Radiogenic Isotopes in Geologic Processes

Transcription

1 Radiogenic Isotopes in Geologic Processes

2 Radiogenic Isotopes in Geologic Processes Sergei V. Rasskazov Sergei B. Brandt Ivan S. Brandt Institute of the Earth s Crust, Siberian Branch of the Russian Academy of Sciences, Irkutsk, Russia 2123

3 Prof. Sergei V. Rasskazov Prof. Sergei B. Brandt Institute of the Earth s Crust Institute of the Earth s Crust Russian Academy of Sciences Russian Academy of Sciences Lermontov Str. 128 Lermontov Str. 128 Irkutsk, Russia Irkutsk, Russia Irkutsk State University Lenin Str. 3 Irkutsk, Russia rassk@crust.irk.ru Dr. Ivan S. Brandt (deceased) ISBN e-isbn DOI / Springer Dordrecht Heidelberg London New York Library of Congress Control Number: c Springer Science+Business Media B.V No part of this work may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission from the Publisher, with the exception of any material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. Cover design: Volcanoes in Middle Gobi (photograph taken by S. Rasskazov) and a fragment of Fig in this book. Revised and updated edition of the book in Russian Radioizotopnaya geologiya v zadachakh I primerakh, Novosibirsk, Academicheskoye izdatel stvo Geo, S.V. Rasskazov, S.B. Brandt, I.S. Brandt et al./e.v. Sklyarov, S.V. Rasskazov (eds), 2005 Printed on acid-free paper Springer is part of Springer Science+Business Media (

4 Ivan S. Brandt Ivan S. Brandt graduated from Moscow State University with a degree in physics and mathematics. At the Institute of the Earth s Crust SB RAS he studied kinetics of radiogenic isotopes in geologic processes, both theoretically and experimentally. He died of cancer when the manuscript of this book was under preparation.

5 Preface Do we actually understand geologic processes? New technology brings new information and perceptions, which sometimes overturn imaginations based on simple observation and estimation, in conjunction with common sense inference. In , Pierre Curie and Ernest Rutherford first formulated the idea of using radioactive transformation of nuclides as a geologic chronometer. After a century of working with such tools, geology has advanced from a descriptive science to an analytic science that formulates conclusions based on exact values. The technology of radiogenic isotope geology has created a branch of science that considers the Earth as a planet generated within a Solar system and studies the subsequent evolution of geologic processes that has resulted in the present formation of our planet s continents and oceans. The physicist Vitaly Ginsburg, Nobel Prize laureate, wrote recently: If Kepler had been given information on orbital parameters of planets with modern precision, he would not have been able to formulate his laws. Indeed, after development of laws of celestial mechanics, methods of measurements became so advanced and such numerous secondary distortion effects were found that to describe an orbit of a cosmic body by a curve of the second order would appear impossible. But it does not mean that Kepler s laws are cancelled ; they still occupy an honorable place in courses on celestial mechanics. A reasonable division into basic and secondary phenomena is accepted and the latter are entered as variations in the basic equations. Application of radiogenic isotopes for study of geologic processes showed that accumulation of daughter substances, according to the Rutherford Soddy Law of Radioactivity, in some cases was interfered with by other circumstances. Authors often gave only verbal explanations of these phenomena without appropriate differential equations. Present-day radiogenic isotope geology, however, expresses results in values that can claim a high degree of accuracy. Similar to the study of celestial mechanics, understanding of natural isotopic systems requires a deliberate selection in geologic processes of the main and secondary effects. New data, technological breakthroughs, and changes of paradigms inevitably result in changes of approaches in geochronology and isotope geochemistry. Only the requirements for selection of material for an isotope analysis remain constant from the geologic point of view. A geologist aspires to obtain data most appropriate to vii

6 viii Preface satisfactory geochronological interpretations using isochron or Concordia Discordia diagrams. Additional complexities arise in cases of deviations from the statistically substantiated models. Factually, every isotopic measurement is to some degree a compromise between a strict model and the probabilities of alternatives. In this book, we designate, therefore, results based on the principle approaches and on interpretations on a level of more or less approximate constructions. Achievements of radiogenic isotope geology have been highlighted in several monographs (Faure 1989, 2001; Geyh and Schleicher 1990; Dickin 1997; McDougall and Harrison 1999; Ozima and Podosek 2002 etc.). Monographs that were published in Russian on this topic were listed by Shukolyukov in introductory comments to the Russian edition of the book on principles of isotope geology by Gunter Faure (1989). A few books have been published recently (Kotlyar et al. 2001; Titaeva 2005 etc.). Our Russian book Radiogenic isotope geology in problems and examples (Rasskazov et al. 2005a) was intended for students and geologists interested in modern approaches to applications of the principle geochronometric systems. Respectively, comprehensive information on the theory, experiments, models and methods is presented in the first three chapters of this book, exhibiting modified compilations from the Russian monograph mentioned above. In the further chapters, some aspects of radiogenic isotope behavior are developed for understanding geologic processes. On the one hand, we substantiate theoretically the geochronological consequences of (1) possible fractionation of argon isotopes in the terrestrial atmosphere, (2) distribution of radiogenic argon in a mineral grain, (3) particularities of Ar Ar spectra, (4) radiogenic isotope losses in an exocontact zone of an intrusive body and in a terrane affected by long-term elevated temperatures provided a Laplace regime, (5) decreasing isotope losses in a cooling dike with transition from an open to a closed isotope system, and (6) separation of leads in ore deposits according to the Concordia Discordia model. On the other hand, we highlight geochronological data on processes of (1) the early Earth, (2) important Phanerozoic boundaries, and (3) the late Phanerozoic. The book was prepared at the laboratory for isotopic and geochronological studies of the Institute of the Earth s crust, Siberian branch of the Russian Academy of Sciences, with the ultimate goal of establishing a special course on radiogenic isotopes in geologic processes for students of the Irkutsk State University, Irkutsk. The help of our colleagues occasionally contributed to this work is highly appreciated. For promotion of this publication, we are especially grateful to Gillian R Foulger, Durham University and Petra D van Steenbergen, Springer Senior Publishing Editor. Drawings in the final manuscript version benefited from assistance of Irina Chuvashova and the text from corrections of Edwin Beschler, Hermine Vloemans, and the team at Crest Premedia Solutions (P) Ltd., Pune, India. Irkutsk, Russia Sergei Rasskazov Sergei Brandt Ivan Brandt

7 Contents 1 Theory and Experience The Law of Rutherford Soddy Determination of Decay Constants Mass Spectrometry Isotope Dilution Statistical Estimation of Results Summary Geochronometric Models Isochron Model Isochron Plane Common Rb Sr Isochron Pb Pb Isochron Concordia Discordia Model Summary: Models for Open and Closed Parent Daughter Systems Principle Geochronometric Isotope Systems Th, U Pb Isotope System Decay Constants of 235 U and 238 U U Pb Dating Pb Pb Dating K Ar Isotope System Constants of Radioactive Transformation Conventional K Ar Method Ar/ 39 Ar Method Rb Sr Isotope System Sm Nd Isotope System Re Os Isotope System Isotope Systems of Extinct Nuclides ix

8 x Contents 3.7 Isotope System of 14 C Summary: Reliability of the Modern Isotopic Methods Atmospheric Argon Origin of 40 Ar Variations of 40 Ar/ 36 Ar and 36 Ar/ 38 Ar Summary: Isotopically Inhomogeneous Atmosphere Distribution of Radiogenic Argon Within a Mineral Closing Isotopic System: α-factor The α-factor in the Conventional K Ar Method Influence of Argon Distribution on Diffusion Parameters Application for Dating of Imposed Geologic Processes Rectangular Distribution of Argon (α = 0) Dominating Rectangular Argon Fraction (0 < α < 0.5) Dominating Semi-Sinusoidal Argon Fraction (0.5 < α 1) Summary: Low-Temperature Argon Losses Thermal Spectra of Argon Isotopes Presentation of Argon Argon Spectra on Diagrams Histograms of Argon Release Deduction of an Algebraic Expression for an Isochron Line in Inverse Ar Ar Coordinates Kinetics of Stepwise Heating Separating Air and Radiogenic Argon E cont = E rad = E E cont < E rad = E E cont < E 39 < E rad Loss of Radiogenic Argon in the Geologic Past Excessive Argon Argon of Poly-mineral Aggregates Argon in Two-Component Crystal Structure Irreversible Processes of Argon Release from Mineral Phases Summary: Peculiarities of Ar Ar Spectra Radiogenic Argon in a Cooling Dike Argon Kinetics K Ar Isotopic Balance in a Cooling Dike Closure of K Ar Isotope System Geologic Application Summary: Temporally Varying Diffusion... 95

9 Contents xi 8 Radiogenic Isotopes in an Exocontact Zone of a Magmatic Body Theoretical Consideration Thermal Model Radiogenic Argon Retention in a Mineral under Varying Temperatures Geochronological Application Summary: Theory of Diffusion and Its Application for Study of Exocontact Processes Appendix: Two- and Three-Dimensional Cases Diffusion in a Laplace Regime Deduction of the Laplace Age Equation Geologic Examples Summary: Estimation of the Diffusion Parameter D/h 2 λ Early Earth Initial Processes in the Solar System ( Ga) Classification of Meteorites Dating of Meteorites Significance of Short-Lived Radionuclides Evolution of Asteroids Priscoan Crust ( Ga) Archean Crust ( Ga) Oldest Crust in Asia Summary: Geologic Processes of the Early Earth Important Phanerozoic Boundaries Vendian Cambrian Stratotype Sections Radiogenic Isotope Ages Events and Hypotheses Alkaline Ultramafic Magmatism with Carbonatites in Cissayan, Siberia Permian Triassic Stratotype Sections Radiogenic Isotope Ages Events Hypotheses Siberian Traps Cretaceous Tertiary Definition of the Boundary Radiogenic Isotope Ages

10 xii Contents Events and Hypotheses Deccan Traps Summary: Specifications of the Important Phanerozoic Boundaries Late Phanerozoic Magmatic Evolution of Asia Hierarchy and Periodicity of Magmatic Events Middle Late Cretaceous: First-Order Processes Dating Geologic Background Early Tertiary: First-Order Processes Thanetian Magmatic Events Spatial Connections Between the Thanetian and Lutetian Magmatic Events Geologic Background Middle Late Tertiary through Quaternary Transition from the First-, through the Second- to the Third-Order Processes Central Mongolia and Adjacent Regions South Primorye and Adjacent East Asia Summary: Magmatic Geodynamics in Asia Late Phanerozoic Magmatic Evolution of North America and Northeast Africa: Comparisons with Asia North America Magmatic Migration of Ma Transition from Intermediate Silicic to Basaltic Magmatism of Ma Predominating Basaltic Magmatism of the Past Ma Geodynamic Models Northeast Africa Earliest Volcanic Events in the Western Rift Hypotheses Bilateral along-axis Propagation of Rift-Related Magmatism Geodynamics of the East African Plateau Comparisons: Africa Eurasia North America Summary: Two Geodynamic Patterns of Rift-Related Magmatic Evolution in Continents Separated Lead Isotopes Dating Problems of Ore Mineralization Definition of Separated Leads Diffusion Discordia Electrical Charge of Pb Separated from U Pb System

11 Contents xiii 14.5 Connected Points of Concordia and Discordia Leads Released by Continuous Diffusion Instant Release of Leads Tuning of T, t, and μ Interpreting Separated Leads from Ore Deposits in Asia The Southern Part of the Siberian Craton The Gargan Block Geologic Implication Summary: Application of Separated Leads for Understanding Timing of Ore-Forming Processes References Index