Earth as an Evolving Planetary System
Earth as an Evolving Planetary System Second Edition
Kent C. Condie
AMSTERDAM • BOSTON • HEIDELBERG • LONDON • NEW YORK OXFORD • PARIS • SAN DIEGO • SAN FRANCISCO • SYDNEY • TOKYO Academic Press is an imprint of Elsevier
Academic Press is an imprint of Elsevier The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, UK Radarweg 29, PO Box 211, 1000 AE Amsterdam, The Netherlands 225 Wyman Street, Waltham, MA 02451, USA 525 B Street, Suite 1900, San Diego, CA 92101-4495, USA Second edition # 2011, 2005 Elsevier Ltd. All Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise without the prior written permission of the publisher. Permissions may be sought directly from Elsevier’s Science & Technology Rights Department in Oxford, UK: phone (þ44) (0) 1865 843830; fax (þ44) (0) 1865 853333; email:
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Contents
Preface
xvii
1. Earth Systems Earth as a Planetary System
1 1
Structure of Earth
3
Plate Tectonics
5
Is the Earth Unique?
7
Interacting Earth Systems
8
Further Reading
2. The Crust
10
11
Introduction
11
Seismic Crustal Structure
11
The Moho
11
Crustal Layers
13
Complexities in the Lower Continental Crust
14
Crustal Types
Oceanic Crust
17 17
Seismic Features
17
Ocean Ridges
18
Ocean Basins
19
Volcanic Islands
20
Trenches
20
Back-Arc Basins
20
Transitional Crust Oceanic Plateaus
20 20
Arcs
21
Continental Rifts
22
Inland-Sea Basins
23
Continental Crust Shields and Platforms Orogens
23 23 24
Continent Size
25
Heat Flow
26
Heat Flow Distribution
26
Heat Production and Heat Flow in the Continents
27
Age Dependence of Heat Flow
30
v
Contents
vi
Exhumation and Cratonization
Unraveling Pressure-Temperature-Time Histories Some Typical P-T-t Paths Cratonization Processes in the Continental Crust
Rheology The Role of Fluids and Crustal Melts Crustal Composition
Approaches Seismic Wave Velocities Seismic Reflections in the Lower Continental Crust Sampling of Precambrian Shields Use of Fine-Grained Detrital Sediments Exhumed Crustal Blocks Crustal Xenoliths An Estimate of Crustal Composition Continental Crust Oceanic Crust Complementary Compositions of Continental and Oceanic Crust
32 33 34 35 37 37 38 39 39 40 42 44 44 45 47 48 48 50
Further Reading
51 51 55 57 57
3. Tectonic Settings
59
Crustal Provinces and Terranes Crustal Province and Terrane Boundaries The United Plates of America
Introduction Ocean Ridges
Ocean Ridge Basalts Ophiolites General Features Tectonic Setting and Emplacement of Ophiolites Formation of Ophiolites Precambrian Ophiolites Tectonic Settings Related to Mantle Plumes
Large Igneous Provinces Oceanic Plateaus and Aseismic Ridges Rifted Continental Margins Continental Flood Basalts Hotspot Volcanic Islands Giant Mafic Dyke Swarms Continental Rifts
General Features Rock Assemblages Rift Development and Evolution
59 60 60 61 61 63 65 66 66 66 67 68 70 71 73 75 75 77 78
Contents
Cratons and Passive Margins Arc Systems
Subduction-Related Rock Assemblages Trenches Accretionary Prisms Forearc Basins Arcs Back-Arc Basins Remnant Arcs Retroarc Foreland Basins Arc Processes High-Pressure Metamorphism Igneous Rocks Compositional Variation of Arc Magmas Orogens
Three Types of Orogens Collisional Orogens Accretionary Orogens Intracratonic Orogens Orogenic Rock Assemblages Tectonic Elements of Collisional Orogens Sutures Foreland and Hinterland Basins The Himalayas Uncertain Tectonic Settings
Anorogenic Granites General Features Associated Anorthosites Tectonic Setting Archean Greenstones General Features Greenstone Volcanics Greenstone Sediments Granitoids Mineral and Energy Deposits
Mineral Deposits Ocean Ridges Arc Systems Orogens Continental Rifts Cratons and Passive Margins Archean Greenstones Energy Deposits Plate Tectonics with Time Further Reading
vii
80 81 81 81 82 83 84 84 85 85 86 89 89 91 92 92 93 95 95 96 97 99 99 1 00 1 01 1 01 1 01 1 03 1 03 1 04 1 04 1 06 1 08 1 10 1 11 111 111 1 13 1 14 114 114 115 115 117 118
Contents
viii
4. The Mantle Introduction Seismic Structure of the Mantle
Upper Mantle Lower Mantle Mantle Upwellings and Geoid Anomalies Temperature Distribution in the Mantle The Lithosphere
Oceanic Lithosphere Continental Lithosphere Composition Seismic Velocity Constraints Mantle Xenoliths Chemical Composition Thickness SubductabiI ity Age of Subcontinental Lithosphere The Low-Velocity Zone The Transition Zone
The 41 0-km Discontinuity The 520-km Discontinuity The 660-km Discontinuity The Lower Mantle
General Features Descending Slabs The D" Layer Spin Transitions Water in the Mantle Plate Driving Forces Mantle Plumes
Hotspots PI ume Characteristics Tracking Plume Tails Plume Sources Mantle Geochemical Components
Identifying Mantle Components Summary Depleted Mantle HIMU Enriched Mantle Helium Isotopes Archean Geochemical Components Mixing Regimes in the Mantle Overview Convection in the Mantle
The Nature of Convection Passive Ocean Ridges
1 21 1 21 1 21 1 21 1 23 1 23 1 26 1 30 1 31 1 32 1 32 1 32 1 34 1 36 1 39 1 41 1 41 1 44 1 45 1 45 1 47 1 48 1 49 1 49 1 50 1 52 1 55 1 55 1 56 1 57 1 58 1 61 1 63 1 64 1 65 1 66 1 66 1 66 1 68 1 69 1 70 1 71 1 71 1 73 1 74 1 74 1 75
Contents
Layered Convection Model Toward a Convection Model for Earth Further Reading
5. The Core Introduction Core Temperature The Inner Core
Anisotropy of the Inner Core Inner Core Rotation Composition of the Core Age of the Core Generation of Earth's Magnetic Field
The Geodynamo Fluid Motions in the Outer Core Fueling the Geodynamo How the Geodynamo Works What Causes Magnetic Reversals? Origin of the Core
Segregation of Iron in the Mantle Siderophile Element Distribution in the Mantle Growth and Evolution of the Core What the Future Holds Further Reading
6. Earth's Atmosphere, Hydrosphere, and Biosphere The Modern Atmosphere The Primitive Atmosphere The Post-Collision Atmosphere
Composition of the Early Atmosphere Growth Rate of the Atmosphere The Faint Young Sun Paradox The Precambrian Atmosphere The Carbon Cycle The Carbon Isotope Record
General Features The 2200-Ma Carbon Isotope Excursion The Sulfur Isotope Record Phanerozoic Atmospheric History The Hydrosphere
Sea Level The Early Oceans Changes in the Composition of Seawater with Time Marine Carbonates The Dolomite-Limestone Problem Evaporites
ix
176 178 180
181 181 182 182 182 185 186 188 189 1 89 1 89 191 1 92 1 93 1 94 1 94 1 95 1 96 1 96 1 97
199 199 201 203 203 204 205 206 208 209 209 211 212 214 216 217 219 220 220 222 223
Contents
X
Banded Iron Formation The Biochemical Record of Sulfur Sedimentary Phosphates The Temperature of Seawater Ocean Volume through Time Euxinia in the Proterozoic Oceans Paleoclimates
Paleoclimatic Indicators Long-Term Paleoclimatic Driving Forces Glaciation Precambrian Climatic Regimes Phanerozoic Climatic Regimes Glaciation The Biosphere
Appearance of Eukaryotes Origin of Metazoans Stromatalites Neoproterozoic Multicellular Organisms The Cambrian Explosion Evolution of Phanerozoic Life-Forms Biological Benchmarks Mass Extinctions
Episodic Distributions Glaciation and Mass Extinction Impact-Related Extinctions Environmental Changes Earth-Crossing Asteroids Comets The Triassic Extinction Impact and a 580-Ma Extinction Epilogue Further Reading 7. Crustal and Mantle Evolution Introduction The Hadean
Extinct Radioactivity Hadean Zircons Origin of the First Crust Composition of the Primitive Crust Felsic Models Anorthosite Models Basalt and Komatiite Models Earth's Oldest Rocks Crustal Origin How Continents Grow
224 224 225 226 229 229 231 231 233 233 235 238 238 240 240 242 242 244 244 247 249 250 251 253 254 254 256 257 257 258 258 259
261 261 261 261 263 266 267 268 268 269 269 273 275
xi
Contents
General Features
275
Growth by Mafic Underplat i n g
276
Oceanic Plateaus and Continental Growth
277
Growth by Plate Collisions
278
Continental Growth Rat es
280
The Role of Recycling
281
Delamination juvenile Crust
283 284
Oxygen Isotopes
285
Neodymium Isotopes
285
Hafnium Isotopes and Detrital Zircons
288
juve nil e Crust in Extensional Settings
291
Freeboard
291
Continental Growth in the Last 200 Ma
292
Toward a Continental Growth Model
The Appro ac h T h e Model
293 293 294
The 2.4- to 2.2-Ga Crustal Age Gap
295
S ecula r Cha nges in the Continental Crust
297 298
Maj or Elements
Rare Earth and Related Elements
298
Nickel, Cobalt, and Chromium
300
Oceanic Plateaus as Starters for Archean Continents Secular Changes in the Mantle
Tracki ng M ant le Geochemical Componen ts into the Archean
300 301 302
Mantle Lithosphere Evolution
303
Continent al Lithosphere
303
Oce anic Lith osphe re
305
Ophiolites
305
Blueschists and Ultra-High-Pressure M etamorp hic Rocks
307
Earth's Thermal History
308
Magma Oceans
309
How Hot Was the Archean Mantle?
310 314
Thermal Models Furthe r Rea di ng
8. The Supercontinent Cycle
316
317
Introduction
317
Supercontinent Reconstruction
31 Y
Continental Collisions and the Assem bly of Supercontinents
325
The First Supercontinent
326
Later Supercontinents
328
Nuna (Columbia)
328
Rodinia
329
Gondwana and Pangea
331
xii
Contents
The Supercontinent Cycle Episodic Ages
332 332
Patterns of Cyclicity
335
Relationship to Earth History
336
Mineral Deposit Age Patterns
336
Strontium Isotopes in Marine Carbonates
337
Sea Level Variations
339
Supercontinents and Evolution Mantle Superplume Events
340
Superplume Events
342 343
Mantle Plumes and Supercontinent Breakup
344
Episodic LIP Events
349
Slab Avalanches
349
Supercontinents, Superplumes, and the Carbon Cycle Supercontinent Formation
350 351
Supercontinent B reakup
353
Mantle Superplume Events
353
Epilogue
354
Further Reading
354
9. Great Events in Earth History
357
Introduction
357
Event 1: Origin of the Moon
358
How Rare Is the Earth-Moon System?
358
Constraints on Lunar Origin
358
The Fission Model
362
Double Planet Models
362
C aptu re Models
363
Giant Impac tor Model Early Thermal History of the Moon
363 366
Event 2: Origin of Life
367
The Role of Impacts
368
The RNA World
370
Hydrothermal Vents
372
Possible Site for the Origin of Life Experimental and Observational Evidence
372 375
The First Life
375
Evidence of Early Life The Origin of Photos ynthesis
376 378
Anoxygenic Photosynthesis
378
Oxygenic Photosynthesis
379
The Tree of Life
381
The First Fossils
382
Possibility of Extraterrestrial Life
384
Event 3: The Onset of Plate Tectonics Plate Tectonic Indic ators
386 386
Contents
Global Changes at the End of the Archean Decrease in Mantle Temperature Komatiite Abundance MgO Content of Komatiites Ni/Fe Ratio in Banded Iron Formation Degree of Upper Mantle Melting The Growth of Cratons Nb(Th Ratio and Neodymium Isotopes in Basalts Changes in Granitoid Compositions Oxygen Isotopes in Zircons Gold Reserves Thickening of the Archean Lithosphere How Did Plate Tectonics Begin: Thermal Constraints When Did Plate Tectonics Begin?: The Ongoing Saga Conclusions Event 4: The Great Oxidation Event Oxygen Controls in the Atmosphere Geologic Indicators of Ancient Atmospheric Oxygen Levels Banded Iron Formation Redbeds and Sulfates Detrital Uraninite Deposits Paleosols Biologic Indicators Molybdenum in Black Shales Mass-Independent Sulfur Isotope Fractionation The Growth of Atmospheric Oxygen Event 5: The Snowball Earth The Observational Database The Snowball Model Event 6: Mass Extinction at the End of the Permian
General Features Evidence for Impact LIP Volcanism Shallow-Water Anoxia Catastrophic Methane Release Conclusions Event 7: The Cretaceous Superplume Event
Geologic Evidence The Carbon Isotope and Trace Metal Record Seeking a Cause A Possible Superchron-Superplume Connection Event 8: Mass Extinction at the end of the Cretaceous
General Features Seeking a Cause Evidence for Impact Iridium Anomalies Glass Spherules
xiii
389 389 389 389 391 391 392 392 393 394 394 395 397 399 402 402 402 404 404 405 406 406 407 408 408 410 412 413 414 416 416 418 418 419 419 421 421 421 424 425 425 426 426 427 427 427 428
Contents
xiv
Soot Shocked Quartz Stishovite Chromium Isotopes LIP Volcanism Chicxulub and the K/T Impact Site Possibility of Multiple K/T Impacts Conclusions Further Reading
10. Comparative Planetary Evolution Introduction Condensation and Accretion of the Planets
The Solar Nebula Emergence of Planets Homogeneous Accretion Chemical Composition of the Earth and the Moon Accretion of Earth The First 700 Million Years Members of the Solar System
The Planets Mercury Mars Crustal Dichotomy Surface Features Martian History Venus In Comparison to Earth Volcanism The Venusian Core Crustal Plateaus Thermal History The Giant Planets Satellites and Planetary Rings General Features Planetary Rings The Moon Rotational History of the Earth-Moon System Satellite Origin Comets and Other Icy Bodies Asteroids Meteorites Chondrites SNC Meteorites Refractory Inclusions Iron Meteorites and Parent Body Cooling Rates Asteroid Sources
428 428 429 429 429 431 433 433 434 437
437 438 438 439 442 443 444 447 449 449 449 453 453 454 455 458 458 462 462 463 464 466 467 467 468 468 471 472 473 474 476 477 478 478 478 479
Contents
XV
Meteorite Chronology Impact Chronology of the Inner Solar System Volcanism in the Solar System Planetary Crusts Plate Tectonics Mineral Evolution Evolution of the Atmospheres of Earth, Venus, and Mars The Continuously Habitable Zone Comparative Planetary Evolution Extrasolar Planets Further Reading References
Index
480 481 482 484 484 485 486 488 488 492 492 493 559
Preface
Although this book began life in 1976 with the title Plate Tectonics and Crustal Evolution, the subject matter has gradually changed focus with subsequent editions, and especially since the third edition in 1989. In the past decade it has become increasingly clear that the various components of Earth act as a single, interrelated system, often referred to as the Earth System. One reviewer of the fourth edition pointed out that the title of the book was no longer appropriate, since plate tectonics was not a major focus. For this reason, for what would have been the fifth edition of the Plate Tectonics book, I have introduced a new title for the book, Earth as an Evolving Planetary System, which will be continued into still later editions. Since the first edition in 1976, which appeared on the tail end of the plate tectonics revolution of the 1960s, our scientific database has grown exponentially and continues to grow—in fact, much faster than we can interpret it. If one compares the earlier editions of the book with this edition, a clear trend is apparent. Plate tectonics is no longer so exciting, but is now taken for granted. The changing emphasis during the past 30 years is from how one system in our planet works (plate tectonics) to how all systems in our planet work, how they are related, and how they have governed the evolution of the planet. As scientists continue to work together and share information from many disciplines, this trend should continue for many years into the future. Today, more than any time in the past, we are beginning to appreciate the fact that to understand the history of our planet requires an understanding of the various interacting components and how they have changed with time. Although science is made up of specialties, to learn more about how Earth operates requires input from all of these specialties—geology alone cannot handle it. In this Earth System book, the various subsystems of the Earth are considered as vital components in the evolution of our planet. Subsystems include such components as the crust, mantle, core, atmosphere, oceans, and life. As with previous editions, the Earth System book is written for advanced undergraduate and graduate students, and it assumes a basic knowledge of geology, biology, chemistry, and physics, which most students in the Earth Sciences acquire during their undergraduate education. It also may serve as a reference book for various specialists in the geologic sciences who want to keep abreast of scientific advances in this field. I have attempted to synthesize and digest data from the fields of oceanography, geophysics, paleoclimatology,
xvii
xviii
Preface
geology, planetology, and geochemistry and to present this information in a systematic manner to address problems related to the evolution of Earth during the last 4.6 billion years. The second edition of the Earth System book includes some of the new and exciting topics in the Earth Sciences. Among these are results from increased resolution of seismic tomography by which plates can be tracked into the deep mantle and mantle plumes can be detected. High-resolution U/Pb zircon isotopic dating now permits us to better constrain the timing of important events in Earth’s history. We have detrital zircons with ages up to 4.4 Ga, suggesting the presence of felsic crust and water on the planet by this time. New information on the core provides us with a better understanding of how the inner and outer core interact and how Earth’s magnetic field is generated. Two expanding areas of knowledge have also required two new chapters in the second edition: one on the supercontinent cycle and one on great events in Earth history. I appreciate Maya Elrick at the University of New Mexico who invited me to attend her graduate seminar on Great Events in Earth History during spring semester 2010. It was from this seminar that I decided the book really needed a “Great Events” chapter (Chapter 9). Really exciting work on the origin of life and the possibilities of life beyond Earth are discussed in the Great Events chapter. Also I include a section on when plate tectonics began and major changes at the end of the Archean. The continuing saga of mass extinctions and the role of impacts has required more coverage, and the Snowball Earth model is discussed in more detail. The episodic nature of crustal preservation, stable isotope anomalies, giant dyke swarms and other phenomena have been well documented in the past few years, so much so that new sections have been added to cover these subjects. In addition, we have provided an updated interactive CD ROM by the author, titled Plate Tectonics and How the Earth Works, to accompany the new book. This CD, with animations and interactive exercises, can be obtained from Tasa Graphic Arts Inc., Taos, NM (www.tasagraphicarts.com). Kent C. Condie Department of Earth & Environmental Science New Mexico Tech Socorro, NM 87801 USA
[email protected] http://www.ees.nmt.edu/condie
Chapter 1
Earth Systems EARTH AS A PLANETARY SYSTEM A system is an entity composed of diverse but interrelated parts that function as a whole (Kump et al., 1999). The individual parts, often called components, interact with each other as the system evolves with time. Components include reservoirs of matter or energy (described by mass or volume) and subsystems, which behave as systems within a system. Earth is considered to be a complex planetary system that has evolved over 4.6 Ga (46 109 years). It includes reservoirs, such as the crust, mantle, and core and subsystems, such as the atmosphere, hydrosphere, and biosphere. Because many of the reservoirs in Earth interact with each other and with subsystems, such as the atmosphere, there is an increasing tendency to consider most or all of Earth’s reservoirs as subsystems. The state of a system is characterized by a set of variables at any time during the evolution of the system. For Earth, temperature, pressure, and various compositional variables are most important. The same thing applies to subsystems within the Earth. A system is at equilibrium when nothing changes as it evolves. If, however, a system is perturbed by changing one or more variables, it responds and adjusts to a new equilibrium state. A feedback loop is a selfperpetuating change and response in a system to a change. If the response of a system amplifies the change, it is known as a positive feedback loop, whereas if it diminishes or reverses the effect of the disturbance, it is a negative feedback loop. As an example of positive feedback, if volcanism pumps more CO2 into a CO2-rich atmosphere of volcanic origin, this should promote greenhouse warming and the temperature of the atmosphere would rise. If the rise in temperature increases weathering rates on the continents, this would drain CO2 from the atmosphere causing a drop in temperature, an example of negative feedback. Because a single subsystem in Earth affects other subsystems, many positive and negative feedbacks occur as Earth attempts to reach a new equilibrium state. These feedbacks may be short lived over hours or range to tens of thousands of years, such as short-term changes in climate, or they may be long lived over millions or tens of millions of years such as changes in climate related to the dispersal of a supercontinent. Earth as an evolving planetary system # 2011, 2005 Elsevier Ltd. All Rights Reserved.
1
2
Earth as an evolving planetary system
The major driving force of planetary evolution is the thermal history of a planet, as discussed in Chapter 4. The methods and rates by which planets cool, either directly or indirectly, control many aspects of planetary evolution. In a silicate-metal planet like Earth, thermal history determines when and if a core will form (Figure 1.1). It determines if the core is molten, which in turn determines if the planet will have a global magnetic field (which is generated by dynamo-like action in the outer core; see Chapter 5). The magnetic field, in turn, interacts with the solar wind and with cosmic rays, and it traps high-energy particles in magnetic belts around the planet. This, of course, also affects life since life cannot exist in the presence of intense solar wind or cosmic radiation. Planetary thermal history also strongly influences tectonic, crustal, and magmatic history (Figure 1.1). For instance, only planets that recycle lithosphere into the mantle by subduction, as Earth does, appear capable of generating continental crust, and thus having collisional orogens. Widespread felsic and andesitic magmas can only be produced in a plate tectonic regime. In contrast, planets that cool by mantle plumes and lithosphere delamination, as perhaps Venus does today, should have widespread mafic magmas, with little felsic to intermediate component. They also appear to have no continents. So where does climate come into these interacting histories? Climate reflects complex interactions of the atmosphere–ocean system with tectonic and magmatic components, as well as interactions with the biosphere. In addition, solar energy and asteroid or cometary impacts can have severe effects on climatic evolution (Figure 1.1). The thermal history of a planet directly or indirectly affects all other systems in the planet, including life. Earth has two kinds of energy sources: those internal to the planet and those external to the SOLAR WIND COSMIC RAYS
IMPACT
CLIMATIC HISTORY
LIFE
Tectonic History
CORE
Crustal Evolution
THERMAL HISTORY
Magmatic History
Magnetic Field
FIGURE 1.1 Major relationships between Earth’s thermal and climatic histories.
CHAPTER
1
Earth Systems
3
planet. In general, internal energy sources have long-term (>106 y) effects on planetary evolution, whereas external energy sources have short-term (1000
H2, He, CH4
Neptune
–220 to –200
>1000
H2, He, CH4
Pluto
–235 to –210
0.005
CH4, N2
This reaction occurs at heights of 30–60 km with most ozone collecting in a relatively narrow band at about 25–40 km (Figure 6.1). Ozone, however, is unstable and continually breaks down to form molecular oxygen. The production rate of ozone is approximately equal to the rate of loss, and thus the ozone layer maintains a relatively constant thickness in the stratosphere. Ozone is an important constituent in the atmosphere because it absorbs UV radiation from the Sun, which is lethal to most forms of life. Hence, the ozone layer provides an effective shield that permits a large diversity of living organisms to survive on Earth. It is for this reason we must be concerned about the release of synthetic chemicals into the atmosphere that destroy the ozone layer. The distributions of N2, O2, and CO2 in the atmosphere are controlled by volcanic eruptions and by interactions between these gases and the solid Earth, the oceans, and living organisms.
THE PRIMITIVE ATMOSPHERE Although most investigators agree that the present atmosphere, except for oxygen, is chiefly the product of degassing, whether a primitive atmosphere existed and was lost before extensive degassing began is a subject of controversy. Three possible sources have been considered for an early atmosphere on Earth (Halliday, 2003; Williams, 2007): (1) residual gases remaining after planetary accretion, (2) extraterrestrial sources such as the Sun, and (3) early degassing of Earth by volcanism. Models suggest that all three may have played roles, although the relative importance of each remains problematic
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Earth as an evolving planetary system
(Shaw, 2008). One line of evidence supporting the existence of an early atmosphere is the fact that volatile elements should collect around planets during their late stages of accretion. This follows from the very low temperatures at which volatile elements condense from the solar nebula (Chapter 10). The distribution of volatile elements in Earth is very different from that in the Sun (Figure 6.2). Although the relative abundances of volatiles are similar to those found in carbonaceous chondrites (a low-temperature group of meteorites), the absolute abundances are smaller. The depletion in rare gases in Earth compared to carbonaceous chondrites and the Sun indicates that if a primitive atmosphere collected during accretion, it must have been lost (Pepin, 1997; Halliday, 2003). The reason for this is that gases with low atomic weights (e.g., CO2, CH4, He, H2) that probably composed this early atmosphere should be lost even more readily than rare gases with high atomic weights (Cl, Br, I, Kr, Xe) and greater gravitational attraction. Xenon is peculiar in that it is depleted relative to other noble gases in Earth, but not in meteorites, a feature often referred to as “Earth’s missing xenon” (Figure 6.2). Just how a primitive atmosphere may have been lost is not clear, but xenon isotopic data suggest that >99% of this early atmosphere was lost in the first 100 Ma of Earth’s history. One possibility is by a T-Tauri solar wind Carbonaceous Chondrites Earth
Atomic Abundance/Solar Abundance
0.1
0.001
10−5 Missing Xenon 10−7
10−9
H
C
N
Cl
Br
I
Ne
Ar
Kr
Xe
FIGURE 6.2 Volatile element abundances in carbonaceous chondrites and Earth relative to solar abundances. Modified after Kramers (2003).
CHAPTER
6
Earth’s Atmosphere, Hydrosphere, and Biosphere
203
(Ozima & Podosek, 1999) (see Chapter 10). If the Sun evolved through a T-Tauri stage during or soon after (
