Methods in Rock Magnetism and Palaeomagnetism. Techniques and instrumentation

Transcription

1 Methods in Rock Magnetism and Palaeomagnetism Techniques and instrumentation

2 Methods in Rock Magnetism and Palaeomagnetism Techniques and instrumentation D. W. Collinson Department of Geophysics and Planetary Physics University of Newcastle upon Tyne, UK SPRINGER-SCIENCE+BUSINESS MEDIA. B.V.

3 1983 D. W. Collinson Originally published by Chapman and Hall in 1983 ISBN All rights reserved. No part of this book may be reprinted, or reproduced or utilized in any form or by any electronic, mechanical or other means, now known or hereafter invented, including photocopying and recording, or in any information storage and retrieval system, without permission in writing from the Publisher. British Library Cataloguing in Publication Data Collinson, D. W. Methods in rock magnetism and palaeomagnetism techniques and instrumentation. 1. Magnetism, Terrestrial I. Title 538'.7'028 QC815.5 ISBN ISBN (ebook) DOI / Library of Congress Cataloging in Publication Data Collinson, D. W. (David Wright) Methods in rock magnetism and palaeomagnetism. Bibliography: p. Includes index. l. Rocks-Magnetic properties. 2. Paleomagnetism. I. Title. QE431.6.M3C '.7 ISBN

4 To J.M.C., for her help and encouragement during the writing of this book, and F.J.C., who would have preferred the book to remain unwritten but showed continued if distracting interest in it.

5 Contents Preface 1 Rock magnetism and magnetic minerals Introduction Magnetic minerals Iron and nickel-iron The titanomagnetite series The titanohaematite series Maghemite Pyrrhotite Goethite 13 2 Initial magnetic susceptibility Introduction Diamagnetism and paramagnetism Ferromagnetic susceptibility Superparamagnetism The Hopkinson effect Measurement of initial susceptibility Direct field measurements Alternating field measurements Ballistic methods Calibration Frequency dependence of susceptibility Initial susceptibility of rocks Anisotropic susceptibility Introduction Measurement of anisotropic susceptibility General comments 54 3 High-field measurements Introduction IRM and Bcr measurements Coils Electromagnets 63 xiii vii

6 viii Contents Iron-free solenoids Measurement of IRM of single particles Measurements of -'i, high field susceptibility and hysteresis curves Magnetic balances The vibrating sample magnetometer Torque magnetometers and measurement of rotational hysteresis Hysteresis measurements and loop tracing 98 4 High- and low-temperature measurements High-temperature measurements Equipment Initial susceptibility Curie points and Ji-T curves General comments Ill 4.2 Low-temperature measurements General Low-temperature variation of NRM Other measurements Controlled fields and field-free space Coil systems Solenoids and single coils Helmholtz coils Multicoil systems Coil control systems Magnetic shields Electromagnetic shields (alternating fields) Magnetic shielding against static fields Superconducting shields Miscellaneous techniques Separation methods Non-magnetic methods Magnetic methods Colloid techniques Piezomagnetism and shock effects Viscous remanent magnetization Anhysteretic remanent magnetization Depositional remanent magnetization A brief review of palaeomagnetism Primary magnetization Secondary magnetization 184

7 Contents 7.3 Palaeomagnetic surveys and interpretation Other aspects of palaeomagnetism The sampling of rocks for palaeomagnetism General Sampling techniques Rock sampling Lake sediments Sampling for archaeomagnetism Soft sediments, muds, etc Sample preparation in the laboratory Orientation methods Hand samples Field-drilled cores The Sun compass Measurement of the NRM of rocks Magnetization of rock samples Introduction Magnetic field of non-spherical samples Inhomogeneity of magnetization Other considerations Historical development of rock magnetometers Magnetostatic instruments The astatic magnetometer Parastatic systems Application of feedback Calibration and measurement of NRM Use of fl.uxgate sensors Spinner magnetometers General design features Development of spinner magnetometers Calibration and measurement The cryogenic magnetometer Operating principle The SCT and CCL cryogenic magnetometers Other cryogenic magnetometers Reliability of weak NRM measurements Other magnetometers Archaeomagnetic instruments Long-core measurements Ballistic magnetometers Experimental magnetometers Small-scale magnetometers Measurement of NRM in the field 297 ix

8 X Contents 10 Field and laboratory stability tests Field tests Observational data Fold test Conglomerate test Baked contacts Magnetic profiles Laboratory stability tests Magnetic cleaning techniques Alternating field cleaning Introduction Practical considerations a.f. demagnetizing equipment Some aspects of a.f. demagnetization Thermal demagnetization Introduction Zero field cleaning Stepwise thermal demagnetization Continuous demagnetization General comments Other cleaning techniques Chemical demagnetization Low-temperature demagnetization Pressure demagnetization Presentation and treatment of data Presentation of NRM directions Demagnetization data and their interpretation Analysis of demagnetization data Stability indices Statistics Mean NRM direction, precision and circle of confidence Testing 'goodness of fit' Test of randomness Combination of observations from a rock unit Statistical significance of the fold test Calculation of pole positions Identification of magnetic minerals and carriers of NRM Introduction Thermal analysis Chemical methods The depositional magnetization technique 405

9 Contents 13.5 Additional data on magnetic minerals The magnetic state of magnetic minerals Intensity of NRM Introduction Sediments Igneous rocks Palaeointensity techniques Introduction The Thellier-Thellier method Single-heating methods Mineralogical alteration Anhysteretic remanent magnetization methods Presentation of data 443 Appendix 1 Conversion factors between SI and c.g.s. units 446 Appendix 2 Demagnetizing factor of ellipsoids 448 Appendix 3 Copper wire data 450 Appendix 4 Preparation of colloidal magnetite 451 Appendix 5 Correction ofnrm directions to field reference systems 452 Appendix 6 Measurement of the magnetic field in a coil 456 Appendix 7 The off-centre measurement method with the astatic magnetometer -correction terms for cyl- 458 indrical samples and the upper magnet Appendix 8 Detection of mains asymmetry 460 References Index xi

10 Preface During the last 30 years the study of the magnetic properties of rocks and minerals has substantially contributed to several fields of science. Perhaps the best known and most significant advances have resulted from the study of palaeomagnetism, which led to quantitative confirmation of continental drift and polar wandering through interpretation of the direction of remanent magnetism observed in rocks of different ages from different continents. Palaeomagnetism has also, through observations of reversals of magnetization, ancient secular variation and ancient field intensities provided data relevant to the origin of the geomagnetic field, and other investigations have contributed significantly to large-scale and local geological studies, the dating of archaeological events and artefacts and more recently to lunar and meteoritic studies. Rock and mineral magnetism has proved to be an interesting study in its own right through the complex magnetic properties and interactions observed in the iron-titanium oxide and iron sulphide minerals, as well as contributing to our understanding of remanent magnetism and magnetization processes in rocks. Simultaneous with the development of these studies has been the development of instruments and techniques for the wide range of investigations involved. The need for reliable measurements of the remanent magnetization of more weakly magnetized rocks has resulted in the development of sensitive magnetometers and improvements in the techniques for the removal of unwanted magnetizations. Much effort has been expended in improving methods for deriving ancient magnetic field intensities from terrestrial and extraterrestrial rocks and in devising instruments and techniques for identifying and characterizing magnetic minerals. Along with these developments have been those concerning new methods of presentation of palaeomagnetic data and of techniques for deriving the most reliable estimate of ancient magnetic field directions and pole positions. Details of many of these instruments and techniques are in the literature but scattered rather widely, and others remain unpublished yet are deserving of wider knowledge. It is the purpose of this book to bring together for easy reference the range of instruments and techniques currently in use, and also to describe some of those used in the past and now less used but which may still be of interest and of use for particular investigations. It is hoped that the book xiii

11 xiv Preface will be useful both to those already working in the subject and to those with either a geological or physical background embarking on studies of any of the many aspects of the magnetism of rocks. Although equipment for routine palaeomagnetic and rock-magnetic studies is now commercially available, information is given in the book to enable such equipment to be built in the laboratory and also more specialized equipment for particular studies. The emphasis is on the basic principles of the various instruments and techniques. Signal processing and the use of computers and microprocessors is not described in detail as information on these aspects of instrumentation is available from other sources. Workers in palaeomagnetism and rock magnetism have been among the most reluctant converts to SI units, but an increasing number are now using them. In the author's opinion the use of c.g.s. is now no longer justified and SI units are used in the book. A table of conversion factors between c.g.s. and SI units is given in Appendix 1, together with some notes on the SJ units used in the book. I am grateful to many of my colleagues both in this country and abroad for discussion and information on various topics. Particular thanks are due to M. 1. Gross, F. 1. Lowes, L. Molyneux, 1. H. Parry, W. O'Reilly, 1. S. Rathore, P. V. Sharma, A. de Sa, A. Stephenson, M. Stupavsky and D. H. Tarling. I am grateful to Mrs L. Boon, Miss S. Bell, Miss A. Codling and Mrs L. Whiteford for jointly typing the manuscript and to Mrs D. Cooper for drawing the diagrams. D. W. Collinson Newcastle upon Tyne England