Quantitative Textural Measurements in Igneous and Metamorphic Petrology
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This page intentionally left blank QUANTITATIVE TEXTURAL MEASUREMENTS IN IGNEOUS AND METAMORPHIC PETROLOGY Processes involved in the development of igneous and metamorphic rocks involve some combination of crystal growth, solution, movement and defor- mation, which is expressed as changes in texture (microstructure). Recent advances in the quantification of aspects of crystalline rock textures, such as crystal size, shape, orientation and position, have opened new avenues of research that extend and complement the more dominant chemical and isotopic studies. This book discusses the aspects of petrological theory necessary to under- stand the development of crystalline rock texture. It develops the methodolo- gical basis of quantitative textural measurements and shows how much can be achieved with limited resources. Typical applications to petrological problems are discussed for each type of measurement. The book has an associated webpage with up-to-date information on textural analysis software, both commercial and free. This book will be of great interest to all researchers and graduate students in petrology. M ICHAEL H IGGINS is Professeur des Sciences de la Terre at the Universite´ du Que´bec a` Chicoutimi, Canada. He is a member of the Geological Association of Canada. QUANTITATIVE TEXTURAL MEASUREMENTS IN IGNEOUS AND METAMORPHIC PETROLOGY MICHAEL DENIS HIGGINS Sciences de la Terre, Universite´ du Que´bec a` Chicoutimi, Canada. Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo Cambridge University Press The Edinburgh Building, Cambridge ,UK Published in the United States of America by Cambridge University Press, New York www.cambridge.org Information on this title: www.cambridg e.org /9780521847827 © M. D. Higgins 2006 This publication is in copyright. Subject to statutory exception and to the provision of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First published in print format 2006 - ---- eBook (EBL) - --- eBook (EBL) - ---- hardback - --- hardback Cambridge University Press has no responsibility for the persistence or accuracy of s for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate. To Ester and Zoe Contents Acknowledgementspageix 1Introduction1 1.1 Petrological methods 1 1.2 Qualitative versus quantitative data 2 1.3 What do I mean by texture? 3 1.4 Information density and data sources 4 1.5 Structure of this book 5 1.6 Software applications for quantitative textural studies 5 2 General analytical methods 7 2.1 Introduction 7 2.2 Complete three-dimensional analytical methods 10 2.3 Extraction of grain parameters from data volumes 13 2.4 Destructive partial analytical methods 14 2.5 Surface and section analytical methods 16 2.6 Extraction of textural parameters from images 26 2.7 Calculation of three-dimensional data from two-dimensional observations 33 2.8 Verification of theoretical parameter distributions 35 2.9 Summary 38 3 Grain and crystal sizes 39 3.1 Introduction 39 3.2 Brief review of theory 42 3.3 Analytical methods 74 3.4 Typical applications 102 vii viii Contents 4 Grain shape 135 4.1 Introduction 135 4.2 Brief review of theory 136 4.3 Methodology 143 4.4 Typical applications 154 5 Grain orientations: rock fabric 165 5.1 Introduction 165 5.2 Brief review of theory 166 5.3 Introduction to fabric methodology 170 5.4 Determination of shape preferred orientations 171 5.5 Determination of lattice preferred orientations 174 5.6 3-D bulk fabric methods: combined SPO and LPO 177 5.7 Extraction of grain orientation data and parameters 181 5.8 Typical applications 184 6 Grain spatial distributions and relations 199 6.1 Introduction 199 6.2 Brief review of theory 200 6.3 Methodology 204 6.4 Typical applications 212 7 Textures of fluid-filled pores 216 7.1 Introduction 216 7.2 Brief review of theory 217 7.3 Methodology 222 7.4 Parameter values and display 226 7.5 Typical applications 228 8 Appendix: Computer programs for use in quantitative textural analysis (freeware, shareware and commercial) 232 8.1 Abbreviations 232 8.2 Chapter 2: General analytical methods 232 8.3 Chapter 3: Grain and crystal sizes 235 8.4 Chapter 4: Grain shape 235 8.5 Chapter 5: Grain orientations (rock fabric) 236 8.6 Chapter 6: Grain spatial distributions and relations 236 References 237 Index 260 Acknowledgements The inspiration for this book came from a short course that I gave in September 2001 in Ankara, Turkey. I would like to thank Durmus Boztug, Cumhuriyet University, Bahadir Sahin, MTA Anakara, the Turkish Chamber of Geological Engineers, TUBITAK and MTA, Anakara for making that short course possi- ble. This book was started while I was on sabbatical at Victoria University, Victoria, Canada and I would like to thank the department, and particularly Dante Canil for hosting me. I would like to thank my readers for their com- ments: Marian Holness, Cambridge University, Ned Chown, Paul Be´dard and Pierre Cousineau, Universite´du Que´bec a` Chicoutimi, Dougal Jerram, Durham University, Lawrence Coogan, University of Victoria, Keith Benn, University of Ottawa, Alison Rust, University of British Columbia, and Judit Ozoray. J. M. Derochette drew my attention to the Benson plate. Data were supplied by Ilya Bindeman, Margaret Mangan and Ronald Resmini. Patrick McLaren of Geosea Consulting, Victoria, showed me how sediment sizes are measured. I would also like to acknowledge Bruce Marsh, Johns Hopkins University, and Kathy Cashman, University of Oregon, without whom CSD research would never have moved along so fast. The Natural Science and Engineering Research Council of Canada and the Fondation de l’UQAC have supported my research and have enabled many of the studies presented here. Last I would like to thank my colleagues at UQAC, my students and my family. ix 1 Introduction I do not know what I may appear to the world, but to myself I seem to have been only like a boy playing on the sea-shore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me. Sir Isaac Newton Newton’s ‘Ocean of Truth’ seems to me more like a landscape: the plains are densely populated with information and most researchers work there. It is not easy to get a perspective on such a mass of information without climbing the surrounding hills. Valleys in the mountains may be difficult to find, and sparsely populated, but some lead to new basins of information ready to be explored. Other valleys may be so deep that we can glimpse what they contain, but cannot explore them closely, or even at all. This book is a guide to a country set in that landscape. Like real countries, its name varies according to who you ask, and the borders do not always follow geographic features. And like most travel writers, my description of the landscape is coloured by where I come from and what I have done. 1.1 Petrological methods In petrology1 we generally examine the results of natural experiments and have to tackle the inverse problem of what happened to what starting material to produce the rock that we observe. This problem is complex and does not usually have a unique solution: we must decide what the most likely starting material was and what the dominant processes were. The general approach is to choose a number of parameters and quantify them in the final rock. A starting material is proposed and various processes applied to this material. The results of this modelling are then compared to the rock in question. 1 2 Introduction All rocks comprise an assemblage of mineral grains of different sizes, orientations and shapes, which may be observed in many different ways. Early petrological studies of rocks used observations in the field and with a polarising microscope. The value of chemical analyses was appreciated, but the effort involved to analyse a small number of samples was considerable. The development of instrumental methods of chemical and isotopic analysis chan- ged this and now such methods have become very important. In fact, many geologists view igneous and metamorphic petrology as simply applied mineral, chemical and isotopic analysis. This is a rather narrow view and in this book I hope to show that quantitative analysis of textures can also help unravel the evolution of materials. In a way, textural methods are more direct than geochemical or isotopic methods as the most important processes involve changes in the size, shape, position and orientation of the crystals. However, it is better to view all these methods as complementary. This book is aimed at researchers in igneous and metamorphic petrology at all levels. Some workers may not be aware of the potential of textural studies to solve problems that they have previously attacked only with chemical or isotopic methods. Others may have started textural studies, but are unaware of what has been done in other subfields of petrology. An ideal petrological study would combine relevant theory, appropriate analytical methods and application to real rocks. However, petrologists are human and in many cases one aspect is developed to the detriment of the other: the siren calls of theory and methods seem to be heard more often than application to rocks. I hope that readers will be able to combine both high- quality measurements with well-grounded theory to advance our understand- ing of the origin of rocks. 1.2 Qualitative versus quantitative data Although the patterns and structures of rocks have been observed and recorded since early times, it was only in the nineteenth century that the development of the polarising microscope enabled a vast increase in the detail and number of petrological studies.