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Laser argon microprobe measurements of chlorine zonation in hydrous silicates
80 FLUID FLOW AND CHEMICAL REACTION KINETIC METAMORPHIC SYSTEMS
IN
Antonio C. LASAGA (Dept. Geology and Geophysics, Yale U n i v e r s i t y ) The treatement and effects of chemical reaction kinetics during metamorphism are developed along with the incorporation of fluid flow, diffusion and thermal evolution. Two simple kinetic models are introduced which illustrate clearly some of the new concepts that must be dealt with, one chemical kinetics are introduced into the modeling of metamorphic systems. A first general result is the existence of a fixed overstepping of equilibrium and the ability to do a back-of-the-envelope calculation of the overstepping o f u n i v a r i a n t T-Xco 2 r e a c t i o n s under
general contact or r e g i o n a l metamorphic events. The overstepping can range by many orders o f magnitude from 0.01°C (or l e s s ! ) t o more than 100°C using c u r r e n t k i n e t i c data. The second major r e s u l t i s the i n t r o d u c t i o n of "kinetic i s o g r a d s " , which s a t i s f y a l l the evidence from the f i e l d but which are completely u n r e l a t e d t o the r e a c t i o n s t a k i n g place at i n v a r i a n t p o i n t s . Such " k i n e t i c i s o g r a d s " may be more commonplace than are c u r r e n t l y b e l i e v e d .
C~OCWR~¢AL SELF-ORG~CIZATION
P . J . ORTOLEYA (Prof~mistry and Geology, I n d i a n a U n i v e r s i t y , Bloomington, I n d i a n a , USA). 0eochemlcal self-orgaalzation is the development of periodic or other repetitive variations of texture or composition without the imposition of a template for these patterns from the environment. This phenomenon, which can occur in all rock types, results from feedback in the network of reaction and transport processes. ~athemmtlc&l reaction-transport modeling of two ex~mples--metmaorphic differentiation and reaction front scalloping-~ will he presented.
A model f o r metamorphic d i f f e r e n t i a t i o n set forth,
growth/dlssolution, and diffusion of pore 'fluid' solutes. Applications to metamorphic layering, clevnge, spotted gneiss, and more complex pattern geneses axe presented, A number of distinct types of mechano-chemical feedback processes sure shown to operate in various domains of temperature, depth, and shear r a t e
spare. The morphological stability of mater-rock reaction fronts is ~nalysed via a computer reaction-transport code. Front fingering is seen to r e s u l t ~rom a flow focusing mechanism. In multiple mineral systems, complex two spatial dimensional p a t t e r n s axe shown to a r i s e spontaneously when fluids of s specified range of composition and inlet velocity are injected into an initially uniform rock.
K I N E T I C S OF T R A C E INTERACTIONS.
ELEMENTS
BEEIAVIOR
DURING
WATER-ROC,
H. P A U W E L S , G. M I C H A R D a n d P. Z U D D A S T~b~[~rre de q e o c h i m i e des eaux, U n i v e r s i t e and l.P.G.P.,Paris)
LASER ARGON MICROPROBE MEASUR~IENTS ZONATION IN HYDROUS SILICATES
OF CHLORINE
T. C. ONSTOTT and D. PHILLIPS (Dept. of Geol. and Geonhys. Sciences, University, Princeton, NJ 08544)
Princeton
Activation of chlorine by slow neutrons yields 38Ar in sufficient quantities to allow measurement by gas mass spectrometry. Utilizing a laser microprobe, this nrocedure is potentially more sensitive and precise (10% at 10 Dnm) than the electron microprobe. The detection of chlorine concentration gradients in several different occurrences of otherwise chemically homogenous hydrous minerals indicates that chlorine zoning may be a common phenomenon. Experimental data on biotite suggests that the zonatlon probably reflects partial reequillbration through binary anion exchange accomplished by diffusion parallel to the interlayer sites at a rate greater than that of oxygen diffusion and with a radius of diffusion comparable to the gralnsize. If the activation energy for chlorine diffusion in biotite is equivalent to that of oxygen diffusion, then the chlorine blocking temperature in biotite for a 5°C/Me cooling rate varies from 210 ° to 273°C for grain radii of 0.1 to 1.0 rmm.
is
including effects of rock flow,
stress-dependent grain free energy, grain
Parls
K i n e t i c s of the i n t e r a c t i o n s between alumino-sllicat mlnerals and water is g e n e r a l l y studied f a r frol equilibrium. Using the trace e l e m e n t s behavior, w e hay, studied feldspars dissolution and secondary mineral~ p r e c i p i t a t i o n in aqueous, s o l u t i o n c l o s e to equilibriu~ w i t h these phases. We s t u d i e d the Rb and Sr b e h a v i o r during tht dissolution of a sanidine, a labradorite and al anorthite. Synthetics minerals and aqueous solutions a r e pours: into a P.T.F.E. b o t t l e i n s e r t e d i n t o a s t a i n l e s s s t e e ~ a u t o c l a v e at 180oC. During tbe experiments, the silica concentration i~ stable. A l u m i n i u m c o n c e n t r a t i o n r e a c h e s a s t e a d y stat~ after 3 m o n t h s . These concentrations are close t~ s a t u r a t i o n w i t h r e s p e c t to q u a r t z and kaolinite. The b e g i n i n g of t h e d i s s o l u t i o n is a p p r o x i m a t e l , stoechiometric. After afe w days the solution i~ relatively enriched in t r a c e e l e m e n t s (Rb, St) w i t h respect to the major ones (K, Ca). M a j o r ale meal c o n c e n t r a t i o h in s o l u t i o n r e a c h e s a m a x i m u m - a f t e r 3c days for K - f e l d s p a r - and then d e c r e a s e s r e f l e c t i n g it Incorporatlon in a n e w f o r m e d p h a s e . Starting wit], sanidine, the n o w f o r m e d phase is a n o t h e r K-feldspar. D u r i n g the p l a g i o c l a s e s - w a t e r interactions, zeoliti~ phases precipitate. The ( t r a c e ~ m a j o r ) 1 t ratir
reaohes a steady stat~ after tb.... ont~ Fo#%%,idin,
and l a b r a d o r i t e e x p e r i m e n t a l dissolutions. This stead~ state set up m o r e r a p i d l y (i mountb) for the anorthit," dissolution. The p a r t i t i o n c o e f f i c i e n t D does not d e p e n d upon th~ c o m p o s i t i o n of the s t a r t i n g m i n e r a l . On t h e r e v e r s e , its d e p e n d a n c e u p o n t h e i n i t i a l t r a c e c o n t e n t in the aqueous solution suggests that the partition coefficient is a " k i n e t i c c o e f f i c i e n t " . But w i t h ,~ a d e q u a t e initial trace concentration, it is p o s s i b l e t,, o b t a i n a p a r t i t i o n c o e f f i c i e n t r e l a t i v e l y c l o s e to th, e q u i l i b r i u m p a r t i t i o n coefficient.
IN
Antonio C. LASAGA (Dept. Geology and Geophysics, Yale U n i v e r s i t y ) The treatement and effects of chemical reaction kinetics during metamorphism are developed along with the incorporation of fluid flow, diffusion and thermal evolution. Two simple kinetic models are introduced which illustrate clearly some of the new concepts that must be dealt with, one chemical kinetics are introduced into the modeling of metamorphic systems. A first general result is the existence of a fixed overstepping of equilibrium and the ability to do a back-of-the-envelope calculation of the overstepping o f u n i v a r i a n t T-Xco 2 r e a c t i o n s under
general contact or r e g i o n a l metamorphic events. The overstepping can range by many orders o f magnitude from 0.01°C (or l e s s ! ) t o more than 100°C using c u r r e n t k i n e t i c data. The second major r e s u l t i s the i n t r o d u c t i o n of "kinetic i s o g r a d s " , which s a t i s f y a l l the evidence from the f i e l d but which are completely u n r e l a t e d t o the r e a c t i o n s t a k i n g place at i n v a r i a n t p o i n t s . Such " k i n e t i c i s o g r a d s " may be more commonplace than are c u r r e n t l y b e l i e v e d .
C~OCWR~¢AL SELF-ORG~CIZATION
P . J . ORTOLEYA (Prof~mistry and Geology, I n d i a n a U n i v e r s i t y , Bloomington, I n d i a n a , USA). 0eochemlcal self-orgaalzation is the development of periodic or other repetitive variations of texture or composition without the imposition of a template for these patterns from the environment. This phenomenon, which can occur in all rock types, results from feedback in the network of reaction and transport processes. ~athemmtlc&l reaction-transport modeling of two ex~mples--metmaorphic differentiation and reaction front scalloping-~ will he presented.
A model f o r metamorphic d i f f e r e n t i a t i o n set forth,
growth/dlssolution, and diffusion of pore 'fluid' solutes. Applications to metamorphic layering, clevnge, spotted gneiss, and more complex pattern geneses axe presented, A number of distinct types of mechano-chemical feedback processes sure shown to operate in various domains of temperature, depth, and shear r a t e
spare. The morphological stability of mater-rock reaction fronts is ~nalysed via a computer reaction-transport code. Front fingering is seen to r e s u l t ~rom a flow focusing mechanism. In multiple mineral systems, complex two spatial dimensional p a t t e r n s axe shown to a r i s e spontaneously when fluids of s specified range of composition and inlet velocity are injected into an initially uniform rock.
K I N E T I C S OF T R A C E INTERACTIONS.
ELEMENTS
BEEIAVIOR
DURING
WATER-ROC,
H. P A U W E L S , G. M I C H A R D a n d P. Z U D D A S T~b~[~rre de q e o c h i m i e des eaux, U n i v e r s i t e and l.P.G.P.,Paris)
LASER ARGON MICROPROBE MEASUR~IENTS ZONATION IN HYDROUS SILICATES
OF CHLORINE
T. C. ONSTOTT and D. PHILLIPS (Dept. of Geol. and Geonhys. Sciences, University, Princeton, NJ 08544)
Princeton
Activation of chlorine by slow neutrons yields 38Ar in sufficient quantities to allow measurement by gas mass spectrometry. Utilizing a laser microprobe, this nrocedure is potentially more sensitive and precise (10% at 10 Dnm) than the electron microprobe. The detection of chlorine concentration gradients in several different occurrences of otherwise chemically homogenous hydrous minerals indicates that chlorine zoning may be a common phenomenon. Experimental data on biotite suggests that the zonatlon probably reflects partial reequillbration through binary anion exchange accomplished by diffusion parallel to the interlayer sites at a rate greater than that of oxygen diffusion and with a radius of diffusion comparable to the gralnsize. If the activation energy for chlorine diffusion in biotite is equivalent to that of oxygen diffusion, then the chlorine blocking temperature in biotite for a 5°C/Me cooling rate varies from 210 ° to 273°C for grain radii of 0.1 to 1.0 rmm.
is
including effects of rock flow,
stress-dependent grain free energy, grain
Parls
K i n e t i c s of the i n t e r a c t i o n s between alumino-sllicat mlnerals and water is g e n e r a l l y studied f a r frol equilibrium. Using the trace e l e m e n t s behavior, w e hay, studied feldspars dissolution and secondary mineral~ p r e c i p i t a t i o n in aqueous, s o l u t i o n c l o s e to equilibriu~ w i t h these phases. We s t u d i e d the Rb and Sr b e h a v i o r during tht dissolution of a sanidine, a labradorite and al anorthite. Synthetics minerals and aqueous solutions a r e pours: into a P.T.F.E. b o t t l e i n s e r t e d i n t o a s t a i n l e s s s t e e ~ a u t o c l a v e at 180oC. During tbe experiments, the silica concentration i~ stable. A l u m i n i u m c o n c e n t r a t i o n r e a c h e s a s t e a d y stat~ after 3 m o n t h s . These concentrations are close t~ s a t u r a t i o n w i t h r e s p e c t to q u a r t z and kaolinite. The b e g i n i n g of t h e d i s s o l u t i o n is a p p r o x i m a t e l , stoechiometric. After afe w days the solution i~ relatively enriched in t r a c e e l e m e n t s (Rb, St) w i t h respect to the major ones (K, Ca). M a j o r ale meal c o n c e n t r a t i o h in s o l u t i o n r e a c h e s a m a x i m u m - a f t e r 3c days for K - f e l d s p a r - and then d e c r e a s e s r e f l e c t i n g it Incorporatlon in a n e w f o r m e d p h a s e . Starting wit], sanidine, the n o w f o r m e d phase is a n o t h e r K-feldspar. D u r i n g the p l a g i o c l a s e s - w a t e r interactions, zeoliti~ phases precipitate. The ( t r a c e ~ m a j o r ) 1 t ratir
reaohes a steady stat~ after tb.... ont~ Fo#%%,idin,
and l a b r a d o r i t e e x p e r i m e n t a l dissolutions. This stead~ state set up m o r e r a p i d l y (i mountb) for the anorthit," dissolution. The p a r t i t i o n c o e f f i c i e n t D does not d e p e n d upon th~ c o m p o s i t i o n of the s t a r t i n g m i n e r a l . On t h e r e v e r s e , its d e p e n d a n c e u p o n t h e i n i t i a l t r a c e c o n t e n t in the aqueous solution suggests that the partition coefficient is a " k i n e t i c c o e f f i c i e n t " . But w i t h ,~ a d e q u a t e initial trace concentration, it is p o s s i b l e t,, o b t a i n a p a r t i t i o n c o e f f i c i e n t r e l a t i v e l y c l o s e to th, e q u i l i b r i u m p a r t i t i o n coefficient.