Minerals, Rocks and Inorganic Materials

Transcription

1 Minerals, Rocks and Inorganic Materials Monograph Series of Theoretical and Experimental Studies 5 Edited by W. von Engelhardt, Tiibingen. T. Hahn, Aachen R. Roy, University Park, Pa.. P. J. Wyllie, Chicago, Ill.

2 Subseries: Isotopes in Geology

3 G.Faure JL.PoweU Strontium Isotope Geology With 51 Figures Springer-Verlag Berlin. Heidelberg New York 1972

4 Professor Gunter Faure Department of Geology The Ohio State University Columbus, Ohio/USA Professor James L. Powell Department of Geology Oberlin College Oberlin, Ohio/USA ISBN-13: e-isbn-13: DOl: / This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically those of translations, reprinting, re-use of illustrations, broadcasting, reproduction by photocopying machine or similar means, and storage in data banks. The use of general descriptive names, trade names, trade marks, etc. in this publication, even if the former are not especially identified, is not to be taken as a sign that such names, as understood by the Trade Marks and Merchandise Marks Act, may accordingly be used freely by anyone. Under 54 of the German Copyright Law where copies are made for other than private use, a fee is payable to the publisher, the amount of the fee to be detennined by agreement with the publisher. by Springer-Verlag Berlin Heiddberg Library of Congress Catslog Card Number Softcover reprint of the hardcover 1st edition 1972

5 Preface Since the end of World War II isotope geology has grown into a diversified and complex discipline in the earth sciences. It has progressed by the efforts of a relatively small number of specialists, many of whom are physicists, chemists, or mathematicians who were attracted to the earth sciences by the opportunity to measure and to interpret the isotopic compositions of certain chemical elements in geological materials. The phenomenal growth of isotope geology during the last 25 years is an impressive indication of the success of their efforts. We have now entered into a new phase of development of isotope geology which emphasizes the application of the new tools to the solution of specific problems in the earth and planetary sciences. This requires the active participation of a new breed of geologists who understand the nature and complexity of geological problems and can work toward their solution by a thoughtful application of the principles of isotope geology. It is therefore necessary to explain these principles to earth scientists at large to enable them to make use of the new information which isotope geology can offer them. This book was written to provide this kind of an introduction to the isotope geology of strontium. The isotopic composition of this chemical element, which is widely dispersed in most terrestrial and extraterrestrial rocks, is changed continuously as a result of the radioactive decay of naturally-occurring 87Rb to stable 87Sr. The resulting changes in the isotopic abundance of 87Sr can be used to date rocks and to obtain information about their origin. We have tried to explain the theoretical basis for these interpretations in such a way that they can be understood by nonspecialists in the earth sciences. Although this subject is relatively young, its literature is voluminous and widely scattered. We have aimed for comprehensive explanations of ideas and concepts, but make no claim to an exhaustive review of the literature. In the early 1960' s attempts to understand the distribution of strontium isotopes in nature were hampered by poor analytical precision and the resulting interlaboratory discrepancies. However, by about 1964 most laboratories were reporting results for an interlaboratory strontium-isotope standard that agreed within about ± 0.15 %. In this book the geologic conclusions drawn from strontium isotope data are generally based on

6 VI Preface many analyses made at several different laboratories; they do not depend on small differences in 87Sr abundance for a small number of analyzed samples. Therefore, the data employed here generally have not been corrected to a common value for an interlaboratory strontium-isotope standard. We dedicate this book to the memory of ARTHUR HOLMES, who was one of the first earth scientists to appreciate the importance of radioactivity and the resulting variation of isotopic compositions to geology. Many applications ofisotopic studies to the solution of geological problems which he foresaw are now becoming a reality. This book contains only a small number of the results of scientific research in isotope geology which he stimulated. We have benefited from helpful comments and discussions of several of our colleagues. We are especially grateful to ROBERT J. PANKHURST, who read the entire manuscript and helped us to improve it. We were also helped by PATRICK M. HURLEY, NORMAN K. GRANT, GERALD J. WASSERBURG, and WARREN HAMILTON, who read specific chapters and offered constructive comments. However, we take sole responsibility for any errors or omissions. This book is a joint venture, and while our names appear in alphabetical order, we share equally in the responsibility for its contents. It is a pleasure to acknowledge the friendly relationship which we have enjoyed with our patient editor, Dr. PETER J. WYLLIE. We set and passed many deadlines without once being taken to task for the delays in completing the manuscript. We thank our efficient secretaries who typed this manuscript: At Ohio State University we thank Miss CARLA MORRIS and Mrs. HELEN JONES, and at Oberlin College we acknowledge Mrs. JEANNE BOWMAN and Mr. HUBERT BATES. Above all, we hope that this book will be useful to advanced undergraduate and graduate students in geology departments everywhere. GUNTER FAURE JAMES L. POWELL

7 Contents 1. The Geochemistry of Rubidium and Strontium Introduction Rubidium Strontium The Rubidium/Strontium Ratio Summary II. Measurement of Geologic Time by the Rubidium-Strontium Method 9 1. Introduction Radioactive Decay of 87Rb to 87Sr The Half-life of 87Rb The Growth of Radiogenic 87Sr in Rocks and Minerals The Rubidium-Strontium Isochron Method of Dating A Useful Approximation Analytical Methods Summary ill. Uses of Strontium Isotopes in Petrogenesis Introduction Evolution of Strontium in the Continental Crust and the Upper Mande Applications to Petrology Summary IV. Volcanic Rocks Introduction The Strontium Isotopic Data for Volcanic Rocks. 28 a) General Comment b) Basaltic Rocks from Oceanic Islands 30 c) Sea-Floor Basalts d) Andesites of Volcanic Arcs e) Antarctic and Tasmanian Dolerites Variations in the Initial 87Sr/86Sr Ratio among and within Geologic Provinces a) Oceanic Islands b) Continental Volcanic Rocks Possible Causes of Variations in Isotopic Composition among Cogenetic Rocks Summary

8 VIII Contents V. Granitic Rocks Introduction The Granite Problem Observed Initial 87Sr/88Sr Ratios of Granitic Rocks Interpretation and Discussion of Initial nsr/88sr Ratios of Granites 46 a) Granites with Initial Ratios within the Basalt Field. 46 The Salisbury Pluton, North Carolina b) Granites with High Initial 87Sr/86Sr Ratios The Heemskirk Granite, Western Tasmania c) Granites with Intermediate Initial 87Sr/86Sr Ratios 50 TheIsle of Skye Phanerozoic Batholiths of North America Summary VI. Alkalic Rocks and Carbonatites Introduction Carbonatites Alkalic Rocks.. 59 a) Potassic Rocks 62 b) Sodic Rocks.. 64 c) Comparison of North American and African Alkalic Rocks Summary VII. Ultramafic and Related Rocks Introduction Sr/86Sr Ratios of Ultramafic Rocks. 66 a) The Layered Gabbro-Norite-Peridotite Association in Major Intrusions Anorthosites b) The Alpine-Type Peridotite-Serpentinite Association c) Ultramafic Rocks in Differentiated Basic Sills and in Minor Intrusions d) Kimberlites e) UltramaficNodules Summary VIII. Sedimentary Rocks and the Oceans 1. Introduction Marine Carbonate Rocks and the Oceans 3. Marine Evaporites Dating of Detrital Sedimentary Rocks. 5. Glauconite Nonmarine Carbonate Rocks. 7. Summary..... IX. Isotopic Homogenization of Strontium in Open Systems. 1.. Introduction,.. 2. Contact Metamorphism

9 Contents 3. Regional Metamorphism Open-System Behavior of Total-Rock Samples. 5. Chemical Weathering. 6. Summary X. Meteorites. 1. Introduction. 2. Rubidium and Strontium in Stony Meteorites 3. The Ages of Stony and Iron Meteorites The Initial 87Sr/86Sr Ratios of Stony Meteorites. 5. Summary XI. The Moon. 1. Introduction. 2. Lunar Basalt. 3. Initial nsr/86sr Ratios. 4. Luny Rock 1 and Sample The Soil. 6. Luna Summary IX XII. The Evolution of the Isotopic Composition of Terrestrial Strontium Introduction Evolution of Strontium in the Mantle Initial 87Sr/86Sr Ratios of Gabbro and Basalt 4. Continental Growth 5. Summary Appendix... References for Appendix Bibliography. Author Index Subject Index