Mid-Ocean Ridges

320 Pages·2004·56.215 MB·English

Geophysical Monograph Series
Including
IUGG Volumes
Maurice Ewing Volumes
Mineral Physics Volumes
Geophysical Monograph Series
112 Mechanisms of Global Climate Change at Millennial 130 Atmospheres in the Solar System: Comparative
Time Scales Peter U. Clark, Robert S. Webb, and Lloyd Aeronomy Michael Mendillo, Andrew Nagy and J. H.
D. Keigwin (Eds.) Waite (Eds.)
113 Faults and Subsurface Fluid Flow in the Shallow Crust 131 The Ostracoda: Applications in Quaternary Research
William C. Haneberg, Peter S. Mozley J. Casey Moore, Jonathan A. Holmes and Allan R. Chivas (Eds.)
and Laurel B. Goodwin (Eds.) 132 Mountain Building in the Uralides Pangea to the
114 Inverse Methods in Global Biogeochemical Cycles Present Dennis Brown, Christopher Juhlin, and Victor
Prasad Kasibhatla, Martin Heimann, Peter Rayner, Puchkov (Eds.)
Natalie Mahowald, Ronald G. Prinn, and Dana E. 133 Earth's Low-Latitude Boundary Layer Patrick T. Newell
Hartley (Eds.) and Terry Onsage (Eds.)
115 Atlantic Rifts and Continental Margins Webster 134 The North Atlantic Oscillation: Climatic Significance
Mohriak and Manik Talwani (Eds.) and Environmental Impact James W. Hurrell, Yochanan
116 Remote Sensing of Active Volcanism Peter J. Mouginis- Kushnir, Geir Ottersen, and Martin Visbeck (Eds.)
Mark, Joy A. Crisp, and Jonathan H. Fink (Eds.) 135 Prediction in Geomorphology Peter R. Wilcock and
117 Earth's Deep Interior: Mineral Physics and Tomography Richard M. Iverson (Eds.)
From the Atomic to the Global Scale Shun-ichiro
136 The Central Atlantic Magmatic Province: Insights from
Karato, Alessandro Forte, Robert Liebermann, Guy
Fragments of Pangea W. Hames, J. G. McHone, P.
Masters, and Lars Stixrude (Eds.)
Renne, and C. Ruppel (Eds.)
118 Magnetospheric Current Systems Shin-ichi Ohtani,
137 Earth's Climate and Orbital Eccentricity: The Marine
Ryoichi Fujii, Michael Hesse, and Robert L. Lysak (Eds.)
Isotope Stage 11 Question Andre W. Droxler, Richard
119 Radio Astronomy at Long Wavelengths Robert G. Z. Poore, and Lloyd H. Burckle (Eds.)
Stone, Kurt W. Weiler, Melvyn L. Goldstein, and Jean-
138 Inside the Subduction Factory John Filer (Ed.)
Louis Bougeret (Eds.)
139 Volcanism and the Earth's Atmosphere Alan Robock
120 GeoComplexity and the Physics of Earthquakes John
and Clive Oppenheimer (Eds.)
B. Rundle, Donald L. Turcotte, and William Klein (Eds.)
140 Explosive Subaqueous Volcanism James D. L. White,
121 The History and Dynamics of Global Plate Motions
John L. Smellie, and David A. Clague (Eds.)
Mark A. Richards, Richard G. Gordon, and Rob D. van
141 Solar Variability and Its Effects on Climate Judit M. Pap
der Hi 1st (Eds.)
and Peter Fox (Eds.)
122 Dynamics of Fluids in Fractured Rock Boris
142 Disturbances in Geospace: The Storm-Substorm
Faybishenko, Paul A. Witherspoon, and Sally M.
Relationship A. Surjalal Sharma, Yohsuke Kamide, and
Benson (Eds.)
Gurbax S. Lakhima (Eds.)
123 Atmospheric Science Across the Stratopause David E.
143 Mt. Etna: Volcano Laboratory Alessandro Bonaccorso,
Siskind, Stephen D. Eckerman, and Michael E.
Sonia Calvari, Mauro Coltelli, Ciro Del Negro, and
Summers (Eds.)
Susanna Falsaperla
124 Natural Gas Hydrates: Occurrence, Distribution, and
144 The Subseafloor Biosphere at Mid-Ocean Ridges
Detection Charles K. Paull and Willam P. Dillon (Eds.)
William S. D. Wilcock, Edward F. DeLong, Deborah S.
125 Space Weather Paul Song, Howard J. Singer, and
Kelley John A. Baross, and S. Craig Cary (Eds.)
George L. Siscoe (Eds.)
145 Timescales of the Paleomagnetic Field James E. T.
126 The Oceans and Rapid Climate Change: Past, Present, Channell, Dennis V. Kent, William Lowrie, and Joseph
and Future Dan Seidov, Bernd J. Haupt, and Mark G. Meert (Eds.)
Maslin (Eds.)
146 The Extreme Proterozoic: Geology, Geochemistry, and
127 Gas Transfer at Water Surfaces M. A. Donelan, W. M. Climate Gregorys. Jenkins, Mark A. S. McMenamin,
Drennan, E. S. Saltzman, and R. Wanninkhof (Eds.) Christopher P. McKay, and Linda Sohl (Eds.)
128 Hawaiian Volcanoes: Deep Underwater Perspectives 147 Earth's Climate: The Ocean-Atmosphere Interaction
Eiichi Takahashi, Peter W. Lipman, Michael O. Garcia, Chuzai Wang, Shang-Ping Xie, and James A. Carton
Jiro Naka, and Shigeo Aramaki (Eds.) (Eds.)
129 Environmental Mechanics: Water, Mass and Energy 148 Mid-Ocean Ridges: Hydrothermal Interactions
Transfer in the Biosphere Peter A. C. Raats, David Between the Lithosphere and Oceans Christopher
Smiles, and Arthur W. Warrick (Eds.) German, Jian Lin, and Lindsay Parson (Eds.)
Geophysical Monograph 148
Mid-Ocean Ridges: HYdrothermal Interactions
Between the Lithosphere and Oceans
Christopher R. German
Jian Lin
Lindsay M. Parson
Editors
American Geophysical Union
Washington, DC
Published under the aegis of the AGU Books Board
Jean-Louis Bougeret, Chair; Gray E. Bebout, Carl T. Friedrichs, James L. Horwitz, Lisa A. Levin, W. Berry Lyons,
Kenneth R. Minschwaner, Andy Nyblade, Darrell Strobel, and William R. Young, members.
Library of Congress Cataloging-in-Publication Data
Mid-ocean ridges : hydrothermal interactions between the lithosphere and oceans / Christopher
R. German, Jian Lin, Lindsay M. Parson, editors.
p. cm.- (Geophysical monograph ; 148)
Includes bibliographical references.
ISBN 0-87590-413-0 (alk. paper)
1. Hydrothermal circulation (Oceanography). 2. Mid-ocean ridges-Research. 3. Hydrothermal
vents. 4. Sea-floor spreading. 5. Earth-Crust. I. German, Christopher R. II. Lin, Jian. III. Parson,
Lindsay M.
GC171.M53 2004
55.1/36'2-dc22
2004062292
ISBN 0-87590-413-0
ISSN 0065-8448
Copyright 2004 by the American Geophysical Union
2000 Florida Avenue, N.W.
Washington, DC 20009
Cover: The black smoker chimney "Candelabra" located within the Logatchev hydrothermal field,
Mid-Atlantic Ridge 15°N. Photo taken with the Breman Quest 4000 ROV during cruise M60/3 of
the German Research vessel Meteor (Chief Scientist Thomas Kuhn). Copyright Universitat Bremen
2004 (courtesy of Colin Devey, InterRidge).
Figures, tables, and short excerpts may be reprinted in scientific books and journals if the source
is properly cited.
Authorization to photocopy items for internal or personal use, or the internal or personal use of specific
clients, is granted by the American Geophysical Union for libraries and other users registered with the
Copyright Clearance Center (CCC) Transactional Reporting Service, provided that the base fee of $ 1.50
per copy plus $0.35 per page is paid directly to CCC, 222 Rosewood Dr., Danvers, MA 01923. 1526-
8448/04/$01.50+0.35.
This consent does not extend to other kinds of copying, such as copying for creating new collec­
tive works or for resale. The reproduction of multiple copies and the use of full articles or the use
of extracts, including figures and tables, for commercial purposes requires permission from the
American Geophysical Union.
Printed in the United States of America.
CONTENTS
Preface
Christopher R. German, Jian Lin, and Lindsay M. Parson vii
The Thermal Structure of the Oceanic Crust, Ridge-Spreading and Hydrothermal Circulation:
How Well Do We Understand Their Inter-Connections?
Christopher R. German and Jian Lin 1
Geophysical Constraints Upon the Thermal Regime of the Ocean Crust
Martin C Sinha and Rob L. Evans 19
The Rheology and Morphology of Oceanic Lithosphere and Mid-Ocean Ridges
R. C. Searle and J. Escartfn 63
Modeling the Thermal State of the Oceanic Crust
Yongshun John Chen 95
Some Hard Rock Constraints on the Supply of Heat to Mid-Ocean Ridges
Mathilde Cannat, Joe Cann, and John Maclennnan 111
Effects of Hydrothermal Cooling and Magma Injection on Mid-Ocean Ridge
Temperature Structure, Deformation, and Axial Morphology
Mark D. Behn, Jian Lin, and Maria T. Zuber 151
Experimental Constraints on Thermal Cracking of Peridotite at Oceanic Spreading Centers
Brian deMartin, Greg Hirth, and Brian Evans 167
Submarine Lava Flow Emplacement at the East Pacific Rise 9° 50'N: Implications for
Uppermost Ocean Crust Stratigraphy and Hydrothermal Fluid Circulation
Daniel Fornari, Maurice Tivey, Hans Schouten, Michael Perfit, Dana Yoerger, Al Bradley Margo
Edwards, Rachel Haymon, Daniel Scheirer, Karen Von Damm, Timothy Shank, and Adam Soule 187
Hydrothermal Processes at Mid-Ocean Ridges: Results From Scale Analysis and Single-Pass Models
Robert P Lowell and Leonid N. Germanovich 219
On the Global Distribution of Hydrothermal Vent Fields
Edward T. Baker and Christopher R. German 245
Ultramafic-Hosted Hydrothermal Systems at Mid-Ocean Ridges: Chemical and
Physical Controls on pH, Redox and Carbon Reduction Reactions
W. E. Seyfried, Jr., D. I. Foustoukos and D. E. Allen 267
Evolution of the Hydrothermal System at East Pacific Rise 9°50'N:
Geochemical Evidence for Changes in the Upper Oceanic Crust
Karen L. Von Damm 285
Vigorous Venting and Biology at Pito Seamount, Easter Microplate
D. F. Naar, R. Hekinian, M. Segonzac, J. Francheteau, and the Pito Dive Team 305
PREFACE
Mid-ocean ridges play an important role in the plate-tectonic lithosphere. Geophysical techniques allow acoustic and geo­
cycle of our planet. Extending some 50-60,000 km across physical mapping of the fine-scale tectonic and volcanic mor­
the ocean-floor, the global mid-ocean ridge system is the site phology of the ocean floor from both surface ships and
of creation of the oceanic crust and lithosphere that covers deep-submergence vehicles. New seismic and electromag­
more than two thirds of the Earth's exterior. Approximately netic technologies provide us with ever-improving images of
75% of Earth's total heat flux occurs through oceanic crust, the interior of the oceanic crust and upper mantle. In parallel
much of it at mid-ocean ridges through complex processes with the above, direct observations can be used, on the mod­
associated with magma solidification, heat transfer, and cool­ ern seafloor, in materials recovered from ocean drilling and in
ing of young oceanic lithosphere. While the majority of this ophiolites that are outcrops of fossilized mid-ocean ridge crest
heat loss occurs through conduction, approximately one third uplifted and preserved on land. Detailed petrological analyses
of the total heat loss at mid-ocean ridges is influenced by a con­ allow researchers to derive information from the thermody­
vective process: hydrothermal circulation. namic equilibrium between different mineral phases and recon­
Hydrothermal circulation facilitates the cycling of energy struct thermal cooling histories of crust and mantle rocks.
and mass between the solid Earth and the oceans, influenc­ Theoretical modeling and laboratory experiments further inte­
ing the composition of the Earth's lithosphere, oceans and grate different observations into coherent conceptual models
even, albeit largely indirectly, the atmosphere. Hydrother­ of how melts are formed in the mantle and rise to the shallow
mal circulation arises when seawater percolates downward depths at which they form oceanic crust and how the oceanic
through fractured oceanic crust: the seawater is heated and lithosphere is fractured and cracked, providing permeability
undergoes chemical modification through reaction with the conduits for hydrothermal circulation.
host rock, reaching maximum temperatures that can exceed The second area of mid-ocean ridge research that allows us
400°C. At these temperatures the fluids become extremely a better understanding of the coupling of the solid earth and
buoyant and rise rapidly back to the seafloor, where they are oceans is submarine hydrothermal activity. Again, this area
expelled into the overlying water column. Active sites of of research has undergone rapid progress in the past decade
hydrothermal discharge can provide extreme ecological niches using a combination of techniques: remote detection using
that can host unique chemosynthetic fauna, previously surface ships and deep-tow vehicles, direct submersible-based
unknown to science. observations at the seafloor, instruments installed on the sea­
The first identification of submarine hydrothermal vent­ floor for substantial periods of time, numerical modeling and
ing and their accompanying chemo synthetically-based com­ complementary experimentation in the laboratory. Systematic
munities in the late 1970s remains one of the most exciting surveys, detecting and determining the magnitude of physical
discoveries in modern science. A quarter of a century later, and chemical anomalies imparted to the water column above
however, many of the processes that control magma supply and mid-ocean ridges, permit determination of the geographic
replenishment beneath the axial ridge-crest, the distributions locations of seafloor hydrothermal vent-sites co-registered
of hydrothermal activity around the global ridge system, the with geological information about their volcanic/tectonic set­
timescales over which individual hydrothermal systems remain ting. Sampling and analysis of the chemically-enriched flu­
active or become renewed, and the detailed circulation path­ ids discharging from these vent-sites at the seafloor can provide
ways for hydrothermal fluids within young oceanic crust first interpretations of the detailed mineral-water interactions
remain only incompletely understood. taking place at depth beneath the sea-bed at these sites and
Although such questions are complex, we are beginning to also, through time-series data and repeat sampling programmes,
make important progress in two key fields of mid-ocean ridge how those processes are progressing with time. By conducting
study which, taken together, have allowed us to develop new complementary experimental procedures at high pressure and
understandings. A first key area concerns the structure and high temperature in the laboratory, we can test hypotheses
development of young ocean crust and the mechanisms by developed from direct observations alone to refine our under­
which heat is transferred from the Earth's interior to the standing of the processes active within the inaccessible seafloor.
Numerical modeling has an important role to play here, pro­
viding valuable insight into the extent to which a limited num­
Mid-Ocean Ridges: Hydrothermal Interactions Between the
ber of direct "snap-shot" observations can be used to extrapolate
Lithosphere and Oceans
Geophysical Monograph Series 148 to a longer-term understanding of how hydrothermal systems
Copyright 2004 by the American Geophysical Union may evolve. Those models, in addition, can also help us pre­
10.1029/148GM00 dict what impact hydrothermal circulation may have, in return,
vii
upon the thermal structure and, hence, geologic behaviour of opments within each of two sub-fields of mid-ocean ridge
its host oceanic crust. research that are most relevant to understanding how mag-
The structure of the material within this volume reflects matic heat supply, tectonic deformation and seafloor hydrother­
the two key areas in which most recent progress has been mal circulation might all interact, with an increasing emphasis
made: the 4-D structure (including temporal evolution) of on Earth's less well-understood slow spreading ridges.
mid-ocean ridges and submarine hydrothermal activity. To The volume derives primarily from presentations and dis­
illustrate where our current understanding begins—and more cussions at the first InterRidge Theoretical Institute that was
importantly, ends—Chapter 1 investigates the extent to which held in Italy in September 2002. The majority of the chapters
coupling of hydrothermal activity and formation and devel­ derive directly from a two-day short course held at the Uni­
opment of young oceanic crust are currently understood. The versity of Pavia at the start of that meeting and establish the
key message here is not how much we know but how much current international state-of-the-art. Additional chapters have
there remains for us to know. been contributed by workshop participants based on discus­
This is followed by a series of chapters (Chapters 2-6) that sions that ensued during the later workshop phase of the meet­
explain what we already know about mid-ocean ridge formation ing, at Sestri Levante and provide examples of cutting-edge
and thermal structure and how this information has been research in this active field that expand the breadth and diver­
obtained—from geophysical remote sensing, from sea-floor sity of the finished volume.
imaging and mapping, from theoretical modeling, and from As editors, we are particularly grateful to InterRidge, the
petrological sampling coupled with mineralogical and geo- European Science Foundation, and the US NSF-sponsored
chemical analysis. Chapter 7 describes an important laboratory RIDGE 2000 Program for their support of this endeavour and
study of thermal cracking of rocks and discusses its implica­ particularly to our co-organisers of the 1st InterRidge Theo­
tions for rock fracture and thermal structure at mid-ocean ridges. retical Institute: Agnieszka Adamczewska and Kensaku Tamaki
The second major section of the book (Chapters 8-13) (Japan), Ricardo Tribuzzio (Italy), Mathilde Cannat (France)
details our current state-of-the-art concerning on-axis and Andy Fisher (USA). We are, of course, very grateful to all
hydrothermal circulation along the global ridge-crest. Impor­ the authors for this volume, who worked diligently to meet our
tant chapters here include an understanding of the geologic deadlines and did all required to bring this project to a suc­
controls of hydrothermal activity on a fast-spreading ridge cessful outcome. We particularly appreciate the efficiency,
and, a particularly new development, the temporal variability understanding and expertise of the AGU staff who have worked
of hydrothermal activity at any given vent-site. A further con­ so effectively with us to ensure the timely publication of this
tribution describes current knowledge of the distributions of book. We are grateful, too, to our reviewers who devoted
hydrothermal activity along the global ridge-crest and how much time and effort to ensure the high quality of the final vol­
this may vary with spreading rate, an important, increasingly ume. Finally, we would like to pay particular tribute to Allan
recognized theme, which is further elucidated in chapters on Graubard, our acquisitions editor, for his support and encour­
flow mechanisms within different hydrothermal systems and agement throughout the entire project.
a specific consideration of the processes driving ultramafic-
hosted hydrothermal systems. The latter are likely to be unique
Christopher R. German
to slow- and ultraslow-spreading ridge crests.
Challenger Division for Seafloor Processes
The book builds directly on two earlier AGU volumes
Southampton Oceanography Centre, UK
although we do not attempt to reconstruct the full breadth of
scope of either of those volumes: Seafloor Hydrothermal Sys­ Jian Lin
tems: Physical, Chemical, Biologic and Geological Interac­ Department of Geology and Geophysics
tions (Humpris, S.E., Zierenberg, R.A., Mullineaux, L.S. and Woods Hole Oceanographic Institution, USA
R.E.Thomson, Editors, AGU, 1995) and Faulting and Mag-
matism at Mid-Ocean Ridges (Buck, W.R., Delaney, P.T., Kar- Lindsay M. Parson
son, J.A. and Lagabrielle, Y., Editors, AGU, 1998). Rather, the Challenger Division for Seafloor Processes
work presented here pulls together just those most recent devel­ Southampton Oceanography Centre, UK
viii
The Thermal Structure of the Oceanic Crust,
Ridge-Spreading and Hydrothermal Circulation:
How Well Do We Understand Their Inter-Connections?
Christopher R. German
Southampton Oceanography Centre, Southampton, UK
Jian Lin
Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA
Understanding the complex interplay between the geological processes active at
mid-ocean ridges and the overlying ocean through submarine hydrothermal circu­
lation remains a fundamental goal of international mid-ocean ridge research. Here
we reflect on some aspects of the current state of the art (and limits thereto) in our
understanding of the transfer of heat from the interior of the Earth to the ocean.
Specifically, we focus upon the cooling of the upper oceanic crust and its possible
relationship to heat transfer via high-temperature hydrothermal circulation close to
the ridge axis. For fast- and intermediate-spreading ridges we propose a simple
conceptual model in which ridge extension is achieved via episodic diking, with a
repeat period at any given location of ca. 50 years, followed by heat removal in
three stages: (a) near-instantaneous generation of "event plumes" (-5% of total heat
available); (b) an "evolving" period (ca. 5 years) of relatively fast heat discharge
(-20%); (c) a decadal "quiescent" period (-75%). On the slow-spreading Mid-
Atlantic Ridge, our understanding of the mechanisms for the formation of long-
lived, tectonically-hosted hydrothermal vent fields such as TAG and especially
Rainbow are more problematic. We argue that both hydrothermal cooling which
extends into the lower crust and heat release from serpentinisation could contribute
to the required heat budget. Slow-spreading ridges exhibit much greater irregular­
ity and episodic focussing of heat sources in space and time. This focussing may sus­
tain the high heat flow required at the Rainbow and TAG sites, although the detailed
processes are still poorly understood.
1. INTRODUCTION ocean-floor, the global mid-ocean ridge system is the site of
the creation of oceanic crust and lithosphere at a rate of-3.3 km2
Mid-ocean ridges play an important role in the plate-tectonic yr1 [Parsons, 1981; White etal, 1992]. Approximately 75% of
cycle of our planet. Extending some 50-60,000 km across the the total heat flux from the interior of the Earth (ca. 32 of 43 TW)
occurs through this oceanic crust, which covers some two-thirds
Mid-Ocean Ridges: Hydrothermal Interactions Between the
of the surface of the planet. Much of this heat is released along
Lithosphere and Oceans
Geophysical Monograph Series 148 mid-ocean ridges through complex processes of magma solid­
Copyright 2004 by the American Geophysical Union ification and rapid cooling of the young oceanic lithosphere.
10.1029/148GM01 While the majority of this heat loss occurs through conduc-
1
2 HOW WELL DO WE UNDERSTAND OCEAN CRUST—HYDROTHERMAL INTERACTIONS?
tion, approximately one third of the total heat loss at mid-ocean ume]; complete mapping of volcanic and tectonic fabrics of
ridges is effected by a convective process termed hydrother­ selected ridge spreading segments at all spreading rates, includ­
mal circulation [e.g., Stein and Stein, 1994]. ing the ultra-slow spreading Gakkel Ridge under the Arctic
Hydrothermal circulation plays a significant role in the ocean [Michael et al., 2003] and the Southwest Indian Ridge
cycling of energy and mass between the solid Earth and the [Sauter et al, 2002; Lin et al., 2002; Dick et al., 2003]; dis­
oceans (see syntheses by Humphris et al. [1995] and German covery of previously unrecognized forms of seafloor mor­
and Von Damm [2003] for detailed reference-lists). This cir­ phological features that indicate inherent and complex
culation, which is driven by the heat loss at mid-ocean ridges, magmatic/tectonic cycles of seafloor spreading, including
affects the composition of the Earth's oceans and, indeed, amagmatic ridges in ultra-slow spreading ridges and long-
atmosphere. Hydrothermal circulation occurs when seawater lived detachment faults in slow and other magma-starved
percolates downward through fractured oceanic crust along the ridges [Tucholke and Lin, 1994; Cann et al., 1997; Tucholke
volcanic mid-ocean ridge system: the seawater is first heated et al., 1998; Searle et al., 2003]; and direct sampling of the
and then undergoes chemical modification through reaction mantle rocks along a segment of the Mid-Atlantic Ridge near
with the host rock as it continues downward, reaching maxi­ the Fifteen-Twenty fracture zone [Kelemen et al, 2003]. Land-
mum temperatures that can exceed 400°C . At these temper­ based laboratory and theoretical studies have likewise pro­
atures the fluids become extremely buoyant and rise rapidly duced valuable models and testable hypotheses on the focusing
back to the seafloor, where they are expelled into the overly­ of 3-D mantle and melt flows beneath mid-ocean ridges [Par-
ing water column. Active sites of hydrothermal discharge pro­ mentier and Phipps Morgan, 1990; Lin and Phipps Morgan,
vide an extreme ecological niche that is home to a variety of 1992; Magde et al, 1997]; the thermal state of the oceanic crust
quite unique chemo synthetic fauna, previously unknown to as a function of magma supply and hydrothermal circulation
science [Van Dover, 2000]. The first identification of sub­ at ridge crests [Phipps Morgan and Chen, 1993; Shaw and
marine hydrothermal venting and their accompanying Lin, 1996; Chen and Lin, 2004, Chen, this volume]; inte­
chemosynthetically-based communities in the late 1970s grated petrological models on the compositional variations
remains one of the most exciting discoveries in modern sci­ among mid-ocean ridge crustal and mantle rocks [Langmuir
ence. A quarter of a century later, however, many of the et al. 1992; Michael and Cornell, 1998]; predictions of the
processes that control the distributions of hydrothermal activ­ varying styles of seafloor faulting and earthquakes and their
ity around the global ridge crest, the detailed circulation path dependence on the degree of tectonic extension and rock types
for hydrothermal fluids within young oceanic crust and how [Searle and Escartin, this volume]; the causes of the dramatic
such processes vary with time, all remain only incompletely differences in ridge crest topography between fast- and slow-
understood. The purpose of this volume, which arose from spreading ridges; and the potentially important role of water
the first InterRidge Theoretical Institute (IRTI) held in Italy and/or serpentinites in controlling the rheology and, thus, the
in September 2002, is to bring together expertise from two tectonic deformation of the oceanic lithosphere [Hirth et al,
discrete communities, studying mid-ocean ridge geological 1998; Searle and Escartin, this volume].
processes and submarine hydrothermal systems, respectively; The second community involved in the development of
these communities had been progressing their research largely both the IRTI workshop and the ideas presented in this volume
independently over the preceding decade. were co-ordinated through the InterRidge Working Group
The first of these groups had been tasked through an Inter- studying Global Distributions of Seafloor Hydrothermal Vent­
Ridge Working Group to investigate the 4-D (space and time) ing. Previously, a state-of-the-art volume detailing many of the
Architecture of Mid-Ocean Ridges. This research commu­ interacting subdisciplines of modern hydrothermal research had
nity focused on fundamental geological processes of the build­ been compiled by Humphris et al. [1995] based on a US Ridge
ing of oceanic crust and lithosphere at ridges through a Theoretical Institute addressing physical, chemical, biologi­
combination of acoustic and geophysical mapping of the ocean cal and geological interactions in seafloor hydrothermal vent­
floor, seismic/electro-magnetic imaging, near- and on-bot­ ing. More recently, German and Von Damm [2003] have
tom geological observations, rock sampling and geochemi- synthesised core information available at the time of publi­
cal/petrological analyses, ocean drilling, laboratory cation of that earlier volume, together with new information
experiments, and theoretical and numerical modelling and that has only been obtained during the intervening decade.
syntheses. The highlights of the achievements of this com­ There have been two key discoveries in this time that have
munity in the last decade include the direct measurements of quite revolutionised our assumptions about seafloor venting
the depth and dimension of magma lenses beneath fast-spread­ to the oceans. The first of these has been the recognition that
ing, intermediate-rate, and hotspot-influenced ridges [Sinha et temporal evolution of any given seafloor hydrothermal system
al., 1998; Detrick et al., 2002a; Sinha and Evans, this vol­ can vary profoundly [e.g., Von Damm, this volume] while the
GERMAN AND LIN 3
second is that hydrothermal activity may be far more wide­ Considering the above, the timeliness of this volume
spread than had previously been recognised occurring along becomes self-evident. While the important role of hydrother­
ridges of all spreading rates and, hence, being present in all mal circulation in regulating ocean (hence, atmospheric) com­
ocean basins [e.g., Baker and German, this volume]. For the positions may be well established, what remains unclear is
decade following discovery of high-temperature venting, what how that hydrothermal circulation is regulated itself. This is
was remarkable was the constancy in temperature and com­ not something that can become understood just by studying
position of erupting vent fluids even though individual sites hydrothermal flow alone. Instead, an integrated approach is
might exhibit quite distinct compositions. The best example required in which the transfer of heat throughout the ocean-
of this constancy was the first high-temperature system dis­ ridge system is taken as the key master variable that drives
covered at 21 °N on the East Pacific Rise, which appears to all other aspects of ridge and hydrothermal interactions dur­
have remained unperturbed for -20 years [Von Damm et aL, ing its transfer from the interior of the Earth. That is the over­
2002]. Another example is the TAG hydrothermal field at the arching theme of the chapters that follow. In the remainder
Mid-Atlantic Ridge which has thus-far maintained a near- of this chapter, however, we use some key case studies to
constant end-member fluid composition from 1986 until 2003, review our current understanding of the processes that sustain
even after being subject to ODP drilling [Edmonds et al., these hydrothermal systems and to highlight key problems
1996]. What has become apparent in the past decade, however, where our level of understanding can be described as incom­
is that such constancy may not be the norm. Both the tem­ plete, at best.
perature and composition of fluids exiting any given vent site
can change significantly with time, apparently in direct 2. CASE STUDY I: THE FAST-SPREADING
response to volcanic extrusion and/or dike intrusion beneath NORTHERN EAST PACIFIC RISE AND THE
the seafloor [e.g., Butterfield andMassoth, 1994; Butterfield INTERMEDIATE-RATE JUAN DE FUCA RIDGE.
etal, 1997; Von Damm etal, 1995, 1997]. Perhaps the best-
studied such site, to date, is that at 9°50'N on the East Pacific 2.1. The East Pacific Rise at 9°-10°N
Rise, which last underwent a volcanically eruptive episode
in April 1991 [Haymon et al, 1993]. A key question of active The East Pacific Rise (EPR) between 9° and 10°N has
current debate is: what has controlled the continuing evolution become one of the most intensively studied sections of ridge
of fluid temperatures and compositions at this site over the crest anywhere on Earth due primarily to the attention received
subsequent decade? during the US RIDGE programme over the past decade or
A major discovery in the past ten years of hydrothermal more - a commitment renewed with the selection of this area
research has been the recognition that hydrothermal activity as an integrated study site that will continue to be studied in
can occur in reasonable abundance along slow-spreading particular detail during the new RIDGE 2000 initiative
ridges [e.g., German et al., 1996b] as well as along faster (http://r2k.bio.psu.edu). Of interest to the discussion we pres­
ridges that are supplied by significantly greater magmatic ent below is that this region also lies within the northern most
heat fluxes. Previously, Baker et al. [1996] had predicted that limits of a NOAA Acoustic Hydrophone Array (AHA), which
the abundance of venting along any section of ridge crest continuously monitors seismic activity along the entire ca.
should scale directly with the available magmatic heat flux 2,000 km of the EPR between 10°S and 10°N [Fox et al.,
and, hence, with ridge spreading rate. The discovery of 2001; http://www.pmel.noaa.gov/vents/acoustics.html].
hydrothermal activity along some of the world's slowest spread­ The ridge at this location has a full spreading rate of 110
ing ridges - notably the SW Indian Ridge [German et al, mm/yr [Carbotte and Macdonald, 1992] and is underlain by
1998; Bach et al. 2002] and the Gakkel Ridge in the Arctic a seismic low-velocity zone, interpreted to represent a melt lens
Ocean [Edmonds et al., 2003] - appears to contradict, or at at a depth of ca. 1.5 km below the seafloor [Detrick et al.,
least modify, this hypothesis. Where does this "additional" 1987]. The area attracted particular attention from hydrother­
hydrothermal flux originate? Recent discoveries on the Mid- mal researchers in the early 1990's after it was recognised
Atlantic Ridge have revealed a new class of tectonically- that an episode of dike emplacement and volcanic extrusion
hosted vent-sites including the low-temperature Saldanha and had occurred toward the northern end of the previously sur­
Lost City sites [Barriga et al, 1998; Kelley et al, 2001] and veyed region at 9°09,-9°54,N in April 1991 [Haymon et al.,
the high-temperature Logatchev and Rainbow hydrothermal 1991, 1993]. Time series studies of vent-fluid compositions at
fields [e.g., Charlou et al., 2002]. The latter, in particular, this site (near 9°50'N) have subsequently continued to show
appears to impart exceptionally high fluxes of heat and chem­ evolving chemical compositions, in direct response to this
icals into the surrounding ocean [Thurnherr & Richards, 2001; volcanic episode, over more than a decade [e.g., Von Damm
German et al., submitted]. etal, 1995, 1997; Von Damm, this volume].