm0 Na ῌ K +it

v KῌMg +itvHt o; '5)toH^f§N¨©m(ªc(.c# Zlm .ª«cpq() ; N Fig. + N$.¬­®6Q+ @%)50 ῌ =>  .ª«cpq&N4.GHIῌ0¯M° ±`N«)U (:2²`; 5 '5)G4.&oHtf)&(:2; NaῌK +itv ¤"Fig. + Relationships between ClT and SO.,T concentrationsof thermal waters and condensate collected from theIwodani region and the vicinity of the Hayashida region ofthe Kirishima volcanic area. Maruo data were cited fromFujita and Sakamoto (,**+).

24NaῌK   !" Giggenbach +322 .-/0123 45" ,1*-** #$%&" '()*+,&- NaῌK 678 ῍ - ῌ 9:;67&<1" KῌMg  .-=&/0>?1@ Giggenbach, +322 A KῌMg  .B&/0CAD" EFG HI - /-11" JKLM HI - 2-31 CNOPI JKLM HI EFGHI&QR -STUV @WCXYIKῌMg-KῌCa ῎῍-ῌ-Giggenbach +322 -" Z[ῐ\]^0_Oc&defgh"67&" K`, Ca,`, Mg,`  - abi3jkNl5mC" KῌMg-KῌCa naopq A\]^0&rstg&-" f&uvi&we" xyz{" |}[" xy~[" 5[" yx1@Giggenbach +322 -80" tgC67r
῎*.2 K-mica`*., chlorite`/.. silica`,K` ,.2 K-feldspar`+.0 H, O`Mg,`῏- K-feldspar`CO,`Ca,`K-mica`calcite`0 silica`,K`AItgC1 ῎" ῏ 867r a!I" KῌMg-KῌCa naoC167 Full equilibrium &PI "  KῌCa naQ c&we CO, b" KῌMg naQ  IIAC 12EFGῌJKLM HI K-Ca-Mg nar Fig. - &Giggenbach +322 -" +,-67 CO, bCAD " 67Fig. , Na-K-Mg diagram (after Giggenbach, +322) of thermal waters from the Iwodani region andthe vicinity of the Hayashida region. Full equilibrium line is derived from chemical equilibriabetween thermal water and the assemblage of minerals ; albite, K-feldspar, K-mica, chlorite,silica. Maruo data were cited from Fujita and Sakamoto (,**+).

O /, P 7,**, ABCD !ῌ"#$%&'+,EF6GH&IJKLMN25Fig. - K/Mg-K/Ca diagram (after Giggenbach, +322) of thermal waters from Iwodani region and thevicinity of the Hayashida region. Full equilibrium line is derived from chemical equilibriabetween thermal water and the assemblages of minerals ; K-feldspar, chlorite, K-mica, calcite,silica. The rock dissolution line refers to solutions of +*, +**, +,*** g of an average crustal rockin + kg of water (after Giggenbach, +322). Maruo data were cited from Fujita and Sakamoto(,**+).

Fig. . Relationships between temperatures and SiO, concentrations of thermal waters andcondensate water collected from the Iwodani region and the vicinity of the Hayashida region.

$%&'()*)      !ῌ"#+,-./0 CO, 123456, 7Immaturewater 89 :;<=>  ?@ "#$%&'()*) +, !&'()*) +µ¶O26 CO,    -. +, -. ,    !" #$ CO,  %&'"  ( )*+ ,- ./012,345 627 89:;<=>?@ABC"   D  ) KῌMg-KῌCa 62+EFGH *+ ,-45D I2"H %JK  ( LMMGNO/PQ ῌ ῍  "RSTUVWRSXYZRS[2\JK ] ^_  `a"b cd ef\H -ῌ. ῎῍῏ghM45i ,-jghMk\lmnopq aorpsCtuhC qvwxyz {|D }+ \ ghM~i%i\] SiO, ~i NOi% + ghMNO SiO,Nicholson .+33-3 (%]  SiO,NOi_\D  i jghM/P~iNOiD &' %\ Lῌ i%45GghM~iFig. .   Lῌ  45GghM~i .R,3 \ *./*.0 %+\ i,.,,.. \ )45GghM~ii\+ ¡ ¢H  (   .£¤¥ghM~i\3\JK\ L  45GghM~i\ ,-¦§Z¨MG©nª«ghMk¬­\®¯" \ ©n\° ±²lmnopqXlmnYZ[2\JK  L  7d ,-ῌZ¨©n\° lmnopq ³ 45GghM~i\ (45/P´ H %JKFig. / A schematic presentation of a genetic model of thermal water in the Iwodani region and thevicinity of the Hayashida region of the Kirishima volcanic area (modiﬁed from Fujita et al., ,***).2 /, ¼ ,**, =# 6 ῌ 7 8 ! ,-. ij27. ῌ ῐ Fig. /   !"#$%&' ( )#*+ ,-. /)0 123%4'Na-K-Mg 5/)0

ῌ6 ῌ 7 809 :;<=>?%@ABC'DE@ABC' ( KῌMg80HF9 /-I11J680HF9K'2-I31J6῍

7-.FG0809LFH;MN0O; :;% PQRS TUV(WOX RPY K'

7809LFH;MN0O; :;%7Z[ῌ\H 2TUV(WOX RPY K' (]^_%4'KῌMg-KῌCa 5/)0῎6 ῌ 7 89 R/'PY`?%K'6 ῌ 7 80/?ab> 9 !cdefg^_%h῏

"ij klY mn#$.%.&o'pq (r)stpq u*v+,w9-xRty z ( {.|/90}Rty z 1~/]O~( 1"F.2 /. $P3 4 +- 5 2  ,- 1-67 +3/1 : <8g$#9 d. 0- .--ῐ....

8ῌ:7ῌ;< ,*** : =# E> EFW ! F. /* +ῐ++.
8ῌ:7 ,**+ : =#? 8 ! ,-. / F.
/*,,+ῐ,,3.

Giggenbach, W. F. (+322) : Geothermal solute equilibria. Derivation of Na-K-Mg-Ca geoindicators. Geochim. Cosmochim. Acta, /,, ,1.3ῐ,10/.

@A+33. : =# d¡ij¢BC 03 +23ῐ,*3.

Ingebritsen, S.E. and Sorey, M.L. (+322) : Vapor-dominated zones within hydrothermalsystems : evolution and natural state. J. Geophys. Res, 3-, +-0-/ῐ+-0//.

£¤¥ῌ¦§ῌ~D¨ῌEF7ῌ©ªῌGHIῌE«¬ῌ@~­®ῌ¯°+ῌ±²³ῌkJ ´ῌKL MῌNIµῌ$¶OP·ῌ¸Q¹R +330 : =#ºG>STU»!V  .+ ,+/ῐ,,/.
Nicholson, K (+33-) : Geothermal Fluids. Springer Verlag Berlin Heidelberg, 03ῐ1+.
Ohba, T., Nogami, K. and Hirabayashi, J. (+331) : Hydrothermal System of the Kirishima'(L)28Volcanic Area Inferred from the Chemical and Isotopic Composition of Spring Watersand Fumarolic Gases. Bull. Volcanol. Soc. Japan, .,, +ῌ+/.

ῌ +303 :+31/ : / +   

!"#$%&'()*+, - / . / 0#'(123#456 789):);<= )ῌ>?) / ,/ 2/ῌ+*+.ῌ@ABCDῌEFGD +33* : 78H'(/ IJ'( K +  78H/ +ῌ23.
Yano, Y. and Ishido, T. (+33/) : Numerical Modeling of the Evolution of Two-phase Zone undera Fractured Caprock. Geothermics, ,., /*1ῌ/,+.