Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003
RODINGITISATION OF A BASALTIC DYKE INDUCED BY H
2OCaCl
2-NaCl-H
2-CH
4REDUCED FLUIDS IN THE BELLECOMBE SERPENTINITE, PIEMONTE ZONE, AOSTA VALLEY, I T A L I A N WESTERN ALPS.
O R I O N E , P.1, C O N T E , R.1, F R E Z Z O T T I . M. L.2. F E R R A N D O , S.1 , C O M P A G N O N I , R.1 ' Dipartimento di Scienze Mineralogiche e Petrologiche, Via Valperga Caluso 35 10125 Torino.
2 Dipartimento di Scienze della Terra, Via Laterina, 8 53100 Siena.
E-mail: frezzottiml@unisi.it
W e report fluid inclusion data in gem quality vesuvianite from coarse-grained veins within a fine-grained rodingite of the Piemonte Zone exposed in the Bellecombe serpentinite, Aosta Valley, Italian Western Alps. The fine-grained rodingite, derived f r o m basaltic dykes, is hosted in a Iherzolite-derived antigorite serpentinite. The rodingitic mineral assemblage (garnet + diopside + vesuvianite + titanite + chlorite + opaques ores) is considered to be coeval with the serpentinisation of the host Iherzolite, which occurred during the late-Alpine greenschist-facies metamorphic event. This event is poorly constrained in the range T = 400-500°C and P = 0.2-0.5 GPa (Borghi et al., 1996). A network of at least five generations of polyphase mono- and/or poly-mineralic coarse-grained veins cross-cuts the fine-grained rodingite: chlorite + diopside veins (Type I), grossular- rich garnet veins (Type II), andradite-rich garnet + chlorite ± diopside ± apatite ± titanite ± opaque ores veins (Type III), vesuvianite veins (Type IV), chlorite veins (Type V). The studied vesuvianite occurs as clear gem-quality crystals in Type IV veins, which developed towards the latest phases of the rodingitisation process.
Primary (in the sense of Roedder, 1984) two phase (L+V=L) liquid dominated [df = 0.8; df = L/(L+V)] aqueous inclusions (size = 1 0 - 5 0 |im) have been recognised in vesuvianite. T h e microthermometric measurements show temperatures of freezing for the aqueous fluid (Tf) below -60°C. Eutectic temperatures (Te) are not visible but melting in the inclusions is always observed at -42°C. Hydrohalite melts ( T ^ ) between -34 and -19.8°C, and ice ( T ^ « ) between -19.6 and -5.9°C, with most measurements at -7.7°C. Homogenisation temperatures to the liquid phase (ThL) are between 240 and 381 °C, with most data at 285°C. Such a microthermometric record corresponds to a H20 - N a C l - C a C l2 system, with a mean salinity of 6 wt% NaCl + 6 wt% CaCl2, and density of 1.09 g/cm3 (Bakker, 1999).
Raman analyses in the gas bubble always show the presence of well defined peaks for molecular H2 at 4156 cm (Fig. 1).
An additional peak at 2918 cm"1 indicates that CH4 is also present.
Fig. 1 - R a m a n spectrum of H2 in fluid inclusions in the 4000 cm'1 region.
Quantitative analyses (n. 13) indicate 7 8 - 9 0 mole % H2 and 2 2 - 1 0 mole % CH4. The high-quality of the hydrogen R a m a n signal (Fig. 1) suggests that the molar volume of the gaseous part of the fluid should not be extremely high, and probably in the range 500 - 1000 mole cm"3 (i.e. 0.008 - 0.004 g/cm3). Based on the relative size of the bubble, the H ^ C H i content in the fluid can be tentatively estimated between 1 and 0.4 mole %.
Microthermometry and R a m a n analyses indicate that the fluid in the inclusions is a complex mixture dominated by saline (6 wt % CaCl2 + 6 wt % NaCl) aqueous fluids with minor H2 (probably < 1 mole %) and traces of CH4. Since the precise molar volume of the H2-CH4 gas mixture is unknown, isochores based on the H20 - N a C l - C a C I have been calculated (Bakker, 1999).
These correspond to P = 0.12 GPa at temperatures of 4 0 0 °C: they are not in agreement with the greenschist-facies conditions of T = 400-500°C and P = 0.2-0.5 GPa estimated for the second Alpine tectono-metamorphic event (Borghi et al., 1996). The 151
Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003
e f f e c t of H2 o n the i s o c h o r e s is s i g n i f i c a n t and can b e c a l c u l a t e d f r o m H e n r y ' s l a w e q u a t i o n : 1 m o l e % H2 in a f l u i d w i t h calculated salinity w o u l d shift i s o c h o r e s t o w a r d s higher p r e s s u r e s by 0.1 G P a ( B a k k e r , 1999). T h i s i m p l i e s that t h e p r e s e n c e o f 1 m o l e % of H2 or less in studied fluid inclusions ( a m o u n t e s t i m a t e d f r o m R a m a n s t u d y ) w o u l d g i v e f l u i d i s o c h o r e s in e q u i l i b r i u m with the r o d i n g i t i s a t i o n event.
P r e s e n t d a t a a l l o w us to p r o p o s e that the r o d i n g i t i s a t i o n p r o c e s s is associated to a c o m p l e x H20 - H2- C H4- C a C l2- N a C l f l u i d mixture. In the C - O - H s y s t e m at the inferred P (0.2 G P a ) and T ( 4 0 0 ° C ) , such a fluid c o m p o s i t i o n ( XH 2 O= 0 . 9 8 8 , XH 2= 0 . 0 1 0 Xch4=0.002; H2/ ( H2+ C H t ) = 0 . 8 6 ) is indicative of f0 2 c o n d i t i o n s at Q F M - 2 (log fO 2= - 3 0 . 5 9 ) a n d c a r b o n activities (ac) < 0 . 0 0 1 .
O n l y in e x c e p t i o n a l c a s e s fluid inclusion studies h a v e r e p o r t e d the p r e s e n c e of d e t e c t a b l e H2 f r o m m e t a m o r p h i c e n v i r o n m e n t s (e.g.: Peretti et al., 1992). In fact, to b e s t a b l e in f l u i d s in c o n s i d e r a b l e a m o u n t s , H2 r e q u i r e s not o n l y l o w o x y g e n f u g a c i t y c o n d i t i o n s but also very l o w C activity, o t h e r w i s e C H4 w o u l d b e the stable fluid p h a s e . S i m i l a r c o n d i t i o n s c a n b e a c h i e v e d d u r i n g s e r p e n t i n i s a t i o n p r o c e s s e s . H2 can b e p r o d u c e d t h r o u g h r e a c t i ons b e t w e e n s e r p e n t i n i s i n g a q u e o u s f l u i d s a n d m a f i c r o c k s such as: F e O (silicate) + 0 . 5 H20 = FeOi.5 ( s i l i c a t e / o x i d e ) + 0 . 5 H2. S e r p e n t i n i s a t i o n r e a c t i o n s c a n a l s o a c c o u n t f o r an e n r i c h m e n t of C a2 + in the a q u e o u s fluid through b r e a k d o w n of c l i n o p y r o x e n e a n d C a - b e a r i n g o r t h o p y r o x e n e .
F r o m the results of the present study, it m a y be c o n c l u d e d that the r o d i n g i t i s a t i o n of the B e l l e c o m b e b a s a l t i c d y k e w a s c o e v a l with the serpentinisation of the h o s ti n g u l t r a m a f i c r o c k s and w a s f a v o u r e d by the infiltration of C a - r i c h H2- b e a r i n g r e d u c i n g a q u e o u s solutions d e r i v e d f r o m the s e r p e n t i n i s i n g m e t a m o r p h i c a q u e o u s f l u i d s .
References
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