Tarantola Albert

English

Mullis Josef

De Capitani C.

Lhomee T.

Dubessy Jean

University of Szeged

Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003 THE CH4/H20 (C02) TRANSITION ZONE IN THE EXTERNAL PART OF THE CENTRAL ALPS, SWITZERLAND: A LOW-GRADE METAMORPHIC FLUID EVOLUTION STUDY. T A R A N T O L A . A . ' . M U L L I S , J . 1 , D E C A P I T A N I , C . \ L H O M E E , T. 2 , D U B E S S Y , J . 2 ' M i n e r a l o g i s c h - P e t r o g r a p h i s c h e s Inst i tut , Bernoul l i s t r . 30 , C H - 4 0 5 6 Base l . 2 U M R G 2 R and C R E G U , Univers i t é Henr i Po inca ré , B P - 2 3 9 , F R - 5 4 5 0 6 V a n d o e u v r e - l e s N a n c y C e d e x . E -ma i l : a l exand re . t a r an to l a@un ibas . ch M a n y f lu id inc lus ion s tudies in the Cent ra l Alps , Swi t ze r l and , i.e. (Frey et al . , 1980) ; (Mul l i s , 1979 , 1987) ; ( M u l l i s et a l . , 1994) , d is t inguish fou r d i f f e r en t f lu id z o n e s ( H H C , CH4, H 2 0 and C 0 2 ) f r o m d i agenes i s to m e d i u m - g r a d e m e t a m o r p h i s m . T h e a im of this s tudy is to s h o w the cond i t i ons under which the t ransi t ion f r o m the CH4 to the H 2 0 - ( C 0 2 ) d o m i n a t e d f l u id z o n e occurs . A lp ine cavi t ies and their qua r t z c rys ta l s represen t the bas is of this work . T h e s e w e r e i nves t iga t ed o n severa l c ro s s -sec t ions th rough the external par t of the Cen t r a l Alps . Ac tua l resul ts o n t w o p r o f i l e s in the L in th and S e r n f va l l eys of t he G l a r u s area (a c lass ical f ield for l ow-g rade m e t a m o r p h i c s tudies) a re presented here . T h e y a re s i tua ted in the s a m e l i t h o l o g y (Nor th -He lve t i c F lysch) . F lu ids are t r apped wi thin c rack-sea l f ib re quar tz du r ing p rograde o p e n i n g of A l p i n e f i s su res . Such f lu id i nc lu s ions r e c o r d increas ing t empe ra tu r e cond i t ions e n c o u n t e r e d by the rock dur ing burial and n a p p e s t ack ing . H i g h - d e n s i t y C H 4 - r i c h f lu id inc lus ions will tend to dec rep i t a t e with increas ing t empe ra tu r e . Thus , non -dec rep i t a t ed ea r ly f lu id inc lus ions m u s t h a v e b e e n f o r m e d c lo se to the m a x i m u m t e m p e r a t u r e a t ta ined b y the rock (Mul l i s et al, 2002) . T h e r m a l matura t ion as a f u n c t i o n of inc reas ing t e m p e r a t u r e led to the f o r m a t i o n of d r y gas and to t he so -ca l l ed m e t h a n e zone . T h i s zone is charac te r ized by an over - sa tu ra t ion of me thane . F lu id immisc ib i l i t y is o f t e n o b s e r v e d and f l u id i n c l u s i o n s a re e i ther CH4- o r H 2 0 - r i c h . T h e volat i le part of the me thane - r i ch f lu id inc lus ions is c o m p o s e d of > 9 7 m o l % C H 4 , < 1 m o l % H H C and < 2 m o l % C 0 2 . In cont ras t , the H 2 0 - r i c h f lu id inc lus ions con t a in < 3 m o l % C H 4 . T h e uppe r t e m p e r a t u r e l imi t of f o r m a t i o n of this z o n e is 2 7 0 ° C . Wi th increas ing t e m p e r a t u r e , in the wa te r zone , the total a m o u n t of vo la t i l e s in t he f lu id inc lus ions d rops b e l o w 1 m o l % , whi le m e t h a n e tends to d i sappea r total ly. In r e s p o n s e to that , the h o m o g e n i s a t i o n t e m p e r a t u r e s fall sys temat ica l ly be low 180°C a l though fo rma t ion t empera tu re s are increas ing . Vi t r in i te r e f l e c t a n c e i nves t i ga t i ons a l o n g the s a m e c ross - sec t ions s h o w an evo lu t ion of R ^ f r o m 6.1 % in the m e t h a n e z o n e to 7 .5 % in the wa te r z o n e , c o n f i r m i n g increas ing t empe ra tu r e fo r the t rans i t ion f r o m the C H 4 to the H 2 0 - ( C 0 2 ) zone . T h e t ransi t ion f r o m the m e t h a n e to the water z o n e is marked b y an inc rease of C 0 2 w i th in t he vo la t i l e par t of f lu id inc lus ions , s o m e local i t ies s h o w i n g even m o r e than 15 m o l % of C 0 2 in CH 4 - r i ch f lu id inc lus ions . O x i d a t i o n is t h o u g h t to be the ma in m e c h a n i s m of the C H 4 b r e a k d o w n yie ld ing C 0 2 and H 2 0 . T h e sudden d e c r e a s e of the total a m o u n t o f vo la t i l e c o m p o n e n t s to b e l o w 1 m o l % , i nduc ing a dec rease of the h o m o g e n i s a t i o n and c la th ra te d i s soc i a t i on t e m p e r a t u r e s , is c o n t e m p o r a n e o u s with the prec ip i ta t ion of calci te , f i l l ing comple t e ly the Alp ine f i s su res . T h e ques t ion of the or ig in of 0 2 is a key topic of this s tudy. T h i s will b e d i s c u s s e d with t he he lp of s t ab le i s o t o p e inves t iga t ions . T h e cons tan t sal ini ty a l o n g the prof i les l eads to s u p p o s e a m o r e or less c l o se s y s t e m wi th r e s p e c t to the wa te r . Di lu t ion b y dehydra t ion reac t ions can pra t ica l ly be e x c l u d e d . References FREY, M . . BÛCHER, K. , FRANCK, E. , MULLIS, J. ( 1980) : Alp ine m e t a m o r p h i s m a l o n g the G e o t r a v e r s e B a s e l - C h i a s s o - A rev iew: E c l o g a e geol . Helv . , 7 3 (2) , 5 2 7 - 5 4 6 . MULLIS, J. ( 1979) : T h e sys tem m e t h a n e - w a t e r as a geo log ica l t h e r m o m e t e r and b a r o m e t e r f r o m the ex te rna l par t o f t he C e n t r a l Alps : Bul l . Minéra l . , 102 , 5 2 6 - 5 3 6 . MULLIS, J. (1987) : F lu id inc lus ions s tud ie s du r ing very l ow-g rade m e t a m o r p h i s m : M . F r e y (ed) : L o w t e m p e r a t u r e m e t a m o r p h i s m , Black ie , G l a s g o w , 162-199. MULLIS, J. , DUBESSY, J. , POTY, B. , O'NEIL, J . (1994) : F lu id r eg imes dur ing late s t ages of a con t inen ta l co l l i s ion : p h y s i c a l , c h e m i c a l , and s table i so tope m e a s u r e m e n t s of f luid inc lus ions in f i s su re quar tz f r o m a g e o t r a v e r s e t h r o u g h the C e n t r a l A l p s , Swi tze r l and : G e o c h i m . C o s m o c h i m . Acta , 5 8 (10) , 2 2 3 9 - 2 2 6 7 . MULLIS, J. , RAHN, M . K. , SCHWER, P., DE CAPITANI, C , STERN, W . B. , FREY, M . ( 2 0 0 2 ) : C o r r e l a t i o n of f lu id i n c l u s i o n t empera tu re s wi th illite "c rys ta l l in i ty" da ta and c l ay minera l chemis t ry in s e d i m e n t a r y r o c k s f r o m the ex te rna l pa r t of t he Cent ra l Alps : S c h w e i z . Mine ra l . Pe t rog r . Mit t . , 82 . 3 2 5 - 3 4 0 . 208 

The CH4/H2O-(CO2) transition zone in the external part of the Central Alps, Switzerland : a low-grade metamorphic fluid evolution study

http://acta.bibl.u-szeged.hu/31535/1/mineralogica_as_002_208.pdf

The CH4/H2O-(CO2) transition zone in the xternal part of the Central Alps, Switzerland: : A low-grade metamorphic fluid evolution study

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