Inside the Subduction Factory

315 Pages·2004·28.206 MB·English

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Geophysical Monograph 138
Inside the Subduction Factory
John Eiler
Editor
88 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, Darrell Strobel, and William R. Young, members.
Library of Congress Cataloging-in-Publication Data
Inside the subduction factory / John Eiler, editor,
p. cm. - (Geophysical monograph ; 138)
Includes bibliographical references.
ISBN 0-87590-997-3
1. Subduction zones. I. Eiler, John, 1967 - II. Series.
QE511.46.S82 2003
551.1'36^dc22
ISSN 0065-8448
ISBN 0-87590-997-3
Copyright 2003 by the American Geophysical Union
2000 Florida Avenue, N.W.
Washington, DC 20009
Cover:
Low-oblique aerial photograph of erupting Fukutoku-oka-no-ba and extinct Minami Iwo Jima
volcanoes in the central Izu-Bonin-Mariana arc (24°16'N, 141°29'E). The picture was taken from
a airplane of the Hydrographic and Oceanographic Department of Japan on the morning of Jan.
21, 1986, looking south. Courtesy of the Hydrographic and Oceanographic Department of Japan.
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CONTENTS
Preface vii
Introduction: Inside the Subduction Factory
John M. Eiler 1
Section I: The Subducted Slab
Thermal Structure and Metamorphic Evolution of Subducting Slabs
Simon M. Peacock 7
Tracers of the Slab
Tim Elliott 23
Basic Principles of Electromagnetic and Seismological Investigation of Shallow Subduction Zone Structure
George Helffrich 47
Section II: The Mantle Wedge
Seismological Constraints on Structure and Flow Patterns Within the Mantle Wedge
Douglas A. Wiens and Gideon P. Smith 59
Rheology of the Upper Mantle and the Mantle Wedge: A View From the Experimentalists
Greg Hirth and David Kohlstedf 83
Experimental Constraints on Melt Generation in the Mantle Wedge
Glen A. Gaetani and Timothy L Grove 107
Mapping Water Content in Upper Mantle
Shun-ichiro Karato 135
Section III: Focus Regions
Volcanism and Geochemistry in Central America: Progress and Problems
M. J. Carr, M. D. Feigenson, L C. Patino, and J. A. Walker 153
An Overview of the Izu-Bonin-Mariana Subduction Factory
Robert J. Stern, Matthew J. Fouch, and Simon L Klemperer 175
Along-strike Variation in Lavas of the Aleutian Island Arc: Genesis of High Mg# Andesite and Implications for
Continental Crust
Peter B. Kelemen, Gene M. Yogodzinski, and David W. Scholl 223
CONTENTS
Section IV: Synthesis
Some Constraints on Arc Magma Genesis
Yoshiyuki Tatsumi 277
Thermal Structure due to Solid-State Flow in the Mantle Wedge Beneath Arcs
Peter B. Kelemen, Jennifer L. Rilling, E. M. Parmentier, Luc Mehl, and Bradley R. Hacker 293
PREFACE
Subduction zones helped nucleate and grow the conti­ ings; geochemical data have proliferated and their general
nents, they fertilize and lubricate the earth's interior, they features abstracted in ways analogous to our long-standing,
are the site of most subaerial volcanism and many major relatively deep understanding of volcanism at mid-ocean
earthquakes, and they yield a large fraction of the earth's ridges. The emerging picture is different from what we held
precious metals. They are obvious targets for study— just a few years ago. The present volume articulates this
almost anything you learn is likely to impact important new picture and points the way toward the next set of chal­
problems—yet arriving at a general understanding is noto­ lenges faced by scientists interested in subduction zones.
riously difficult: Each subduction zone is distinct, differing It is common for volumes of this type to divide them­
in some important aspect from other subduction zones; fun­ selves into two types of papers: those that broadly summa­
damental aspects of their mechanics and igneous processes rize a field, much as a text book or review paper might, and
differ from those in other, relatively well-understood parts those that present recent scientific results, much as an arti­
of the earth; and there are few direct samples of some of cle contributed to a journal. We have done something dif­
their most important metamorphic and metasomatic ferent. The papers in this volume can be sub-divided by the
processes. As a result, even first-order features of subduc­ region or discipline they consider (more on this below) and
tion zones have generated conflict and apparent paradox. A contain pedagogical "coverage" of the background to their
central question about convergent margins, for instance— subjects. At the same time, contributing authors advance
how vigorous magmatism can occur where plates sink and new ideas that consider recent discoveries or advances in
the mantle cools—has a host of mutually inconsistent observational or experimental method while focusing on
answers: Early suggestions that magmatism resulted from fundamental questions in subduction zone research: How
melting subducted crust have been emphatically disproved does the earth's mantle move near subducting plates; what
and recently just as emphatically revived; the idea that does this movement mean for the thermal evolution of sub­
melting is fluxed by fluid released from subducted crust is ducting slabs; what, in turn, does this thermal evolution
widely held but cannot explain the temperatures and imply about the metamorphism of, and phases escaping
volatile contents of many arc magmas; generations of kine­ from, the slab; and what, in detail, are the consequences
matic and dynamic models have told us the mantle sinks at (rheological, magmatic, and chemical) of delivering those
convergent margins, yet strong evidence suggests that melt­ escaping phases to the overlying mantle?
ing there is often driven by upwelling. In contrast, our We have organized the volume so that the papers will build
understanding of why volcanoes appear at ocean ridges and upon one another in a logical way. At the same time, the
"hotspots"—although still presenting their own chest­ authors have not slavishly conformed to such a structure, and
nuts—are fundamentally solved problems. you will find several major themes developed in detail
Over the last several years, the "subduction problem" has throughout the book. The introduction briefly summarizes the
changed in response to advances in several sub-disciplines last century of scientific thought about what we now recog­
that collectively redefine, and are likely soon to solve, nize as subduction zones, providing a context for readers with
some of our most deeply entrenched debates. Data on the little background in the field. Section I initiates the discussion
mineral physics and melting properties of hydrous silicate with a set of three papers on the subducted slab: Chapter 1
rocks have improved to the point that one can use them as discusses the thermal structure of slabs, in particular their
detailed constraints on models of convection and magma­ upper surface; Chapter 2 analyzes the geochemistry of slabs,
tism; geophysical data have directly imaged details of sub­ in particular the compositions of fluids and melts that might
duction zones that were previously inferred; numerical be released from them due to heating; and Chapter 3 reviews
models are beginning to capture key, in some cases count­ geophysical methods for interrogating shallow subduction-
er-intuitive, kinematic behaviors and their dynamical forc­ zone structure, including the electromagnetic methods.
vii
In section II, we focus on the mantle wedge—that part of convergent margins advances—and a radical suggestion,
the mantle between the down-going slab and the over-rid­ backed by evidence from igneous and metamorphic petrol­
ing plate. The dynamics and melting behavior of the man­ ogy and thermal models, that subducted slabs are often, if
tle wedge is arguably the centerpiece of current debates not always, heated above their melting points. This is an
about subduction zones, so this section contains the largest idea that was deeply held thirty years ago, discarded almost
number and most mechanistically detailed papers in the entirely for most of the last twenty years, and is apparently
volume. We start, in Chapter 4, with geophysical con­ ready to be revived for a new and more sophisticated look.
straints on the structure and kinematics of the mantle This volume derives from a conference, "The Subduction
wedge, followed by a discussion of experimental and Factory," held in Eugene, Oregon in late summer of 2000.
observational constraints on the rheology of upper mantle The conference was an inspiration to many of those who
rocks in Chapter 5—a specialized problem but one that attended because it brought together key players in the
underlies some of the most important issues facing subduc­ study of subduction zones at a special time of discovery
tion zone research. Then, in Chapter 6, we discuss experi­ and conceptual advances in their respective fields. A major­
mental constraints on the way in which the mantle wedge ity of speakers and a few particularly active attendees were
melts in response to the introduction of slab-derived asked to contribute chapters to this volume, reporting on
hydrous phases and end this section with Chapter 7—a par­ the key results they presented, summarizing the contextual
ticularly forward-looking proposition for a means of geo- background of their relevant sub-disciplines and, perhaps
physically mapping hydrogen concentration (arguably the most importantly, reflecting on relationships between their
controlling parameter for rheology and melting properties) advances and what they learned from others at the meeting.
in the upper mantle, including the mantle wedge. In the two years that have passed since this meeting, the
Section III brings together a set of papers on another side ideas it generated have retained much of their immediacy
of subduction zone research: the need to come to grips with and matured as authors reflected on one another's work.
historical and geographic idiosyncrasies of individual sub­ We thank, first and foremost, the authors who con­
duction zones. These papers explore the geology, geo­ tributed to this volume, both for the thought and work they
physics, and geochemistry of three subduction zones that put into their own contributions and for their patience with
are the focus of much recent research, including two that the collective effort. We are also grateful to the many
are 'focus areas' identified by the Margins program of the reviewers who helped improve these chapters—particular­
National Science Foundation. These are the Central ly those asked to deal with the longer and more technically
American arc, the Izu-Bonin-Mariana subduction zone sys­ detailed contributions and those who had to work under a
tem in the western Pacific, and the Aleutian island arc— demanding deadline. Similarly, we thank Allan Graubard,
Chapters 8, 9, and 10, respectively. It is important to note our acquisitions editor, Bethany Matsko, our production
that these papers develop arguments that are certain to editor, and the rest of the AGU book production staff, who
prompt future research on subduction zones generally and provided much needed guidance and repeatedly demon­
are not simply reviews of prior work. strated the patience of saints. Finally, we thank the organ­
The book concludes with two papers in section IV that izers of the "Subduction Factory" conference and the
advance focused arguments with broad implications for the administrators and staff in NSF's Margins program that
behavior of subduction zones, based on syntheses of much supported both that meeting and this volume.
of the material discussed in more detail throughout the rest
of the volume. These include a proposal for the processes
that control the location of subduction zone volcanoes—a John M. Eiler
problem worth re-visiting whenever our understanding of California Institute of Technology
viii
Introduction
John M. Eiler
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California
Geological models of subduction zones impact thinking about many of the central
problems in the structure, dynamics, chemistry, and history of the solid earth. Should
those models change, the effects will reach across the earth sciences. We are cur­
rently in the midst of such a change, brought on by several causes. First, the earth sci­
ence community recently began an organized, multi-disciplinary study of subduction
zones by way of the National Science Foundation's "Margins initiative." This has
improved the quality of our descriptions of key focus areas and should continue to
do so for the next several years. Less purposefully but of equal importance, several
of the debates in subduction zone research have evolved in recent years, making us
revisit both recent and old observations in a new light. This book consists of descrip­
tions of these advances, written by the some of the leading participants. I describe the
origin and organization of the book in the preface; here I review the context of pre­
vious thought regarding convergent margins, with particular focus on the link
between tectonics and magmatism, and point out questions raised in the following
chapters.
ANCIENT HISTORY logically active belts surrounding the Pacific ocean basin.
These belts were collectively referred to as the "the andesite
For much of geology's modern era, models of subduction line" and interpreted as evidence for a specialized kind of
zones have been inseparable from "the andesite problem"; magmatism that occurs only at the boundary between conti­
that is, the question of the origin of andesitic rocks that are nental land masses and ocean basins (Marshall, 1911). The
abundant in many magmatic arcs. This specialized petro- earliest widely remembered explanation of andesite genesis
logic problem is disproportionately important both because was Bowen's hypothesis that they, like many other silicic
andesite makes up a large fraction of subduction zone prod­ magmas, are liquids residual to crystallization-differentia­
ucts, and because andesites compositionally resemble aver­ tion of basalt (Bowen, 1928). By this interpretation,
age continental crust (e.g., Rudnick and Fountain, 1995). andesites are ultimately products of basaltic magmatism
Moreover, the literature on the "andesite problem" has been and, to project his ideas into the plate tectonic world-view,
a clearing-house for ideas about the geology of convergent therefore are close relatives of volcanic rocks from mid-
margins generally. ocean-ridges and intraplate volcanic centers. More detailed
It was recognized near the beginning of the 20th century and geological interpretations of andesites and associated
that andesitic rocks are highly concentrated into young, geo- rocks at this time called on geosynclinal theory and seem
peculiar and non-physical when looked at with the benefit
of hindsight. Figure la is the author's attempt to synthesize
several of these ideas. A noteworthy weakness of such mod­
Inside the Subduction Factory
els is that they offered no clear explanation of why andesites
Geophysical Monograph 138
are common in Pacific-rim magmatic arcs while rare else­
Copyright 2003 by the American Geophysical Union
10.1029/138GM01 where.
1
2 INTRODUCTION
PRE-PLATE-TECTONIC HUNCHES became a centerpiece of modern thinking about arc magma­
tism. However, the idea that andesites are primary partial
Several discoveries over the course of the 1950s and melts of hydrous mantle, while containing elements of the
1960s pointed out the special character of andesites from modern understanding of subduction zones, conflicted with
magmatic arcs and the link between tectonics, deep seis- evidence that andesites are complexly evolved rocks that
micity, and magmatism. First, it was recognized that seis- interact extensively with the crust through which they erupt
micity associated with volcanic arcs is organized into (see Turner and Verhoogen, 1960, and Ringwood, 1975, for
Benioff zones, suggesting that oceanic crust founders into contemporary summaries of these criticisms). Hence the
the mantle at what we now recognize as convergent mar­ "andesite problem": the secondary origin suggested by
gins {Benioff, 1954). Moreover, experimental and applied Bowen is unable to explain why andesites are common in
petrology provided controversial but growing evidence magmatic arcs and rare elsewhere (and gets details of the
that mafic magmas are derived from partial melting of petrology and chemistry wrong); a primary origin explains
peridotite, that peridotite is the major constituent of the the association of andesites with Benioff zones, but gets the
upper mantle, that water lowers the melting points of sili­ chemistry and petrology substantially wrong.
cates, and that crystallization-differentiation of anhydrous
basalt, as envisioned by Bowen's earliest work, produces THE LAST BIG THING
typical andesites only under special circumstances (see
Ringwood, 1975, for a review of these early experiments). Responses to this dilemma played out during the formu­
The unique compositional character (i.e., high silica con­ lation of plate tectonic theory, and therefore both built on
tent and proportion of andesite) of lavas from magmatic the preceding arguments and integrated them with new
arcs and similarities between andesite and the continents insights about earth's plates. The following geophysical,
as a whole were recognized (Polvedaart, 1955). Finally, geochemical, and experimental observations were particu­
there were growing suspicions that partial melting of peri- larly important: Magmatic arcs were shown to be clearly
dotites to low degrees and in the presence of water pro­ associated with convergent margins and subduction of
duces unusually siliceous melts. Polvedaart (1955) made ocean lithosphere; active convergent-margin volcanism was
among the earliest such suggestion, although he did so found to consistently occur ca. 80 to 150 km above the top
based on the outlandish contemporary belief that the prim­ of the down-going slab, suggesting control by a single pres­
itive crusts of the moon and mars consist almost entirely sure and/or temperature-dependent reaction in the slab; par­
of rhyolite pumice. O'Hara (1965) made a similar but tial melting of high-pressure mafic rocks (e.g., eclogites)
more well-founded suggestion based on the phase equilib­ were shown to have compositions broadly similar to those
ria of idealized systems. of convergent-margin andesites; and the new tools of iso­
The relationships among these discoveries and their over­ tope and trace-element geochemistry demonstrated that
all significance were not widely recognized until the plate convergent-margin magmas sample the top of the subduct­
tectonic revolution was underway (for instance, a standard ing plate (see Gill, 1981, for review of these developments).
petrologic text of this era, Turner and Verhoogen, 1960, These new constraints (and a variety of abstruse petrolog­
does not link Benioff zones and andesitic volcanism). ic arguments) were used to suggest that magmatism at con­
However, speculative inklings appeared surprisingly early. vergent margins is ultimately driven by melting of basalt in
For example, Polvedaart (1955) suggested that andesites the down-going plate. Simple versions of this hypothesis
are partial melts of rocks in the hanging walls of Benioff- equated andesites with slab melts (Hamilton, 1969; Oxburgh
zone faults (i.e., the mantle wedge), formed in the presence and Turcotte, 1970); more sophisticated versions called
of water released from foundered (i.e., subducted) ocean upon reaction between slab-derived melts and the overlying
crust, and silicic compared to basalts because of the effect mantle wedge, upwelling and decompression melting of that
of water on the melting behavior of peridotite. A charitable hybrid mantle to produce basaltic and andesitic melts, and
interpretation of his hypothesis is illustrated in Figure lb; complex processes of differentiation, mixing and crustal
similar models with sounder petrologic basis and more contamination prior to eruption (Figure lc; Ringwood,
explicit ties to the concept of subduction would crop up as 1975). These suggestions obviously require that the top of
plate tectonic theory took form (McBirney, 1969; Wyllie, subducted lithosphere melts, placing lower limits on its con­
1971). Perhaps the greatest long-term impact of this hypoth­ ductive and frictional heating (e.g., Oxburgh and Turcotte,
esis was to inspire experimental studies of peridotite-water 1970), and makes predictions about the compositions of
systems (e.g., Kushiro et al, 1968), which eventually residues of melting in deeply subducted slabs, which stir