Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003
THE CH4/H
20 (C0
2) TRANSITION ZONE IN THE EXTERNAL PART OF THE CENTRAL ALPS, SWITZERLAND: A LOW-GRADE METAMORPHIC FLUID E V O L U T I O N 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 Institut, Bernoullistr. 3 0 , C H - 4 0 5 6 B a s e l .
2 U M R G 2 R and C R E G U , U n i v e r s i t é H e n r i P o i n c a r é , 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 - m a i l : a l e x a n d r e . t a r a n t o l a @ u n i b a s . c h
M a n y fluid inclusion studies in the Central Alps, S w i t z e r l a n d , i.e. (Frey et al., 1 9 8 0 ) ; ( M u l l i s , 1 9 7 9 , 1987); ( M u l l i s et al., 1994), distinguish f o u r d i f f e r e n t fluid z o n e s ( H H C , CH4, H20 and C 02) f r o m d i a g e n e s 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 i m of this study is to s h o w the c o n d i t i o n s u n d e r w h i c h the transition f r o m the CH4 to the H20 - ( C 02) d o m i n a t e d f l u i d z o n e o c c u r s . A l p i n e cavities and their q u a r t z c r y s t a l s r e p r e s e n t the basis of this w o r k . T h e s e w e r e i n v e s t i g a t e d o n s e v e r a l c r o s s - s e c t i o n s t h r o u g h the external part of the C e n t r a l Alps. A c t u a l results o n t w o p r o f i l e s in the L i n t h a n d S e r n f v a l l e y s of t h e G l a r u s area (a classical field for l o w - g r a d e m e t a m o r p h i c studies) a r e presented h e r e . T h e y a re situated in the s a m e l i t h o l o g y ( N o r t h - H e l v e t i c F l y s c h ) .
F l u i d s are t r a p p e d within crack-seal f i b r e quartz d u r i n g p r o g r a d e o p e n i n g of A l p i n e f i s s u r e s . S u c h f l u i d i n c l u s i o n s r e c o r d increasing t e m p e r a t u r e c o n d i t i o n s e n c o u n t e r e d by the r o c k d u r i n g burial and n a p p e s t a c k i n g . H i g h - d e n s i t y C H4- r i c h f l u i d i n c l u s i o n s will tend to d e c r e p i t a t e with increasing t e m p e r a t u r e . T h u s , n o n - d e c r e p i t a t e d e a r l y fluid i n c l u s i o n s m u s t h a v e b e e n f o r m e d c l o s e to the m a x i m u m t e m p e r a t u r e attained b y t h e r o c k (Mullis et al, 2 0 0 2 ) .
T h e r m a l m a t u r a t i o n as a f u n c t i o n of i n c r e a s i n g 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 g a s a n d to t h e s o - c a l l e d m e t h a n e z o n e . T h i s z o n e is c h a r a c t e r i z e d by an o v e r - s a t u r a t i o n of m e t h a n e . Fluid i m m i s c i b i l i t y is o f t e n o b s e r v e d a n d f l u i d i n c l u s i o n s are either CH4- or H20 - r i c h . T h e volatile part of the m e t h a n e - r i c h fluid i n c l u s i o n s is c o m p o s e d of > 9 7 m o l % C H4, < 1 m o l % H H C a n d < 2 m o l % C 02. In contrast, the H20 - r i c h f l u i d i n c l u s i o n s c o n t a i n < 3 m o l % C H4. T h e u p p e r t e m p e r a t u r e limit of f o r m a t i o n of this z o n e is 2 7 0 ° C . W i t h increasing t e m p e r a t u r e , in the w a t e r z o n e , the total a m o u n t of v o l a t i l e s in t h e f l u i d i n c l u s i o n s d r o p s b e l o w 1 m o l % , w h i l e m e t h a n e tends to d i s a p p e a r totally. 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 s y s t e m a t i c a l l y b e l o w 180°C a l t h o u g h f o r m a t i o n t e m p e r a t u r e s are increasing. Vitrinite r e f l e c t a n c e i n v e s t i g a t i o n s a l o n g t h e s a m e c r o s s - s e c t i o n s s h o w an e v o l u t i o n 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 w a t e r z o n e , c o n f i r m i n g increasing t e m p e r a t u r e f o r the transition f r o m the C H4 to the H20 - ( C 02) z o n e .
T h e transition f r o m the m e t h a n e to the water z o n e is m a r k e d b y an i n c r e a s e of C 02 w i t h i n t h e volatile part of f l u i d inclusions, s o m e localities s h o w i n g e v e n m o r e than 15 m o l % of C 02 in C H4- r i c h f l u i d i n c l u s i o n s . O x i d a t i o n is t h o u g h t to b e the m a i n m e c h a n i s m of the C H4 b r e a k d o w n yielding C 02 and H20 . T h e s u d d e n d e c r e a s e of the total a m o u n t o f v o l a 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 n d u c i n g a d e c r e a s e of the h o m o g e n i s a t i o n a n d c l a t h r a t e d i s s o c i a t i o n 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 p r e c i p i t a t i o n of calcite, filling c o m p l e t e l y the A l p i n e f i s s u r e s .
T h e q u e s t i o n of the origin of 02 is a key topic of this study. T h i s will b e d i s c u s s e d with t h e h e l p of s t a b l e i s o t o p e investigations. T h e c o n s t a n t salinity a l o n g the profiles l e a d s to s u p p o s e a m o r e or less c l o s e s y s t e m with r e s p e c t to the w a t e r . Dilution b y d e h y d r a t i o n react io n s c a n pratically be e x c l u d e d .
References
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