Timescales of Magmatic Processes E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 670

Veröffentlichungsjahr: 2011

Beschreibung

Quantifying the timescales of current geological processes is critical for constraining the physical mechanisms operating on the Earth today. Since the Earth's origin 4.55 billion years ago magmatic processes have continued to shape the Earth, producing the major reservoirs that exist today (core, mantle, crust, oceans and atmosphere) and promoting their continued evolution. But key questions remain. When did the core form and how quickly? How are magmas produced in the mantle, and how rapidly do they travel towards the surface? How long do magmas reside in the crust, differentiating and interacting with the host rocks to yield the diverse set of igneous rocks we see today? How fast are volcanic gases such as carbon dioxide released into the atmosphere? This book addresses these and other questions by reviewing the latest advances in a wide range of Earth Science disciplines: from the measurement of short-lived radionuclides to the study of element diffusion in crystals and numerical modelling of magma behaviour. It will be invaluable reading for advanced undergraduate and graduate students, as well as igneous petrologists, mineralogists and geochemists involved in the study of igneous rocks and processes.

Leseprobe

Contents

List of Contributors

Introduction to the Timescales of Magmatic Processes, 1Anthony Dosseto, Simon P. Turner, Fidel Costa and James A. Van Orman

1: Extinct Radionuclides and the Earliest Differentiation of the Earth and Moon

SUMMARY

INTRODUCTION

SYSTEMATICS AND REFERENCE PARAMETERS FOR SHORT-LIVED RADIONUCLIDES

HF-W CHRONOLOGY OF THE ACCRETION AND EARLY DIFFERENTIATION OF THE EARTH AND MOON

146,147SM-142,143ND CHRONOLOGY OF MANTLE-CRUST DIFFERENTIATION ON THE EARTH, MARS, AND THE MOON

CONCLUSIONS

ACKNOWLEDGMENTS

REFERENCES

2: Diffusion Constraints on Rates of Melt Production in the Mantle

SUMMARY

INTRODUCTION

SIMPLE MODEL

MORE REALISTIC MODELS

DIFFUSIVE FRACTIONATION OF U-SERIES ISOTOPES

CONCLUDING REMARKS

REFERENCES

3: Melt Production in the Mantle: Constraints from U-series

SUMMARY

INTRODUCTION

SIMPLE VS. COMPLEX MODELS FOR MELT PRODUCTION IN THE MANTLE

HOW DO WE DETERMINE THE MELTING RATE?

MELTING RATES AND GEODYNAMIC PROCESSES

POROSITY OF THE MELTING REGION

CONCLUSIONS

REFERENCES

4: Formulations for Simulating the Multiscale Physics of Magma Ascent

SUMMARY

INTRODUCTION

GRAIN-SCALE PROCESSES

POROUS FLOW IN A DEFORMABLE MEDIA

VEIN NETWORKS

DYKING

DIAPIRISM

ACKNOWLEDGMENT

REFERENCES

5: Melt Transport from the Mantle to the Crust - Uranium-Series Isotopes

SUMMARY

INTRODUCTION

INCEPTION OF MELTING

POROUS FLOW

CHANNEL FLOW

EVIDENCE FROM U-SERIES ISOTOPES

CONCLUDING REMARKS

REFERENCES

6: Rates of Magma Ascent: Constraints from Mantle-Derived Xenoliths

SUMMARY

INTRODUCTION

THE SIGNIFICANCE OF XENOLITHS

ENTRAINMENT OF XENOLITHS - BRITTLE FRACTURE IN THE UPPER MANTLE

ASCENT VELOCITY: CONSTRAINTS FROM THE TRANSPORT OF XENOLITHS

GEOCHEMICAL AND MICROSTRUCTURAL CONSTRAINTS

DISCUSSION

CONCLUSIONS

ACKNOWLEDGMENTS

REFERENCES

7: Time Constraints from Chemical Equilibration in Magmatic Crystals

SUMMARY

INTRODUCTION

RE-EQUILIBRATION OF CHEMICAL ZONING IN CRYSTALS BY DIFFUSION

THE DIFFUSION COEFFICIENT

MEASURING THE CHEMICAL GRADIENT

MODELING THE ZONING PATTERNS OF MINERALS WITH CASE STUDIES

RELATION OF TIMESCALES DETERMINED FROM MODELING THE ZONING PATTERNS WITH OTHER TECHNIQUES

CONCLUDING REMARKS

ACKNOWLEDGMENTS

REFERENCES

8: Magma Cooling and Differentiation - Uranium-series Isotopes

SUMMARY

INTRODUCTION

FRACTIONATION OF U-SERIES ISOTOPES DURING MAGMA DIFFERENTIATION

TIMESCALES OF DIFFERENTIATION FROM U-SERIES ISOTOPES IN CRYSTALS

TIMESCALES OF DIFFERENTIATION FROM WHOLE ROCK SAMPLES

PHYSICAL IMPLICATIONS FOR MAGMA DIFFERENTIATION

CONCLUDING REMARKS

APPENDIX A – CALCULATION OF CRYSTALLIZATION AGES USING 226RA-230TH

APPENDIX B – DISCRETE MAGMA RECHARGE, CONTINUOUS CRUSTAL ASSIMILATION AND CRYSTALLIZATION

REFERENCES

9: Defining Geochemical Signatures and Timescales of Melting Processes in the Crust: An Experimental Tale of Melt Segregation, Migration and Emplacement

SUMMARY

INTRODUCTION

MELT GENERATION AND MOBILITY IN THE MID-TO DEEP CRUST: INSIGHT THROUGH EXPERIMENTATION

MELTING AND ASSIMILATION IN THE UPPER CRUST

CONCLUDING REMARKS

REFERENCES

10: Timescales Associated with Large Silicic Magma Bodies

SUMMARY

INTRODUCTION

HOW TO ESTIMATE TIMESCALES IN LARGE SILICIC MAGMA CHAMBERS?

RADIOMETRIC DATING

MODELING DIFFUSIVE RE-EQUILIBRATION OF CHEMICAL ELEMENTS IN MINERALS AND GLASSES

MODELING TIME-DEPENDANT PHYSICAL PROCESSES

CONCLUDING REMARKS: HOW SOON WILL THE NEXT SUPERERUPTION OCCUR? THE CASE OF YELLOWSTONE

ACKNOWLEDGMENTS

FURTHER READING

REFERENCES

11: Timescales of Magma Degassing

SUMMARY

THE IMPORTANCE OF DEGASSING

THE FIRST STEP: MAKING A BUBBLE

OPEN AND CLOSED SYSTEM DEGASSING

GAS SEGREGATION

TIMESCALES OF DEGASSING

LONG-TERM CONSEQUENCES OF DEGASSING

SYNTHESIS

REFERENCES

Index

Colour plates appear in between pages 154 and 155

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Library of Congress Cataloguing-in-Publication Data

Timescales of magmatic processes: from core to atmosphere / edited by Anthony Dosseto, Simon P. Turner, James A. Van Orman.p. cm.Includes index.ISBN 978-1-4443-3260-5 (cloth) - ISBN 978-1-4443-3261-2 (pbk.) 1. Magmatism. 2. Magmas. 3. Volcanic ash, tuff, etc. 4. Geological time. I. Dosseto, Anthony. II. Turner, Simon P. III. Van Orman, James A. QE461.T526 2010551.1’3-dc22

2010023309

A catalogue record for this book is available from the British Library.

This book is published in the following electronic formats: eBook [9781444328516]; Wiley Online Library [9781444328509]

Set in 9/11.5pt TrumpMediaeval by SPi Publisher Services, Pondicherry, India

1 2011

List of Contributors

OLIVIER BACHMANNDepartment of Earth and Space Sciences, University of Washington, Johnson Hall, room 435, Mailstop 351310, Seattle, WA 98195-1310, USA

KIM BERLODepartment of Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, QC H3A 2A7, Canada

JONATHAN D. BLUNDYDepartment of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, UK

BERNARD BOURDONInstitute of Geochemistry and Petrology, ETH Zurich, NW D 81.4, 8092 clausiusstrasse 25, Switzerland

G. CAROCRPG-CNRS, Nancy Universite, 15 rue Notre-Dame-des-Pauvres, Vandoeuvre-les- Nancy 54501 cedex, France

FIDEL COSTADepartamento de Geofisica y Georiesgos, Institut de Ciencies de la Terra Jaume Almera’, C/Lluis Sole i Sabaris s/n 08028, CSIC, Barcelona, Spain and Earth Observatory of Singapore, Nanyang Technological University, Singapore 637879

ANTHONY DOSSETOGeoQuEST Research Centre, School of Earth and Environmental Sciences, University of Wollongong, Northfields Avenue, Wollongong, NSW 2522, Australia

TIM ELLIOTTBristol Isotope Group, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, UK

JAMES E. GARDNERDepartment of Geological Sciences, University of Texas at Austin, 1 University Station C1100, Austin, TX 78712, USA

W. L. GRIFFINGEMOC ARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Talavera Road, Sydney, NSW 2109, Australia

T. KLEINEInstitut fur Planetologie, Westfalische Wilhelms- UniversitatMunster, Wilhelm-Klemm- Str. 10, 48149 Munster, Germany

KURT KNESELDepartment of Earth Sciences, University of Queensland, Brisbane, Qld 4072, Australia

DANIEL MORGANSchool of Earth and Environment, University of Leeds, LS2 9JT, UK

CRAIG O’NEILLGEMOC ARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Talavera Road, Sydney, NSW 2109, Australia

SUZANNE Y. O’REILLYGEMOC ARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Talavera Road, Sydney, NSW 2109, Australia

TRACY RUSHMERGEMOC ARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Talavera Road, Sydney, NSW 2109, Australia

ALBERTO E. SAALDepartment of Geological Sciences, Brown University, 324 Brook Street, Box 1846, Providence, RI 02912, USA

MARC SPIEGELMANLamont-Doherty Earth Observatory of Columbia University, PO Box 1000, Palisades, NY 10964, USA

SIMON P. TURNERGEMOC ARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Talavera Road, Sydney, NSW 2109, Australia

JAMES A. VAN ORMANDepartment of Geological Sciences, 112 A.W. Smith Building, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106-7216, USA

Introduction to the Timescales of Magmatic Processes

ANTHONY DOSSETO1, SIMON P. TURNER2, FIDEL COSTA3,4 AND JAMES A. VAN ORMAN5

1GeoQuEST Research Centre, School of Earth and Environmental Sciences, University of Wollongong, Wollongong, Australia

2GEMOC ARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW, Australia

3Institut de Ciencies de la Terra ‘Jaume Almera’, CSIC, Barcelona, Spain

4Earth Observatory of Singapore, Nanyang Technological University, Singapore

5Department of Geological Sciences, 112 Case Western Reserve University, Cleveland, OH, USA

The publication in 1928 of The Evolution of Igneous Rocks by Norman L. Bowen laid the foundation to understand the formation and evolution of igneous rocks. Most of the magmatic processes that he proposed are still used and discussed today, and his search for the physical parameters to understand these processes are major themes of current research. At that time, radioactivity had been known for 30 years, but there were as yet no applications to dating of magmatic rocks. Thus any quantification of the timescales of the processes that he proposed was not possible. Determining accurate isotope ratios of natural rocks for the purposes of dating and understanding their formation would come decades later (Paterson, 1956). An example of how critical the quantification of timescales is, and how radiometric dating revolutionized our understanding of the world around us, is the determination of the age of the Earth. Early scientific estimates varied widely (Dalrymple, 1994), but by measuring the trace abundance of isotopes produced by radioactive decay of uranium and thorium, it was possible to demonstrate that the Earth is actually 4.55 billion years old.

Great progress in understanding magmatic processes has been made by a combination of field-work, geochemical analysis, experiments and numerical models (Young, 2003). On the one hand, there has been an avalanche of geochemical data (e.g. databases PETDB, GEOROC and NAVDAT) that have allowed the relationship between plate tectonics and magma genesis and allowed the formation and differentiation of the different reservoirs of the Earth to be explored (Wilson, 1989). The detailed petrological and geochemical studies of individual rocks and crystals have shown that magmas can carry a mixture of components from various sources and with various ages (Davidson et al., 2007). On the other hand, experiments, theory and numerical models have brought a robust understanding of the conditions and intensive variables under which magmas can be generated, differentiated, and may erupt (Carmichael & Eusgter, 1987; MELTS algorithm of Ghiorso & Sack, 1995). Numerical models built upon robust physical properties of multicomponent silicate melts (viscosity, density and diffusivities; Stebbins et al., 1995) can be used to investigate the dynamics of magmatic processes (Nicholls & Russell, 1990; Spera & Bohrson, 2001).

Early dating of rock and mineral suites by various radioactive decay systems (e.g.14C, Rb-Sr, K-Ar, Sm-Nd, U-Pb) provided a calibrated sequence of magmatic and volcanic events (eruption or emplacement of plutonic bodies) that could be used for the first time to estimate rates of magma production and evolution. However, the errors resulting from the large amount of sample required and the precisions of the measurements did not allow distinction between eruption and crystallization ages, or for dating a series of eruptions from the same vent, which may record the magmatic differentiation (Faure, 1986).

A turning point in understanding the dynamics of magmatic processes has occurred in the last 10 to 20 years, with technological advances in mass spectrometry and the accurate analysis of short-lived radioactive isotopes of the U and Th decay series (Bourdon et al., 2003), as well as the possibility of in-situ dating of extremely small amounts of minerals (diameter of several tens of μm (Hanchar & Hoskin, 2003)). Furthermore, a large amount of data on the kinetics of element migration in minerals together with natural observations has led to the realization that the chemical heterogeneities in crystals can also be used to extract time information on mag-matic processes. Thus, it is now possible to study magmatic processes over timescales ranging from a few minutes to millions of years. This allows us to tackle critical questions about the evolution of Earth systems: How long is a magma stored in a reservoir prior to eruption? How long does it take to evolve from mafic to felsic magma compositions? How long does it take for a magma generated in the mantle to reach the surface? How long after the Earth was created did a metallic core form?

In this book we bring together syntheses of work aimed at tackling these questions. Although the age of the Earth is now well known, until recently little was known about how the seemingly homogeneous proto-Earth differentiated into different envelopes (core, mantle, crust). In the first chapter, Caro and Kleine show how extinct radionuclides can be used to demonstrate that differentiation of the Earth’s core and the initiation of plate tectonics all occurred very early in the Earth’s history. The differentiation of the Earth into various reservoirs is still a work in progress. Material is continuously transferred from the mantle towards the surface and surface material is recycled back into the mantle at subduction zones. To understand the mechanisms that allow melting of mantle rocks and the transfer of produced magmas to the surface, it is necessary to quantify the timescales of production and transport of magmas below mid-ocean ridges and at hotspots. Two complementary approaches can be used to constrain the times-cales of magma production in the mantle: uranium-series disequilibrium and the diffusion of trace elements in minerals.

Uranium and thorium-series isotopes compose the decay chains that start with 238U, 235U and 232Th, and end with stable isotopes 206Pb, 207Pb and 208Pb, respectively (Figure 0.1). These decay chains are composed of radioactive systems where a parent nuclide decays into the daughter by alpha or beta emission. For instance, 230Th decays into 226Ra by alpha emission. For systems closed for more than 1 Myr, all radioactive systems in the decay chains are in secular equilibrium: the activities (i.e. rates of decay) of parent and daughter nuclides are equal. For instance, (230Th) = (226Ra), or (226Ra/230Th) = 1 (where parentheses denote activities). This is the case for mantle rocks prior to melting. During geological processes, such as partial melting or fractional crystallization, the different radionuclides behave differently inducing radioactive disequilibrium. Thus, because 226Ra is usually more incompatible than 230Th (i.e. it partitions preferentially into the magma relative to the residue during partial melting), magmas are often characterized by (226Ra/230Th) >1. Once radioactive disequilibrium is produced, the system returns to secular equilibrium by radioactive decay over a times-cale that is about five times the half-life of the

Fig. 0.1238U and 235U decay chains. See Plate 0.1 for a colour version of this image.

shorter-lived daughter nuclide. For example, if a magma with (226Ra/230Th) >1 behaves as a closed system once the disequilibrium is acquired, secular equilibrium will be attained after –8,000 years (226Ra half-life is 1,602 years (Bourdon et al. 2003)) (Figure 0.2). Thus, because U-series isotopes fractionate during magmatic processes and their ratios are time-dependent, they provide a critical tool for constraining the timescales of magmatic processes.

In Chapter 3, Bourdon and Elliott show that the study of the U-series isotope composition of basaltic rocks can be used to constrain the conditions and timescales of mantle melting. It is

Fig. 0.2 Decay of radioactive dis-equilibria for different systems. The timescale of return to secular equilibrium is directly a function of the daughter nuclide half-life in the system considered: –8,000 years for 226Ra-230Th, 300,000 years for 230Th-238U. See Plate 0.2 for a colour version of this image.

shown that the porosity of the matrix undergoing melting needs to be as low as a few per mil, in order to account for observed U-series isotope compositions. Moreover, the rate of melting beneath oceanic islands (e.g. Hawaii, Reunion) is inferred to be related to the upwelling rate of the hotspot (i.e. mantle instability). They show that melt production is greater at large hotspots, and that melting beneath oceanic islands occurs faster than beneath mid-ocean ridges. During partial melting of mantle rocks, trace elements partition between minerals and the newly formed magma and it is generally assumed that a partitioning equilibrium is reached between minerals and magma, as trace elements are believed to diffuse rapidly into the magma. Nevertheless, in Chapter 2, Van Orman and Saal show that diffusion of trace elements can be very slow, leading to an incomplete equilibrium between minerals and magma. They also show that different rates of diffusion for 238U, 230Th, 226Ra and 210Pb can explain most of the observed radioactive disequilibria in basaltic rocks. Thus, diffusion of trace elements (including radionuclides) needs to be taken into account when inferring rates of magma production.

Mantle melting operates at variable rates between different tectonic environments (hots-pots, mid-ocean ridges, arcs). In all cases, once a significant amount of melt is produced, it will rise towards the surface, because it is less dense than the surrounding rocks. In Chapter 4, O’Neill and Spiegelman show how numerical modeling can provide some understanding of the mechanisms and timescales involved in the transfer of magmas from their source region towards the surface. They suggest that melt transport can occur over a wide range of timescales: from several millimeters per year when a pluton intrudes into the brittle crust, up to several tens of kilometers per hour during dyke propagation and elastic fracturing.

Because radioactive disequilibrium between U-series isotopes is created during mantle melting and any disequilibrium decreases will disappear within up to 300,000 years after production, U-series isotopes can also be used to constrain the timescales of magma transport. For instance, in Chapter 5, Turner and Bourdon show that magma ascent rates must be at least –1 to 20 m/yr in order to preserve radioactive disequilibria produced in the mantle and observed in many basaltic rocks. Furthermore, because any 226Ra-230Th disequilibrium vanishes after –8000 years, the observation of 226Ra-230Th disequilibrium in island arc basalts implies magma ascent rates in subduc-tion zones as high as 70 m/yr. Magma transport in channels and/or rapidly propagating fractures is required to account for such large ascent rates. The study of xenoliths (i.e. fragments of rock detached from the deep lithosphere and transported in a magma) can also provide information on the timescales of magma ascent. Because xenoliths are denser than their host magma, the magma is required to have a minimum velocity in order to transport them to the surface and prevent their settling. Moreover, the chemical composition of xenoliths is not in equilibrium with that of the host magma, and this disequilibrium disappears with time. Thus, the study of xeno-liths and host magma compositions can be used to determine magma ascent rates. In Chapter 6, O’Reilly and Griffin show that ascent rates for (volatile-rich) alkali basalts range from 0.2 to 2 m/s, i.e. 6 orders of magnitude faster than that of calc-alkaline basalts, inferred from U-series isotopes. Furthermore, kimberlites are believed to erupt at near-supersonic speeds (≥300 m/s). To summarize, melt transport can occur over a wide range of timescales that reflect the variety of mechanisms involved (e.g. slow porous vs. fast channel flow). Moreover, magma transport can be very rapid, with transfer from the Earth’s mantle to the surface in only a few hours in the case of alkali basalts.

Magmas do not always reach the surface directly from the region of their production, but frequently stall in the crust where they differentiate, leading to the observed diversity of igneous rock compositions. To understand how magma differentiation produces such a wide range of compositions, it is important to constrain the rates of magma differentiation to discriminate between physical models of magma emplacement in the crust, cooling and interaction with the country rock. In Chapter 7, Costa and Morgan show how the study of elemental and isotopic composition profiles in minerals can be used to infer rates of magma cooling. The approach is based on several concepts:

• A gradient of concentrations for a given element exists between crystals and the magma they are derived from;

• Concentrations between the crystal and the melt will equilibrate with time by diffusion: and

• Diffusion rates are different for each element. Thus, because chemical equilibration between a melt and a crystal takes time, concentration and isotope composition profiles can be used to infer the timescales of magma evolution. Diffusion studies suggest that characteristic timescales of magma evolution in the crust (crystal fractiona-tion, crust assimilation, magma recharge, etc.) are typically up to a few hundred years.

Similarly to previous chapters presenting the use of U-series isotopes to investigate rates of magma production and transfer, Chapter 8 by Dosseto and Turner shows how the U-series isotope composition of co-genetic volcanic rocks can be used to infer timescales of magma differentiation. Two approaches are presented: the first one is based on a study of the U-series isotope composition of minerals. It is shown that depending on the degree of differentiation of the volcanic rock hosting the minerals (i.e. more or less silica-rich), timescales of differentiation can vary from less than one thousand years to several hundred thousand years. However, minerals from the same rock can also yield contrasting timescales. This is most likely explained by the complex history of minerals during magmatic evolution; for instance, minerals carried by a volcanic rock may not be in equilibrium with the host magma and have formed from an earlier magmatic batch. To circumvent this problem, it would be necessary to analyse profiles of the U-series isotope compositions in minerals. This represents a major analytical challenge, although recent advances in in-situ geochemical techniques promise more detailed investigations of mineral isotope composition in the near future. Another approach is to study the U-series isotope composition of whole

Fig. 0.3 Schematic drawing summarizing the timescales of magmatic processes. Early Earth differentiation occurs shortly after the beginning of the solar system (less than a few 100 million years). Inset (a) shows melt production at different tectonic settings (from left to right: subduction, divergent plates and hotspot). Timescales of magma production and transfer towards the surface are inferred to be very short, of the order of a few 1,000 years or less. Inset (b) shows magma differentiation in the crust (regardless the tectonic setting). Crystal fractionation, crustal assimilation or magma degassing occur over short timescales, typically a few 1,000 years or less. Note however that some magma bodies can exist for several 100,000 years, but the actual process of differentiation of the magma into a wide range of composition is very short. Moreover, it takes 100,000’s of years to heat up the crust to allow crustal assimilation, but once the required thermal regime is reached, production and assimilation of crustal melts into mantle-derived magmas is very rapid. See Plate 0.3 for a colour version of these images.

rock samples. It is shown in Dosseto and Turner’s chapter that changes in U-series isotope composition of volcanic rocks with differentiation indices yield timescales of differentiation of the order of a few thousand years or less, even when crustal assimilation and frequent recharge of the magma body are taken into account.

Silicic magma bodies are often characterized by timescales of differentiation of the order of several hundreds of thousands of years, as indicated by U-series isotopes in crystals. In Chapter 10, Bachmann explores the timescales related to large eruptions of silicic magma bodies such as Yellowstone Caldera. Large silicic eruptions can have dramatic consequences for life and there is a clear inverse relationship between the frequency of these eruptions and their size. Although it is crucial to be able to predict such events, our understanding of the mechanisms involved is still incomplete. The techniques presented in this volume carry the hope of further advances in the near future.

As mantle-derived magmas are emplaced into the crust, they interact with the host rock and can induce significant melting of the surrounding rock. In Chapter 9, Rushmer and Knesel show how melting experiments and the geochemistry of melts and restites (the residual solid from partial melting) shed light on the mechanisms and times-cales of crustal melting. It is shown that the composition of melts produced from the crust depends not only on the composition of the starting material (i.e. the crustal rock that undergoes melting) but also on the conditions of melting (pressure, temperature, presence or not of deformation). Rushmer and Knesel show that crustal melting and ‘contamination’ of mantle-derived magmas occur over timescales of a few hundred years.

Finally, when magmas erupt, they release a wide range of gases in variable abundances. This phenomenon is of primary importance since it is linked to the origin and evolution of the Earth’s atmosphere. It can also represent a significant hazard, as major degassing is generally associated with the most destructive eruptions. In Chapter 11, Berlo et al. discuss how the timescales of degassing can be quantified, in particular using short-lived radionuclides such as 210Pb. It is shown that degassing is a complex process where some magma bodies can accumulate significant volumes of gas derived from the degassing of other volumes of magma. Timescales of magma degassing are very short compared to timescales of differentiation and are generally on the order of a few years.

In conclusion, magmatism on Earth encompasses a variety of processes – from partial melting, to transport, to crystallization and degassing in magma chambers – that operate on vastly different timescales (Figure 0.3). Our understanding of these processes and their timescales, in various tectonic settings, relies on integration of observational, theoretical and experimental constraints from many sub-disciplines in geochemistry and geophysics. This book provides an introduction to the various approaches that are used to study magmatic timescales, and the key constraints that have been derived from each. By introducing the reader to all of the aspects that should be treated to gain a better understanding of mag-matic processes, we hope that this book opens new doors onto the future.

REFERENCES

Bourdon B, Henderson GM, et al. 2003. Uranium-series Geochemistry. Washington Geochemical Society – Mineralogical Society of America, Washington DC.

Bowen NL. 1921. Diffusion in silicate melts. Journal of Geology30: 295–317.

Bowen NL. 1928. The Evolution of Igneous Rocks. Princeton University Press, Princeton, 332 pp.

Carmichael ISE, Eugster HP. 1987. Thermodynamic modeling of geological materials: minerals, fluids and melts. Reviews in Mineralogy and Geochemistry17: 500 pp.

Dalrymple GB. 1994. The Age of the Earth. Stanford University Press, Stanford, 492 pp.

Davidson JP, Morgan DJ et al. 2007. Microsampling and isotopic analysis of igneous rocks: implications for the study of magmatic systems. Annual Reviews of Earth Planetary Science35: 273–311.

Faure G. 1986. Principles of Isotope Geology. John Wiley & Sons, New York, Chichester, Brisbane, Toronto, Singapore.

Ghiorso MS, Sack RO. 1995. Chemical mass transfer in magmatic processes IV. A revised and internally consistent thermodynamic model for the interpolation and extrapolation of liquid-solid equilibria in mag-matic systems at elevated temperatures and pressures. Contributions to Mineral Petrology119: 197–212.

Hanchar JM, Hoskin PWO. 2003. Zircon. Reviews inMineralogy and Geochemistry53: 500 pp.

Nicholls J, Russell JK. 1990. Modern methods of igneous petrology: understanding magmatic processes. Reviews in Mineralogy and Geochemistry24: 314 pp.

Paterson C. 1956. Age of meteorites and the Earth. Geochimica et Cosmochimica Acta10: 230–237.

Spera FJ, Bohrson WA. 2001. Energy-Constrained Open-System Magmatic Processes I: General Model and Energy-Constrained Assimilation and Fractional Crystallization (EC-AFC) Formulation. Journal of Petrology42: 999–1018.

Stebbins JF, McMillan PF, Dingwell DB. 1995. Structure, dynamics and properties of silicate melts. Reviews in Mineralogy and Geochemistry32: 616 pp.

Wilson M. 1989. Igneous Petrogenesis: A Global Tectonic Approach. Springer-Verlag, Berlin, 466 pp.

Young DA. 2003. Mind over Magma. Princeton University Press, Princeton NJ, 686 pp.

1

Extinct Radionuclides and the Earliest Differentiation of the Earth and Moon

G. CARO1 AND T. KLEINE2

1CRPG-CNRS, Nancy Université, Vandoeuvre-les-Nancy, France 2Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Münster, Germany

SUMMARY

The extinct 182Hf-182W and 146Sm-142Nd systems provide key chronological constraints on the major episodes of planetary differentiation. Both chronometers can be considered extinct after approximately 5–6 half-lives (i.e., after 50 Myr and 400 Myr respectively) and are therefore selectively sensitive to early events. Application of 182Hf-182W chronometry shows that segregation of the Earth’s core may have been complete no earlier than 30 Myr after formation of the solar system and probably involved at least partial re-equilibration of newly accreted metallic cores within the terrestrial magma ocean. The current best estimate for the termination of the major stage of Earth’s accretion and segregation of its core is provided by the age of the Moon, which formed as a result of a giant collision of a Mars-sized embryo with the proto-Earth. According to 182Hf-182W systematics this event occurred 50–150 Myr after CAI formation. As the Earth cooled down following the giant impact, crystallization of the magma ocean resulted in the formation of the earliest terrestrial crust. While virtually no remnant of this protocrust survived in the present-day rock record, the isotopic fingerprint of this early event is recorded in the form of small 142Nd anomalies in early Archean rocks. These anomalies show that magma ocean solidification must have taken place 30–100 Myr after formation of the solar system. In contrast, 146Sm-142Nd systematics of lunar samples show that the lunar mantle may have remained partially molten until 300 Myr after CAI formation. Therefore, extinct chronometers indicate that accretion and differentiation of the Earth proceeded rapidly. The core, mantle and crust were completely differentiated less than 100 Myr after formation of the solar system.

INTRODUCTION

The accretion and earliest history of the Earth was an episode of major differentiation of a magnitude that probably will never be repeated. Frequent and highly energetic impacts during the Earth’s growth caused widespread melting, permitting separation of a metallic core from a silicate mantle in a magma ocean. Soon after the major stages of the Earth’s growth were complete, the magma ocean solidified and a first protocrust formed. Earth is an active planet, however, concealing most of the evidence of its earliest evolutionary history by frequent rejuvenation of its crust. Consequently, there is no direct rock record of the Earth’s origin and earliest evolution, but fortunately witnesses of the Earth’s earliest evolution have been preserved as small isotope anomalies in the terrestrial rock record. The short-lived Hf-W and Sm-Nd isotope systems provide key constraints for understanding the Earth’s accretion and earliest differentiation, and in this chapter, the basic theory of these isotope systems and their application to models of the Earth’s formation and differentiation will be discussed.