Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003
FLUID INCLUSION MODIFICATION IN QUARTZ AS DOCUMENTED IN TEXTURES OBSERVED BY CATHODOLUMINESCENCE TECHNIQUES
V A N D E N K E R K H O F . A. M . ' . K R O N Z , A.1
1 Geoscience Centre Gottingen, University of Gottingen, Goldschmidtstr. 3, D-37077 Gottingen.
E-mail: akerkho@gwdg.de
The interpretation of the rock-forming conditions from fluid inclusion data requires knowledge about the fluid inclusion forming mechanisms and later modifications in lower-crustal rocks during uplift. Cathodoluminescence (CL) techniques are helpful in identifying such modifications as well as other evidence of fluid-mineral interaction (Van den Kerkhof, Hein, 2001).
In this way a wide variety of micro-textures which are not visible in normal transmitted light can be identified in quartz.
Furthermore, information can be obtained about the mechanical and physico-chemical processes during fluid (re)trapping and subsequent changes including the partial leakage of fluid inclusions. Observations in cathodoluminescence show evidence for micron-scale mechanisms, which act at different depths and result in the present appearance of fluid inclusions on the E a r t h ' s surface.
Cathodoluminescence of quartz is essentially caused by point defect structures which are mostly related with the incorporation of trace elements in the quartz crystal lattice. The interaction with fluids may result in the local redistribution of trace elements or the development of secondary quartz with a different trace element content compared to the precursor quartz.
These changes produce variations of the C L wavelengths and intensities, which can be visualized in images.
A n a l y t i c a l m e t h o d s
W e combined CL-imaging and wavelength resolving trace element analysis ( E P M A ) by means of an electron m i c r o p r o b e (JEOL J X A 8900RL) equipped with a CL-detector (200-900 nm). In this way the main trace elements (Al, Ti, Fe, K and Na) in quartz can be measured. Quantitative trace element analysis by E P M A require optimized equipment settings and sample preparation (see also Miiller et al., 2002). The possible errors caused by overlapping of characteristic X-ray emission lines, curved behavior of the background signal ('Bremsstrahlung') and defects in the sample and the surface coating induced by electron beam irradiation, have been taken into account. The ranges of the detection limits (in ppm) are Al (25-74), Ti (18-43), Fe (14-31), K (11-20). Somewhat lower detection limits could be achieved by measuring with a time delay which takes the surface damage caused by the electron beam irradiation into account. Trace element profile lines have been recorded which include both the contrasting textures and host quartz.
Results
A number of C L micro-textures have been distinguished which are indicative of lower crystal order (higher density of defect structures), whereas other textures indicate higher crystal order (healing). Structural water in quartz causes reduced crystal order. Quartz with high concentrations of micropores shows dark contrasts in CL. After some minutes of electron b e a m irradiation micropores are contrasted as diffuse dark spots of 1-5 /¿m in size (e.g. Van den Kerkhof, Grantham, 1999). T h e changes during the measurement are interpreted to be the effect of amorphization by the release of structural water f o r m e d by electron capturing. Intra- and transgranular textures in C L include diffusive zones with lower trace elements, late vein quartz fillings and altered zones along open fractures. Decorated grain boundaries in C L include grain boundary alteration (diffusion) and the formation of intergranular secondary quartz. Both phenomena are often associated with small aqueous fluid inclusions (Fig. 1). The quartz along the grain boundaries is characterized by reduced trace element concentrations.
C L micro-textures around single fluid inclusions (primary or secondary in origin) can be grouped in (a) d i f f u s i o n textures (b) healing textures associated with micro-fracturing, and (c) quartz recovery textures:
(a) Diffusion textures (leaked fluid inclusions) are essentially dislocations around fluid inclusions and visible in C L as fine darkly contrasting lines. These structures may form as a reaction to stress concentration and probably present the p a t h w a y s of partial water loss (Bakker, Jansen, 1991; Hurai, Horn, 1992). T h e C L of quartz around fluid inclusions which s h o w halos of small secondary inclusions -interpreted as a result of implosion-decrepitation- reveal low contrasts (Fig. 2).
(b) Healing textures (secondary quartz) typically consist of a pattern of irregular patches of non-luminescent secondary quartz interconnected by fine healed micro-fractures (Fig. 3). Microfracturing is mainly controlled by the fluid inclusions.
(c) Quartz recovery textures (growth nuclei) have been observed as pure idiomorphic quartz nuclei with low C L intensity (Fig.
4). They show the same crystallographic orientation as the host quartz and have been observed in highly impure host quartz with Al and Ti-concentrations of up to 175 and 190 ppm, respectively. The nuclei are superimposed on brightly contrasting microcracks which are assumed to form at the a - P transformation and evidently formed at lower temperatures. H o w e v e r , they may be transected by healed microfractures and therefore must have formed at higher temperatures than the brittle-ductile transition. Relics of alkali elements in the center may indicate that now disintegrated fluid inclusions may have functioned as a nucleus. The quartz nuclei are assumed to form by volume diffusion of trace elements through lattice defects.
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Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003
Interpretation
W e m a y largely distinguish b e t w e e n f e a t u r e s f o r m e d in the ductile and in the brittle d e f o r m a t i o n r e g i m e . In the d u c t i l e r e g i m e d i f f u s i o n of w a t e r and trace e l e m e n t s are r e s p o n s i b l e for the C L - c o n t r a s t i n g textures, w h e r e a s the brittle r e g i m e is c h a r a c t e r i z e d by the f o r m i n g of p u r e s e c o n d a r y q u a r t z in faults, m i c r o - f r a c t u r e s and cataclastic d o m a i n s . T h e m i c r o - f r a c t u r i n g is largely controlled by t e n s i o n c o n c e n t r a t i o n a r o u n d fluid inclusions. D u r i n g uplift fluid i n c l u s i o n s b e h a v e as large m e t a s t a b l e d e f e c t structures in the q u a r t z : they tend to b e healed b y dissolution-precipitation, d i f f u s i o n and q u a r t z re-crystallization.
C h a n g e s in fluid i n c l u s i o n s k n o w n as 'decrepitation' are actually the result of very d i f f e r e n t p r o c e s s e s w h i c h act at a w i d e r a n g e of p r e s s u r e s and t e m p e r a t u r e s . V a r i o u s f e a t u r e s like grain b o u n d a r y alteration or s e c o n d a r y quartz fillings a re n o r m a l l y a s s o c i a t e d with the t e x t u r e s a r o u n d fluid inclusions. M a n y fluid i n c l u s i o n s w h i c h are c l a s s i f i e d f r o m optical o b s e r v a t i o n as 'decrepitated' should b e actually c o n s i d e r e d as r e - t r a p p e d f l u i d s in s e c o n d a r y quartz.
C L t e c h n i q u e s not o n l y h e l p s in f i n d i n g e v i d e n c e of the t i m i n g of fluid i n c l u s i o n e n t r a p m e n t ( p r i m a r y , s e c o n d a r y inclusions), b u t also s h o w late re-equilibration p h e n o m e n a w h i c h f o r m e d a f t e r the d e v e l o p i n g of the fluid inclusion cavity. T h e v a r i o u s C L textures a r o u n d fluid i n c l u s i o n s s h o w that fluid inclusions can b e g r o u p e d principally in l e a k e d ('decrepitated') i n c l u s i o n s a n d i n c l u s i o n s associated with s e c o n d a r y q u a r t z w h i c h precipitates at the fluid inclusion sites. T h e latter p h e n o m e n o n is v e r y c o m m o n in a l m o s t all r o c k t y p e s and tentatively d e s i g n a t e d as 'retrapping decrepitation', i.e. c o m b i n e d f r a c t u r i n g and recrystallization, associated with the f o r m i n g of p u r e s e c o n d a r y q u a r t z w h i c h s h o w s low C L intensity. T h e result of both m e c h a n i s m s m o s t l y c a n not be d i s t i n g u i s h e d b y n o r m a l m i c r o s c o p i c o b s e r v a t i o n . H o w e v e r , the c o n s e q u e n c e s f o r the fluid i n c l u s i o n d e n s i t y a n d c o m p o s i t i o n are a s s u m e d significant: d e n s i t y and m o l a r v o l u m e of d e c r e p i t a t e d (and selectively leaked) fluid i n c l u s i o n s m a y be in part c o n t r o l l e d by s u r f a c e e f f e c t s a n d therewith not n e c e s s a r i l y in e q u i l i b r i u m with t h e a m b i e n t p r e s s u r e a n d t e m p e r a t u r e . O n the other h a n d r e - t r a p p e d inclusions are e x p e c t e d to re-equilibrate c o m p l e t e l y . F l u i d i n c l u s i o n s and quartz p r e c i p i t a t i o n s m a y f o r m the n u c l e u s for p u r e quartz z o n e s w h i c h r e p l a c e the p r e c u r s o r quartz.
Examples of fluid-inclusion related CL-images
1. Fluid i n c l u s i o n s a l o n g grain b o u n d a r y s e a m e d by quartz with d a r k e r C L c a u s e d b y r e d u c e d trace e l e m e n t c o n c e n t r a t i o n s . 2. Fluid i n c l u s i o n s with h a l o s of satellite i n c l u s i o n s interpreted as a result of i m p l o s i o n - d e c r e p i t a t i o n . T h e i m m e d i a t e s u r r o u n d i n g quartz s h o w s d i f f u s i v e textures within a z o n e with slightly b r i g h t e r C L - c o n t r a s t .
3. Fluid i n c l u s i o n s with p a t c h y s e c o n d a r y quartz. A n e x a m p l e of ' r e - t r a p p i n g d e c r e p i t a t i o n ' .
4. I d i o m o r p h i c q u a r t z nuclei interpreted as a result of p r o g r e s s i v e quartz r e c o v e r y and related fluid inclusion d i s i n t e g r a t i o n .
BAKKER, R. J., JANSEN, J. B. H . (1991): E x p e r i m e n t a l p o s t - e n t r a p m e n t w a t e r loss f r o m s y n t h e t i c C 02- H20 i n c l u s i o n s in natural quartz. G e o c h i m i c a et C o s m o c h i m i c a A c t a , 55, 2 2 1 5 - 2 2 3 0 .
HURAI, V., HORN, E . E. ( 1 9 9 2 ) : A b o u n d a r y l a y e r - i n d u c e d i m m i s c i b i l i t y in naturally r e - e q u i l i b r a t e d H20 - C 02- N a C l i n c l u s i o n s f r o m m e t a n m o r p h i c q u a r t z ( W e s t e r n C a r p a t h i a n s , C z e c h o s l o v a k i a ) . C o n t r i b u t i o n s to M i n e r a l o g y and P e t r o l o g y , 112, 4 1 4 - 4 2 7 .
MULLER, A., KRONZ, A., BREITNER, K. ( 2 0 0 2 ) : T r a c e e l e m e n t and g r o w t h patterns in q u a r t z : a f i n g e r p r i n t of the e v o l u t i o n of the s u b v o l c a n i c P o d l e s i G r a n i t e S y s t e m ( K r u n é H o r y Mts., C z e c h R e p u b l i c ) Bulletin of the C z e c h G e o l o g i c a l S u r v e y , 7 7 , N o . 2, 135-145.
VAN DEN KERKHOF, A. M . , GRANTHAM, G. H. ( 1 9 9 9 ) : M e t a m o r p h i c c h a r n o c k i t e in c o n t a c t a u r e o l e s a r o u n d intrusive e n d e r b i t e f r o m Natal, S o u t h A f r i c a . C o n t r i b u t i o n s to M i n e r a l o g y and P e t r o l o g y , 137, 115-132.
VAN DEN KERKHOF, A. M . , HEIN, U. H. ( 2 0 0 1 ) : Fluid inclusion p e t r o g r a p h y . L i t h o s , 55, 2 7 - 4 7 .
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