Principles of Stable Isotope Geochemistry

About this book

Discussion of stable isotope geochemistry is generally limited to a specific scholarly paper or to cursory coverage in a larger geochemistry text. This is the first dedicated text to cover the basics of a wide range of stable isotope applications in a manner appropriate for someone entering the field. At the same time, it offers sufficient detail - and numerous references and examples - to direct research for further inquiry.

For courses in Geochemistry that emphasize stable isotopes, offered in departments of Geology.

Contents

PREFACE xi ABOUT THE AUTHOR xiii 1 INTRODUCTION 1 1.1 Historical Background 1 1.2 Scope of the Discipline 4 1.2.1 What Are Stable Isotopes? 5 1.2.2 Which Elements and Why? 7 1.3 Abundances of the Rare Isotopes of Light Elements 7 1.4 Characteristics of Elements That Undergo Significant Isotopic Fractionation 7 1.5 Applications in the Earth Sciences 9 1.6 Isotope Effects 10 1.6.1 Kinetic Isotope Effects 10 1.6.2 Equilibrium Isotope Effects 11 References 13 2 TERMINOLOGY, STANDARDS, AND MASS SPECTROMETRY 15 2.1 Overview 15 2.2 Isotopologues, Isotopomers, and Mass Isotopomers 15 2.3 The Delta Value 17 2.4 Isotope Exchange Reactions 20 2.5 The Fractionation Factor 21 2.6 103 ln alpha, DELTA, and the epsilon Value 22 2.7 Reference Standards 24 2.7.1 Hydrogen 25 2.7.2 Carbon 28 2.7.3 Nitrogen 28 2.7.4 Oxygen 29 2.7.5 Sulfur 30 2.8 Isotope Ratio Mass Spectrometry 30 2.8.1 The First Isotope Ratio Mass Spectrometers 30 2.8.2 Modern Conventional Mass Spectrometers 31 2.8.3 Gas Chromatograph Isotope Ratio Mass Spectrometry (GC-IRMS) 33 2.8.4 Gases Measured in Isotope Ratio Mass Spectrometry 33 2.8.5 Relations between Measured and Desired Isotopic Ratios 35 2.8.6 Ion Microprobe Analyses of Stable Isotope Ratios 36 References 39 3 EQUILIBRIUM ISOTOPIC FRACTIONATION 40 3.1 Introduction 40 3.2 Theoretical Determination of Stable Isotope Fractionation Factors 41 3.2.1 Free Energy of Reaction 41 3.2.2 The Internal Energy of a Molecule 42 3.2.3 Vibrational Partition Function 43 3.2.4 Translational and Rotational Partition Function 45 3.2.5 The Complete Partition Function Ratio 46 3.2.6 Extension to More Complex Molecules 46 3.2.7 Relationship to Temperature 46 3.2.8 "Empirical" Theoretical Methods 47 3.3 Experimental Determination of Fractionation Factors 47 3.3.1 Introduction 47 3.3.2 Mineral--Water Exchange Reactions 49 3.3.3 Mineral--Calcite Exchange Reactions 51 3.3.4 Mineral--CO2 Exchange Reactions 51 3.3.5 The Three-Phase Approach 52 3.4 Empirical Determination of Fractionation Factors 52 3.5 Other Potential Factors Controlling Isotope Partitioning 53 3.5.1 Pressure Effect 53 3.5.2 Composition and Structure 54 3.6 So Which Fractionation Factors Are Correct? 56 3.6.1 An Example from Quartz--Calcite Fractionation 56 References 60 4 THE HYDROSPHERE 64 4.1 Overview 64 4.2 Natural Abundances of the Isotopologues of Water 65 4.3 Meteoric Water 67 4.4 The Meteoric Water Line 68 4.4.1 General Features of the GMWL 69 4.4.2 Variations in Slopes and Intercepts of Local MWLs 69 4.4.3 Meteoric Waters in Arid and Semiarid Environments 70 4.5 The Deuterium Excess Parameter 71 4.6 Evaporation and Condensation 74 4.6.1 Evaporation 74 4.6.2 Condensation: Closed-System (Batch) Isotopic Fractionation 75 4.6.3 Condensation: Open-System (Rayleigh) Isotopic Fractionation 78 4.7 Factors Controlling the Isotopic Composition of Precipitation 80 4.7.1 Temperature 80 4.7.2 Distance or Continentality Effect 82 4.7.3 Latitude Effect 83 4.7.4 Altitude Effect 83 4.7.5 Amount Effect 84 4.7.6 Seasonal Effects 86 4.8 Groundwater 86 4.9 Geothermal Systems 88 4.10 Basinal Brines and Formation Waters 89 4.11 Glacial Ice 91 4.11.1 Underlying Bases for Glacial Paleoclimatology 92 4.11.2 Determining the Age of Glacial Ice 93 4.11.3 Thinning of Ice Layers 94 4.11.4 The Example of Camp Century, North Greenland 94 4.11.5 Example of the GRIP Summit Core: Flickering Climates 97 References 100 5 THE OCEANS 103 5.1 Overview 103 5.2 Oxygen Isotope Variations in Modern Oceans 104 5.2.1 Salinity--delta18O Relations in Shallow Marine Waters 104 5.2.2 Salinity--delta18O Relations in Deep Ocean Waters 105 5.3 Depth Profiles in Modern Oceans: delta18O(O2)aq. and delta13C(SIGMACO2) 108 5.4 Isotopic Compositions of Ancient Oceans 109 5.4.1 Primitive Oceans 109 5.4.2 Secular Changes in delta18O of Marine Sediments 111 5.5 Seawater--Basalt Interactions: Buffering the delta18O Value of the Ocean 112 5.5.1 Low-Temperature Alteration 112 5.5.2 High-Temperature Alteration 113 5.5.3 Evidence from Drill Core Material 114 5.5.4 Evidence from Obducted Material 114 5.6 Buffering the 18O/16O Ratio of Ocean Water 116 5.6.1 Summing the Processes Affecting the 18O/16O Ratio of Seawater 116 5.6.2 Model Calculations 116 5.6.3 Unresolved Controversy 117 References 118 6 BIOGENIC CARBONATES: OXYGEN 120 6.1 Introduction 120 6.2 The Phosphoric Acid Method 121 6.2.1 A Major Breakthrough 121 6.2.2 Acid Fractionation Factors 123 6.2.3 Applicability 124 6.3 The Oxygen Isotope Paleotemperature Scale 125 6.4 Factors Affecting Oxygen Isotope Paleotemperatures 129 6.4.1 Variations in delta18O of Ocean Water in Space and Time 130 6.4.2 Vital Effects 131 6.4.3 Diagenesis 133 6.4.4 Ecology of the Organism 138 6.5 Applications of Oxygen Isotope Paleothermometry 139 6.5.1 The Quaternary 139 6.5.2 The Paleogene and Neogene (Cenozoic) 140 6.5.3 Older Samples 140 6.6 Application to Continental Carbonates 141 References 145 7 CARBON IN THE LOW-TEMPERATURE ENVIRONMENT 149 7.1 Introduction 149 7.2 The Carbon Cycle 150 7.2.1 Carbon Isotope Budget of the Earth 153 7.3 Carbon Reservoirs 153 7.3.1 Mantle 153 7.3.2 Plants 153 7.3.3 Organic Carbon in Sediments 157 7.3.4 Methane 159 7.3.5 Atmospheric CO2 160 7.4 delta13C Values of Carbonates 161 7.4.1 Introduction 161 7.4.2 General Characterization of Carbonates 162 7.4.3 The Vital Effect 162 7.4.4 Carbonate Speciation Effects 165 7.4.5 Controls on the delta13C Value of Marine Carbonates over Long Timescales 165 7.4.6 Variations in the delta13C Value of Marine Carbonates at Short Timescales 169 7.5 delta13C Studies of Terrestrial Carbonates 171 References 174 8 LOW-TEMPERATURE MINERALS, EXCLUSIVE OF CARBONATES 179 8.1 Introduction 179 8.2 Phosphates 179 8.2.1 Analytical Techniques 180 8.2.2 Applications to Marine Paleothermometry 181 8.2.3 Application to Mammals: Theory 183 8.2.4 Sample Applications 186 8.3 Cherts 189 8.3.1 Application to Precambrian Chert Deposits 189 8.3.2 Application to Phanerozoic Cherts 190 8.3.3 Diagenesis 191 8.3.4 Application to Recent Sediments 193 8.3.5 Other Silica Applications 194 8.4 Clay Minerals 195 8.4.1 Early "Bulk" Sample Studies 195 8.4.2 Grain-Size Considerations 196 8.5 Iron Oxides 199 References 201 9 NITROGEN 206 9.1 Introduction 206 9.2 The Nitrogen Cycle 207 9.3 Nitrogen Isotope Fractionation 208 9.3.1 Nitrogen Fixation 209 9.3.2 Mineralization 209 9.3.3 Assimilation 210 9.3.4 Nitrification 210 9.3.5 Denitrification 210 9.4 The Characteristic delta15N Values of Various Materials 211 9.4.1 Plants and Soil 212 9.4.2 Other Terrestrial Reservoirs 212 9.4.3 Nitrogen in the Oceans 213 9.5 Nitrogen Isotope Ratios in Animals 216 References 219 10 SULFUR 222 10.1 Introduction 222 10.2 Analytical Techniques 223 10.3 Equilibrium Fractionations and Geothermometry 225 10.4 Sulfate and Sulfide Formation at Low Temperatures: The Sedimentary Sulfur Cycle 228 10.5 Secular Variations in Sulfur 231 10.5.1 Long-Term Variations 231 10.5.2 Alternative Approaches: Barite and Trace Carbonates 232 10.5.3 Time Boundaries 233 10.5.4 Archean Sulfates: Clues to the Early Atmosphere 234 10.5.5 Sulfur Isotope Anomalies: Mass-Independent Fractionation 235 10.6 Sulfur Isotope Ratios in the Terrestrial Environment 238 10.7 Oxygen Isotope Variations in Sulfates 236 References 239 11 IGNEOUS PETROLOGY 242 11.1 Introduction 242 11.2 The Mantle 243 11.2.1 Oxygen 243 11.2.2 Carbon 247 11.2.3 Nitrogen 251 11.2.4 Hydrogen 252 11.2.5 Sulfur 254 11.3 Emplacement of Plutonic Rocks: Interactions with the Crust and Hydrosphere 255 11.3.1 Normal Igneous Rocks 256 11.3.2 Shallow-Level Hydrothermal Alteration by Meteoric Water: Low Plutonic Rocks 256 11.3.3 High--delta18O Igneous Rocks 258 11.4 Calculating Fluid/Rock Ratios 259 11.5 Other Processes: Degassing, Assimilation, and Fractional Crystallization 261 11.5.1 Magmatic Volatiles 261 11.5.2 Assimilation--Fractional Crystallization (AFC) Processes 263 References 266 12 METAMORPHIC GEOLOGY 272 12.1 Introduction 272 12.2 Stable Isotopes as Geochemical Tracers 273 12.2.1 Closed System: Protolith Identification and Alteration 273 12.2.2 Open Systems: Volatilization and Fluid Infiltration Processes 274 12.3 Fluid Sources and Fluid--Rock Interaction 280 12.3.1 Oxygen and Hydrogen 280 12.3.2 Carbon 282 12.3.3 Sulfur 283 12.4 Scales of Equilibration During Metamorphism 284 12.4.1 Regional-Scale Exchange 284 12.4.2 Localized Exchange 285 12.5 Quantifying Fluid--Rock Ratios and Fluid Fluxes 286 12.5.1 Simple Mixing Models: Zero-Dimensional Water--Rock Interaction Models 286 12.5.2 One-Dimensional (Directional) Water--Rock Interaction Models 287 12.6 Thermometry 291 12.6.1 Introduction 291 12.6.2 Oxygen Isotope Thermometry in Metamorphic Rocks: Testing for Equilibrium 293 12.6.3 Applications of Stable Isotope Thermometry 295 12.7 Retrograde Exchange: "Geospeedometry" 296 12.8 State of the Art 300 References 302 13 EXTRATERRESTRIAL MATERIALS 309 13.1 Introduction 309 13.2 Classification of Meteorites 310 13.3 Oxygen Isotope Variations in Meteorites 310 13.3.1 Introduction 310 13.3.2 Discovery of an 17O Anomaly 312 13.3.3 Possible Explanations: Mixing of Two Distinct Reservoirs 313 13.3.4 Mass-Independent Fractionation 316 13.4 Hydrogen 318 13.4.1 Introduction 318 13.4.2 Meteorites 319 13.5 Carbon 320 13.6 Nitrogen 322 References 325 APPENDIX A Standard Reference Materials for Stable Isotopes 329 APPENDIX B Sample Calculation of the Correction Procedure for Adjusting Measured Isotope Data to Accepted IAEA Reference Scales 332 INDEX 334

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