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.