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Pressure Solution and Coble Creep in Rocks and Minerals: a Review

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Pressure Solution and Coble Creep in Rocks and Minerals: a Review Pressure solution and Coble creep in rocks and minerals: a review K. R. McCLAY SUMMARY A brief review of research on pressure solution grained quartz and calcite rocks may give rise is given. The mechanisms of pressure solution to geological strain rates at temperatures from and Coble creep are discussed. Deformation by 2oo-35o°C. Coble creep is expected to give rise diffusive mass transfer processes is generally to geological strain rates in fine-grained galena accompanied by grain boundary sliding and at low temperatures and also in fine-grained this will have important effects on the textures calcite rocks at temperatures around 35o°(I. and microstructures produced during deform- The chemistry of natural rock pressure solution ation. Experiments using relaxation testing systems is expected to have significant effects seem promising in allowing access to the slow and needs further detailed study. Further strain rates necessary to observe pressure research is needed to elucidate the nature and solution phenomena. The thermodynamics of role of grain boundaries during diffusive mass non-hydrostatically stressed solids is discussed transfer. Pressure solution phenomena are and an analysis of possible diffusion paths in important in the compaction behaviour of some rocks is presented. Evaluation of theoretical petroleum reservoir rocks and perhaps in some rate equations for pressure solution and Coble faults where sliding is accommodated by creep indicates that pressure solution in fine- pressure solution. MANY ROCKS DEFORMED IN LOW GRADE METAMORPHIC ENVIRONMENTS, particularly in the range of T = i5o°C--35o°C and at confining pressures of 2--8 kb, i.e. up so lower greenschist facies (Fyfe I974, Turner I968 , p. 366) exhibit textures such as tectonic stylolites, truncated fossils, tectonic overgrowths and striped cleavages. These textures suggest deformation involving diffusive mass transfer processes. Since the pioneering work of Sorby (I863, I865, I9O8 ) and the early research at the turn of the century (e.g. Van Hise I9o4, Becke x9o3, Adams & Coker i91o), little advance was made until research interest was re- newed in the fifties and sixties (e.g. Heald I956 , Plessman I964, Weyl i959, Ramsay i967) and by the discussion of the thermodynamics of non-hydrostatically stressed solids (Correns i949, McDonald i96o , Kamb I96I , McLellan i966 , I968 , I970 and Paterson i973). A Tectonic Studies Group meeting held at Imperial College on 5 November I976 (see the Conference Report following this review) aimed to review current research on pressure solution and geological deformation which is accomplished by diffusive mass transfer. This paper gives a brief review of pressure solution and Coble creep (grain boundary diffusion) as deformation mechanisms in rocks and minerals. It has not been possible to make a complete survey of the literature or of current research work for this review, which is intended mainly as an introduction to the subject and to the synopses of the papers read at the Tectonic Studies Group meeting which follow. 3l geol. Soc. Lond. vol. x:~l, I977, pp. 57-7o, 4 figs, x plate. Printed in Great Britain. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/134/1/57/4885405/gsjgs.134.1.0057.pdf by guest on 30 September 2021 58 If. R. McClay I. Pressure solution and Coble creep (A) MECHANISMS Rock deformation mechanisms may be divided into three broad categoriesg (I) cataclastic processes, (2) intracrystaUine processes involving dislocation move- ments, (3) diffusive mass transfer processes. If a grain boundary is subjected to compressive and tensile stresses, then a chemical potential gradient for vacancy flow is set up (see review by Burton 1977, Nabarro 1948 and Herring 1950). The flux of matter, which is equal and opposite to the vacancy flux, leads to depletion of material at relatively compressive boundaries and deposition at relatively tensile boundaries. If the diffusion of matter is essentially through the grains, i.e. lattice diffusion, the process is termed Nabarro-Herring creep, whereas if it is predominantly around the grain bound- aries, the flow is called Coble creep (Coble 1963). Grain neighbour switching in superplastic flow (Ashby & Verrall 1973, Edington et al. 1976) is essentially grain boundary sliding with accommodation by diffusional creep. Lattice diffusion (Nabarro-Herring creep) is important at high temperatures in metals (see review by Burton 1977) and perhaps in the Earth's mantle (Stocker & Ashby 1973). At lower temperatures, however, lattice diffusivities and solid state grain boundary diffusivities are expected to be too slow to account for the ob- served strains found in low grade metamorphic rocks (see section on rates of deformation later in this paper). Many textures in low grade metamorphic rocks (T = 2oo-35o°C) are interpreted as indicating deformation by diffusive mass transfer (P1. i A, B. C). It is therefore inferred that the presence of a fluid phase in the grain boundaries enhances diffusive mass transfer in low grade metamorphic rocks--hence the term 'pressure solution'. (B) THE EVIDENCE FOR PRESSURE SOLUTION IN ROCKS Evidence for pressure solution in rocks and minerals is readily found, from a macroscopic scale down to the microscopic scale. Since Sorby (I 865) first ascribed the pitting of pebbles (P1. IB) to pressure solution, many authors have discussed the textures and structures attributed to this form of diffusive mass transfer (e.g. Voll I96O , Ramsay 1967, Plcssman 1964, 1972 , Spry 1969, Groshong I975a and b, Trurnit 1968 , Ayrton & Ramsay 1974, Kerrich 1975, Logan & Semeniuk 1976 , Mitra 1976 ) . Limestones display a wide variety of pressure solution phenomena which occur both during diagenesis and deformation. Stylolites are extensively developed in many limestones (Arthaud & Mattauer 1969, 1972 ) with an accumulation of quartz and phyllosilicates in the stylolite and clay seams which often truncate fossils (P1. IA). Logan & Semeniuk (1976) have described in detail pressure solution and stylolite development in the limestones of the Canning Basin, Western Australia. The formation of some rock cleavages is often thought to involve pressure solution (Borradaile I977, Cosgrove 1976, Alvarez et al. 1976 , Williams 1972 , Groshong I975a, Siddans 1972 , Durney i972b ). Detailed high voltage electron microscopic studies by Knipe & White (I977) , however, have shown the corn- Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/134/1/57/4885405/gsjgs.134.1.0057.pdf by guest on 30 September 2021 Pressure solution and Goble creep in rocks 59 plexities of cleavage development and in particular they emphasize the effects of crystallization involving the formation of new phyllosilicates. Striped (or spaced) cleavage in limestones (Alvarez et al. 1976) and in the greywacke rocks of South Devon (P1. I C) has been attributed to pressure solution processes (Ramsay I967). Beach (I974) and Kerrich (1975) have studied the chemical changes resulting from pressure solution of the Devonian greywackes of SW England and found that at least some of the vein minerals are supplied from the surrounding rock. Differentiation of minerals into low pressure zones during folding (Stephansson 1974) has often been attributed to pressure solution, particularly during the formation of crenulation cleavages (Dumey I972a, Williams I972, Cosgrove 1976 ). Tectonic overgrowths (P1. IA) which often have a fibrous appearance (Durney & Ramsay 1973, Mitra 1976 ) and pressure shadows (Stromgard 1973) are also considered as evidence of pressure solution processes in low grade metamorphic rocks. Mitra (1976) has presented a very elegant analysis of deformation mechan- isms in quartzites in which he has been able to separate strain components due to dislocation creep and to pressure solution. A very detailed and historical review of research on pressure solution and of pressure solution structures is given by Kerrich (I977). (C) THE EXPERIMENTAL EVIDENCE Grain boundary diffusional creep was first postulated by Coble (1963) for creep in AltOs. Coble creep was later found to operate in pure magnesium (Jones 1965) and in other metals and ceramics such as copper (Burton & Greenwood 197o ) and cadmium (Crossland I974) , (for details see Burton 1977). Grain bound- ary creep was found to have a lower activation energy than that for lattice dif- fusion and a strain rate dependence on the reciprocal of the grain size cubed (see later section on rate equations). It has been found to be important in metals at low homologous temperatures (,-o o.6 T melting, Burton 1977) whereas Nabarro- Herring creep occurs only at high temperatures (,~ o. 9 T melting). t ~IG. I. A B ,I, C EXIENSION A. Undeformed hexagonal array of grains with marker line. The compressive stress axis is horizontal. B. If diffusional creep occurs without grain boundary sliding (i.e. the centres of the hexagonal grains and the marker line are not displaced) then there would be a volume increase and voids (black areas) would form against the compressive stress. This is an unlikely mechanism and the diffusional creep is accompanied by grain boundary sliding-- I (], C. Diffusional creep with grain boundary sliding resulting in offset of the marker line and no voids forming (constant volume deformation). Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/134/1/57/4885405/gsjgs.134.1.0057.pdf by guest on 30 September 2021 60 If. R. McClay Diffusional creep in polycrystals whether Nabarro-Herring or Coble creep, is generally
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