C H E M ！C A L P R O C E S S E S IN D E F O R M A T IO N A T L O W M E T A M O R P H IC G R A D E S Pressure Solution and Hydraulic Fracturing by A lastair Beach

Pressure solution has long been recognized as an im portant m echanism of de和rm ation, particularly in sedim entary rocks at low m etam orphic grade. G eologists have tended to study only the m ost easily m anaged aspect of pressure solution structures 一their geom etry as a record of rock deform ation. A t the sam e tim e the m ost co m m on pressure solution structures, such as stylolites in lim estones, clearly evolve th rough com plex chem ical processes, as do cleavage stripes and associa ted syntectonic veins w hich are abundant in terrigenous sedim entary rocks that have been d e form ed under lo w grade m etam orphic conditions. This review 和cusses on stripes and veins, draw ing together those concepts that need integrated study in o rd e r to r e a c h a b e t te r understanding a厂pressure solution.

G eological Setting s m a lle r s c a le in s la t e s a n d t h e d e fin it io n o f c h e m ic a l a n d m ineralogical changes associated w ith cleavage developm ent Spaced cleavage stripes are the w idespread result of defor- (K nipe, 1982). F or exam ple, it is suggested that chem ical m ation of sandy sequences. The striping is m uch better changes w ere concentrated in the stripes, w ith the interven- developed in im m ature sedim entary rocks such as greyw ackes (F i2ure 1). and has a com m on genetic relationshii) (Figure 2) ing areas retaining their diagenetic chem ical fingerprint. to the developm ent 01 syntectonic veins kbeacn, i,)/ /). i nus V ery few data are how ever available on chem ical changes a first approach involves exam ination of the m ineral constit- associated w ith fracturing. F racture grow th is often pro- uents of stripes and the intervening lithons in conjunction m oted by the presence of fluids in the crack; chem ical bonds w ith any related veins, and the sim ple observatio n that stripes contain m ore phyllosilicates than the lithons is m ost im portant. The geological conditions for the developm ent of these structures have been defined through oxygen isotope and fluid in c lu sio n stu dies . T h e d o m ina n t m e ch an ism o f d e fo rm a tion below about 400'C is pressure solution, involving dissolution of m ineral grains at points of high stress (Figures 3 and 4), d咐usion through the grain boundary fluid phase, and precipi- tation of new m inerals at points of low stress. Syntectonic veins com m only accom m odate the excess m aterial rem oved from stripes during deform ation, as show n by field relations, and confirm ed by oxygen isotope studies on both veins and the adjacent rock. M ineralogical Evidence T ra ditio naliv . the h ig h er D e rc en taRe o f D hv llo silic a te s in th e Figure 1: P ressure solution stripes in a deform ed greywacke 厂rom N orth D evon. The stripes are seen to be anastom osing cleavage stripes has t)een interpretea as the resuit oi inert accum ulation through rem oval of m ore soluble m aterial, usu- in this cross-section of bedding, and fanning slightly across a ally quartz (Figure 5). C areful m odal analyses dem onstrate fold hinge zone. The stripes appear darker because of the how ever that there m ay be a m ore fundam ental change in increased am ount of clay m ineral com pared w ith the gre.v- m ineralogy during stripe developm ent. For exam ple, B each w a c k e . (1974) show s that in a deform ed greyw acke the am ount of feldspar and_epidote decreased into a zone of increased cleavage striping and the am ount of illite-m uscovite and siderite gradually increased (Figures 6 and 7). Significantly, the am ount of quartz first decreased, then increased again . Su c h d ata c an be ra tion a liz ed in to fo rm a l c he m ica l rea c tion s inferred to have occurred during stripe developm ent (e.g. Beach and K ing, 1978; Beach, 1979). C hem ical Evidence A quantitative assessm ent of the source-sink relation of stripes and veins w as presented by Beach (1974); Stephens and others (1979) present detailed chem ical analyses of stripes and lithons. T hey define losses of N a, Si, A l, M n, F e, M g, and C a from the stripes, m easured relative to the com position of the lithons and assum ing that Ti w as im m obile. F igure 2: P re s su r e solution stripes and syntectonic V e I n S The advent of the transm ission electron m icroscope has developed as related structures in a de阳rm ed greyw acke, perm itted the identification of stripes and lithons on a m uch N o rt h D e v o n .

2 EP ISO D ES, Vol. 1982, N o. 4. constrain the types of reactions that m ay occur under certain conditions of tem perature and fluid com position. T he converse problem to fracture stresses, that is th e stre ss perturbation around a pressure solution stripe, has only recently been tackled by Fletcher and Pollard (1981) w i th w hat they term the anticrack m odel. C om pressive st re s s concentrations exist at stripe tips and the m ode of propaga- tion is considered to be analogous to that for tensile cracks, i.e. stripes propagate along the surface to w hich the m axi- m um com pressive stress is norm al. The m echanism of stripe propagation m ay also be reaction rate controlled and m ay involve any process that responds to local variations in stress, such as preferential solution of m inerals. ion exchange through the fluid, or m ineral reac- tions. It is thus the opposite end ot tne sequence to ine Figure 3: C ongruen t pressure solution of quartz grains. processes that m ay occur at crack tips. D e tr ita l grain rim s are preserved inside early diagenetic overgrow ths, both of w hich are affected by pressure solution at grain contacts. F ield w id th 2 M M .

in the vicinity of the crack tip are broken by hydrolysis, and 1976) by evaluating the variations in chem ical potential of a fracture propagation occurs at stress differences low er than fluid phase w hen a rock is solid phase in equilibruirn w ith a those required for dry fracturing. Barnett and K errich (1930) subjected 1 0 a d iffe re ntia l stre ss. T h e se t r e a tm e n ts a r e have attem pted to relate local changes in m ineral chem istry sim plified by restricting the concept of pressure solution to to this process, w hich is know n as stress c o r ro s io n c r a c k in e . the dissolution of quartz and calcite, w ithout identifying H ow ev e r, it is not possible from t h e ir d a ta to distinguish either solution m echanism s or coupled diffusion processes. b e tw e e n lo c a liz a t io n o f r e a c t io n s a h ea d of fracture tips These latter tw o aspects w ill now be briefly discussed. during stress corrosion cracking and alteration of m inerals along fractures as a result of enhanced access of reactants. Pressure solution com m only involves m ore com plex m inerals than quartz and calcite, not only in the form ation of cleavage Stress D istribution A round C racks A nd Stripes stripes, but also in the grow th of pressure shadow s. It is The m odel of an elliptical flaw in an infinite elastic m edium necessary to identify both the chem ical m echanism of solu- is often used to evaluate geological fracture problem s tion of m inerals such as carbonates, w hite m ica, chlorite and through calculation of the stress distribution around the feldspars, and the process of diffusion of aqueous com ponents of th ese m ine ra ls. crack under biaxial applied stress, w ith or w ithout an internal tO fluid pressure. The role of hydraulic fracturing and sub- A ccording to Helgeson (1969), quartz dissolves in water by critical crack grow th are being increasingly advocated as tw o fo r m th e neutral m olecule H 4SiO 4. This m ay dissociate m odes of form ation of m any naturally form ed fractures (B each, 1977; A nderson and G rew , 1977). A t crack tips, stress concentrations m any tim es the regional stbihema nipnl 9cer, ei aonnsdeizd ta.ht ieoL onvw we rchaoelnl c etohnnetcr eapntHtior anotsfi otnhfe oH ff+ lsu iclidirce aph tienads e oi nlius t higoerne f amltuaeiydr value exist, and m ay be im portant for several reasons. F irst, w a te r m o le c ules attac h th e m selve s to stre tch e d S i-O bo nd s in the crack tip, and bond rupture ensues (stress corrosion sensu pfhoheranmsceein gbiyn r：oclrc oekca amsle a eytq hluoeicl iabmlrliyaug imnic trwuediatesh e o mtfhi ente shroeallu cbrihelieatmcyti iocofanl sq u pianort teazn atdinead-l stricto). Second, gradients in chem ical potential of the solid gradients for silica aw ay from reaction sites. Sim ilar phase in equilibrium w ith the fluid phase exist, and enhanced local increase in equilibrium value of the concentrati4 Na+ in the fluid phase in conjunction w ith a m in e ra l r e a c t io n diffusion of any com ponent m a y occur. T hird, ton exchange silic a in so lu tio n betw een crack fluid and crack w all m ay occur preferentially in c r e a se th e lo c a l c o n c e n tr a tio n o f at crack tips in the area of low er m ean stress w here tensile through the form ation of soluble sodium silicate com plexes. stresses are concentrated. For stress corrosion }in ihe strict Such processes can often be anticipated from a consideration sense, the log crack velocity is proportional to the log stress of the equilibrium betw een a m ineral assem blage and the intensity, and crack tip reaction rate is the controlling factor fluid com position (the latter recorded for exam ple by fluid in fracture grow th (A nderson and G rew , 1977). inclusions), together w ith the likely reaction paths during pressure solution. The geologically form ed hydraulic fracture is the best pos- sib le environm ent for prom oting subcritical crack grow th s i n c e fractures are open and fluid filled , enabling free diffusion of com ponents to and from , reaction sites; elevated tem peratures increase reaction r at e s ; fluid filled cracks undergo stress corrosion a t l o w e r stress inte nsities th a n cra ck s w ith sm all co nc en tra tion s o f w a te r vaDour (M ichalske and F reim an, 1982), and, the fluid is not pure w ater, but a brine w itn varying concentrations of cations and anions (Plus C O A prom ot- ing hydrotherm al-type reactions (H elgeson , 1969) at the crack tip. U nlike m any m etam orphic problem s w here diffusion is con- sidered the rate-contr olling step, subcritical grow th of natu- ral hydraulic fractures is likely to be reaction rate controlled F igure 4: Congruent pressure solution a厂calcite ooliths. The (M ichalske and Freim an, 1982). Thus, although there m ay be 厂ibrous calcite overgrow ths are little affected by the stylo- no record in the rock of a reaction related to crack grow th litic solutio n surfaces separating the grains. Field w id th 2 (but see Barnett and K errich , 1980), it should be possible to m m 。

邓 E PISO D ES, Vol. 1982, N o. 4. P re ssu re So lutio n M ec h an ism s A distinction can be m ade betw een congruent and incon- gruent pressure solution (cf. Beach，-1982). T he form er, w hich has been the subject of m uch theoretical and descriptive w ork. involves the dissolution and precipitation of the sam e phase or phases kFigure 3). t}or exam ple quartz grains dissolve , so quartz overgrow ths form . In contrast, incon- gruent pressure solution, w hich leads to the form ation of m any cleavage stripes, involves the dissolution of one or m ore phases such as feldspar and the precipitation of one or m ore different phases such as m ica and quartz (Figure 8）二 third category arises as an extension of congruent pressure solu tio n w h ere the m in e ra l c o nc ern ed sh ow s so lid so lution changes in com position, for exam ple from dissolved calcite to precipitated ferroan-calcite overgrow ths. Figure 5: P ressure solution cleavage stripe in greyw acke. F ie ld w id th 2 m m . A n im portant question arises at this point. A re the phase and chem ical changes observed m erely a result of deform ation? A re they carried passively by this process, or do they provide the prim e driving force for pressure solution? In the form er case, observation of these changes is m erely a record that pressure solution has occurred, w hile in the latter it is im portant to understand the reaction m echanism in order to evaluate the rate of deform ation by incongruent pressure s o lu t io n . In the m ajority of cases, phase changes provide part of the driving force for deform ation by pressure solution. Both congruent and incongruent processes m ay be represented by chem ical reactions, the form er albeit trivial, w ith a diffusion step from source to sink. The rate of a chem ical process is exponentially proportional to the reciprocal of the energy Figure 6: D etail of texture adjacen t to pressure solution barrier that has to be overcom e, and so can low er this energy stripe show n in Figure 5. Irregular quartz and 介Idspar b a r r ie r . grains, detrital m icas and diagenetic clays are present. F ield T extures in deform ed rocks m ay indicate that the deform a- w id th 0 .1 m m . tion m echanism m aps produced by R utter (1976) and others provide low er lim its only for rates of pressure solution. F or exam ple, it has long been know n that clay seam s prom ote or C alcite dissolves in w ater by ionic dissociation . Local "activate" pressure solution in sandstones (e.g. H eald, 1955), solution, deposition and chem ical potential gradients w ill and a sim ilar process m ay occur in spaced cleavage stripes. depend on the pH and com position of the grain boundary Q uartz grains in direct contact w ith another quartz grain fluid. T he presence of ions in solution calls attention to tw o m ay show little pressure solution, w hereas other grains in further problem s. F irst, it is know n that ions are surrounded contact w ith clay or m ica show strongly pressure-solved by electrostatically coordinated w ater m olecules, and the contacts. W ith the diffusion step rate controlling, the num ber of these , and hence the effective size of the solvated activation energy for diffusion is interpreted to be low ered ion, w ill affect the rate of ion diffusion through the grain alo ne m ic a a nd c la v co nta cts, an d a diffu sio n m e ch a nism fo r boundary f luid during pressure solution. C ations are general- th is h a s b e e n p r o p o s e d b v b e a c h k l J ZSZ ), a n a lo g o u s t o t n a t ly m ore solvated than anions. Second, there is a tendency for in v ok ed by M ic h alske a nd 1,reim a n k1lizSZ ) to r trie pro ble m o i different ions to diffuse at different rates, determ ined by their size and charge. In order to m aintain electrical subcritical crack grow th by stress corrosion. neutrality it m ay be necessary for tw o ions, for exam ple of C alcite show s solid solution variation in com position, and calcium and carbonate, to diffuse at the rate determ ined by under hydrostatic stress the chem ical potential of the C aC 0 3 the m ore slow ly diffusing ion. H ow ever, cooperative or com ponent in equilibrium w ith an aqueous solution w ill de- coupled diffusion m ay be set up w hereby each ion has its ow n diffusion path and rate in a com plex reaction schem e w here the overriding condition is that overall electrical neutrality is m aintained. C om plex schem es w ith diffusion rates enhanced by cross-coupling m ay thus arise. Pressure solution of feldspars, m icas, chlorites and other m inerals involves solution of both ions and m olecules, fluid equilibria betw een these com plexes, and in m any cases pre- cipitation in proportions different to those produced by solution. Such m inerals are com m only involved in the pro- cesses leading to the form ation of slaty cleavage and spaced cleavage stripes, and therefore deserve m ore attention than ha s hith erto be e n de vo ted to th e m in this fie ld . M any of the pressure solution reactions referred to are w ell docum ented in hydrotherm al studies (H elgeson, 1969) and sufficient data exist to evaluate equilibria up to 300'C . S olutio n a n d D reC iDIta tio n m ech a nism s in vo lv e ion s su c h a s F igure 7: D etail of texture w ithin pressure solution stripe H 十，Na', K 十，and 匕a2 十 ， and thus pressure solution depends on s h o w n in Figure 5. T h e in c r e a s e in am ount and th e strong a com plex solid-fluid equilibrium , susceptible to changes in P , orientation of ph yllosilicates com pared w ith Figure 6 is T, PH 2 0 an d ionic concentrations. e v id e n t . F ie ld w id th 0 .1 m m .

2 4 E PISO D E S, Vol. 1982, N o. 4. pend on the com position of the solid solution. In deform ed in o rde r to un d ersta n d th e sc ale o n w h ich th e c h em ic al rocks, solid solution is seen to occur under non-hydrostatic processes involved in pressure solution w ere operating . stress w ith ferroan-calcite over-grow ths on calcite grains (Beach 1982). In these rocks pure calcite w as in local R efere nce s equilibrium w ith the grain boundary fluid at points of high A nderson, O .L. and G rew , P .C ., 1977. Stress corrosion theory stress w here solution occurred, and at points of low stress w here precipitation took place ferroan-calcite w as in local of crack propagation w ith applications to geophysics. equilibrium with the fluid. For the sam e differential stress, R ev. G eophys. Space Phys., v. 15, p. 77-104. chem ical potential gradients w ill be larger in the system Barnett, R .L. and K errich, R ., 1980. Stress corrosion crack- w here com positional variations occur than w here pure calcite ing of biotite and feldspar. N ature, London, v. 283, p. is both dissolved and precipitated, because of the chem ical 18 5 - 18 7 . potential change related to the com positional difference. Beach, A ., 19 74. A geochem ical investigation of pressure C onversely, pressure solution m ay proceed at low er values of solution and the form ation of veins in a deform ed grey- differential stress w here a com positional change can occur. w acke. C ontrib. M ineral. Petrol., v. 46, p. 61-68. In the developm ent of spaced cleavage stripes, both reduction Beach, A ., 1977. Vein arrays, hydraulic fractures and pres- in the activation energy for diffusion (through grain boundary sure solution structures in a deform ed flysch sequence, effects) and enhancem ent of chem ical potential gradients SW England. Tectonophysics, v. 40, p. 201-225. (through com positional changes and chem ical reactions) are thought to have occurred. In general therefore, pressure Beach, A ., 1979. Pressure solution as a m etam orphic process in deform ed terrigenous sedim entary rocks. Lithos, v. 12, 一 炙神少一监 p. 5 1-58. B each, A ., 19 82. D eform ation m echanism s in som e cover thrust sheets from the external French A lps. J. Struc- 诫肇烤零 萝 诀 肠 戈 tural G eol., v. 4, p. 137-149. 醛夔 ． 袱’攀鳄 争 ＿万含一丫几 B each, A . and K ing, M ., 1978. D iscussion on pressure 扩声熟＿厂 鑫 solution. J. G eol. Soc. London, v. 135, p. 649-651. 飞、 F letcher, R .C ., and Pollard, D .D ., 1981. A nticrack m odel for pressure solution surfaces. G eology, v. 9, p. 419-424. ， 升答蘸鬓 扮 嫩 嚷 鹜 窘 介 ︸ H eald, M .T., 1955. Stylolites in sandstones. J. G eol. v. 63, p. 卜铸 爆 香一锐丈 或 扮 16-30 . 骊 壕 撬 H elgeson, H .C ., 1969. T herm odynam ics of hydrotherm al sys- 鲜 鑫 蓄 黔分龚 ； tem s at elevated tem peratures and pressures. A m . J . Sci., v. 267, p. 729-804. Figure 8: Incongruent pressure solution. O ver}qrow ths o厂 K nipe, R .J., 1982. C leavage lam ella border structures and white m ica and quartz on a detrital quartz grain, as a result m icrochem istry . I n : A tlas o f D e fo rm ation al a n d M e ta - of pressure solution breakdow n of feldspar in a d e加rm ed m orphic R ock F a b r ic s B orradaile, G .J. et al. (eds.), greywacke. Field w id th 0.1 m m . Springer-Verlag, B erlin , p. 146-7. solution structures m ay develop at low er differential stresses M cC lay, K .R ., 1977. Pressure solution and C oble creep in than those required by R utter's (1976) analysis. A n exam ple rocks and m inerals: a review . J. G eol. Soc. London, v. is provided by the solution of detrital feldspars and the 134, p. 57-70. ‘ precipitation of phengite/m uscovite-quartz overgrow ths, M ichalske, T .A . and F reim an, S.W ., 1982. A m olecular inter- either pervasively through a rock or in spaced stripes (Beach pretation of stress corrosion in silica. N ature, London, v. and K ing, 19 78; Stephens et al., 1979; Beach, 1982). 295, no. 5849, p. 511一2. C on clusio n R utter, E.H ., 1976. T he kinetics of rock deform ation by Pressure solution structures need to be exam ined geom etri- pressure solution. R . Soc . Lond., P hilos. Trans., v. A 283, cally, m ineralogically, and chem ically. A n increasing atnount p. 203-2 19. of quantitative data on fine-grained m inerals w ilFbe required Stephens, M .B ., G lasson, M .J. and K eays, R .R ., 1979. Struc- from the electron m icroscope, so tha t chem ical reactions tural and chem ical aspects of m etam orphic layering de- occurring during pressure solution m ay be form ulated. H y- velopm ent in m etasedim ents from C lunes, A ustralia. A m . drotherm al solution chem istry w ill aid the definition of J. Sci., v. 279, p. 129一160. re alistic diffu sio n a nd re ac tio n m e ch an ism s. A constraint on pressure solution m ay be im posed by exam in- ing the overall m aterial balance. T here are m any exam ples A B O U T T H E A U T H O R : of the synchronous developm ent of pressure solution stripes A lastair Beach graduated from 味 轰 and veins and B each (1974) attem pted to quantify the balance o xfo rd U niversity in 1968, and of Si, C a and F e in such a system . F letcher and Pollard studied structural geology under (1981) develop the notion that the propagation of the frac- John R am say at Im perial C ol- tures and the stripes are m utally dependent, form ing closed lege, com pleting a Ph.D . there cells w here the crack volum e and the anticrack (i.e. stripe) o n d e fo rm a tion an d m e ta m o r- "antivolum e" are equal. phism in shear zones in the Lew - In term s of the chem ical ideas put forw ard, the propagation isian com plex. F ollow ing ten of cracks and stripes is controlled by the rates of chem ical years of lecturing at Liverpool reactions at their tips (and one m ay be evaluated from the U niversity, he has recently other), w hilst the rate of bulk strain (shortening across seam s joined the exploration team at and extension across veins) is controlled by the diffusion the British N ati o n al O il C orpora- m echanism related to the chem ical reactions. In studying tion (now Britoil), 150 St. Vin- rocks deform ed by pressure solution, an attem pt should cent Street, G lasgow , G 2 5LJ, therefore be m ade to determ ine the overall m aterial balance, U .K .

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