ASPECTS OF PRESSURE SOLUTION AS A DEFORMATION MECHANISM - by - Robert Kerrich Imperial College, London. 1974. CONTENTS ACE I-OVIT,7,DGEITTS L23 inTh1QT =AUER 1 Introduction to concepts of pressure solution . • 1- 10 CHAPTER 2 The historical development of research on pressure solution and related topics 11- 60 PREFACE TO CHAPTERS 3 AND 4 .. • • • • 61- 63 CHAPTER 3 Fluid inclusion studies of tectonic veins 64-120 CHAPTER 4 Oxygen isotope geochemistry applied to tectonic environments 121-174 `CHAPTER 5 Pressure solution and deformation mechanism fields • • • • 175-214 * CHAPTER 6 The interrelationship of pressure solution and tectonic veining 215-220 CHAPTER 8 The distribution of chemical elements and mineral species in the deformed matrix around rigid cylindrical inclusions 231-243 CONCLUSIONS. APPENDIX I Density measurements on fluid inclusions 244 APPENDIX II H20/C02 determinations on fluid inclusions 244 APPENDIX III Measurements of salinity of fluid inclusions 244 APPENDIX IV Preparation of mineral separates for 245-250 isotopic determination • • • • APPENDIX V Fluorination and mass spectrometry 251-254 APPENDIX VI 180/160 values of mineral separates 255 analysed • • • ACKNOWLEDGMENTS I would like to thank my supervisor, Dr N.J. Price, for help and stimulating comment throughout this research. Dr 1. 3uoh made representations of the oxygen isotope project to the 'Universities Stable Isotope Comrnitt 0.1.1.-Lv kindly ap17)7..oved the work. Isotopic analyses were done at n.':`]. R. Aldermarston by J. Durham, to whom my thanks are ex- tended for instruction in fluorination 1. - mass spectrometers. Dr R.D. Beckinsale has given much useful advice concerning applications of stable isotope geochemistr71 and has critically read t'ne manuscript of chapter 4. Carbonate determinations were kindly made by Dr Y. Shackleton and Er E. gall at the department of Pleistocene Research, Cambridge. Mr D. Brig s and Er D. 3ernard provided help and instruc- tion in mineral separation procedure. Facilities at the Institute of Geological Sciences have been used, with the kind permission of Dr IT. Snelling. The 1120 /CO2 determinations on fluid inclusions were done by Dr T. Sheppard of I.G.S., who has been generous in allowing use of his microscope freezing stage apparatus. The X-RF at ire nerial College has been used for geochemi- cal analyses by kind permission of Dr G. Borley, who has also advised on theoretical aspects of X-ray spectrometry and geo- chemistry. Er G. Bulland gave practical instruction in sample .preparation and running of the spectrometer. Data reduction was kindly done by Mr R. Parker. Samples for analysis by XRD were run by Mr R. Curtiss, who helped with interpretation of the output. Thanks are due to Mr J. Blount and Mr D. Bailey for rock cutting and polishing services respectively. Dr G. Borley has given freely of facilities for making photomicrographs. Mrs B. Richardson and her staff in the photographic department have provided help and instruction in producing the plates. Professor J.G. Ramsay, Drs A. Siddans, P.R. Cobbold, A. Ries, S. Schmidt, and M. Casey, A. Bell and C. Taylor have kindly suggested field areas suitable for investigating press ure solution. Much benefit has accrued from discussions with Drs E. Rutter and S. White, to whom I am most grateful for critical reading of the manuscript. In particular I would wish to extend my thanks to Martin Casey. Much of our research has run in parallel, and an in- valuable interchange of ideas has taken place during the two years in which we have shared a room. This research has been undertaken during tenure of a Natural Environment Research Council grant. ABSTRACT Pressure solution, which may be defined as the solution, dif- fusion, and precipitation of rock forming minerals in re- .sponse to sl3ress fields, bas lon7 been reco— ed as an im po tant mode by :hich low grade metamorphic rocks deform. forraton on the PT conditions of crustal environments in which pressure solution is the dominant de- formation mech,sni= has been obtained using oxr:Ren isotoT:e and fluid inclusion thermometry. In addition, data from the former technique has been used to define empirically the thermal stability limits of some metamorphic mineral assemb- lages. Field work for this research has been conducted in the Dalradian Series of Southwest Scotland, and Palaeozoic rocks of the Central Pyrenees. Estimates of the crustal conditions over which pressure solution is an important deformation mechanism in dominantly quartz bearing assemblages are 30°C to 450°C ± 50°C, ir- respective of the crustal depth ( =mean stress). Oxygen isotope geochemistry has been used to assess the equilibrium relations between tectonic veins and their host rocks. In addition, thermometric data on veins, obtained from this method, has been related to the mineral assemblages and deformation mechanisms characteristic of tectonic veins at different crustal levels. Some information is adduced from data on fluid inclusions and oxygen isotopes, in minerals from tectonic veins, concern- ing the nature and sources of fluids in tectonic environments. Attention has been directed towards the problem of non- equilibrium isotopic fractionation in stress fields. Quantitative data relating to the distribution of chemi- cal and mineral species around cylindrical inclusions with pressure shadows, has been obtained from XRF and XRD. The measured distribution of chemical and mineral species is re- lated to stress fields, and to considerations of kinetic effects. Aspects of tectonic striping are discussed with regard to the physics of diffusion. 3. CHAPTER I Introduction to concepts of pressure solution Terms and definitions. Pressure solution is a term which is widely used to de- scribe the phenomenon of solution, diffusion, and precipitation of rock forming minerals. Together, these three elements of the process constitute an intercrystalline mechanism by which rocks may deform. Although use of 'pressure' imparts a genetic sense to the term there can be little serious doubt that 'press- ure effects', or 'stress' play an important role in this process. In addition, 'solution' bears the implication that pore fluids are present in the rock system, and that the diffusive element of pressure solution takes place through such a fluid phase. There is an ill-defined state between pore fluids and water adsorbed at grain boundaries. This has led to use of the term 'water assisted diffusive processes' to describe the pheno- menon, in preference to pressure solution. However, the former term is sufficiently general to include many metamorphic re- )actions, in addition to at least one intracrystalline deforma- tion mechanism. Pressure solution, as understood above, bears a close re- lation to a process known in the metallurgical literature as 'Coble Creep'. Coble creep is a mechanism whereby deformation is achieved through a transfer of atoms from points of high stress to points of low stress along grain boundaries. No fluid phase is involved in this process of 'grain boundary diffusion'. There are disadvantages to the use of either 'Coble Creep' or 'grain boundary diffusion' in reference to the geological • phenomenon. First, there is a certain knowledge that in some instances diffusion takes place through a fluid phase, rather than exclusively along grain boundaries. Second, the latter term bears no inference concerning the role of stress or deforma- tion in this process. These are two factors which have been in- timately associated with pressure solution at a descriptive level, since inception of the concept. Solution transfer also suffers from this inadequacy. Grain boundary diffusion could equally be used to describe the growth of authigenic minerals during diagenesis, or granitisation by grain boundary ichors. In conclusion, it would seem that pressure solution remains the most appropriate term to cover the process of solution, dif- fusion, and precipitation, of rock forming minerals in response to differential stress. It should be noted, however, that some workers do not consider that such a mechanism exists at all. Boundaries of the phenomenon. There is a concensus of opinion in the` literature, arrived at by subjective means, that pressure solution is an important mode by which rocks at a low grade of metamorphism deform. Pressure solution is known also to play a significant role in the diagenesis of some sediments. At deeper crustal levels, - there is no satisfactory distinction between mineral segrega- tion by pressure solution, and what is loosely termed metamorph- ic differentiation. There may in fact be no real difference, because they are probably both the result of diffusion. Such confusion reflects the lack in detailed understanding of physi- cal processes in the earth. ,In this thesis pressure solution is taken as a deformation mechanism that generally leads to small scale mineral segregation, and which is commonly active • in high and intermediate crustal levels. Features such as gneissic layering, ultrabasic pods, or pegmatites, which are formed in high grade metamorphic terrain, are not included with- in the context of pressure solution. The term pressure solution does not extend to include hydrothermal transfer of minerals in a moving fluid s7stem. Pressure solution - the problem. As a prelude to any research in this thesis a comprehensive literature survey on pressure solution and related topics was undertaken. This survey served two main purposes. First, to establish the state of the science with regard to pressure solution. Second, as an aid to providing a broad perspective from which worthwhile research problems could be evaluated. A detailed account of the historical development of re- search on pressure solution and related topics was written directly from the synthesis of literature, and is presented as a review of the subject in chapter 2. Although the survey covers a wide range of topics, some of which are discussed in- dividually in different chapters, it was decided that a self- contained review would make for a more unified presentation of the subject.