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ScienceWeek
SCIENCEWEEK
ScienceWeek
March 7, 2003
Vol. 7 Number 10
An Online Digest of Research in the Sciences
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Science is a community with an attitude: people who rejoice when
a new truth defeats their past confusions, people who would
rather know reality than superstitions, people who believe that
with their minds and hearts and hands they can shape their own
destiny. Since the beginning of human time, this attitude has
threatened those whose life and fortune are based on illusion.
-- Anonymous
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Section 1
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Thematic Issue: Geophysics: Earth's Mantle
1. Introduction
2. Structure of the Mantle
3. Mantle Dynamics
4. Plate Dynamics in the Mantle
5. Mid-Mantle Dynamics
6. Deep-Mantle Dynamics
7. Mantle Plumes
Notices and Subscription Information
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Section 2
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1. INTRODUCTION
EARTH STRUCTURE
"Early in the 20th century it became evident from the study of
seismic waves that the interior of the Earth has a radially
layered structure, like that of an onion. The boundaries between
the layers are marked by abrupt changes in seismic velocity or
velocity gradient. Each layer is characterized by a specific set
of physical properties determined by the composition, pressure
and temperature in the layer. The four main layers are the crust,
mantle and the outer and inner cores. At depths of a few tens of
kilometers under continents and less than ten kilometers beneath
the oceans seismic velocities increase sharply. This seismic
discontinuity, discovered in 1909 by A. Mohorovicic, represents
the boundary between the crust and mantle. R. D. Oldham noted in
1906 that the travel-times of seismic compressional waves that
traversed the body of the Earth were greater than expected; the
delay was attributed to a fluid outer core. Support for this idea
came in 1914, when B. Gutenberg described a shadow zone for
seismic waves at epicentral distances greater than about 105°.
Just as light-waves cast a shadow of an opaque object, seismic
waves from an earthquake cast a shadow of the core on the
opposite side of the world. Compressional waves can in fact pass
through the liquid core. They appear, delayed in time, at
epicentral distances larger than 143°. In 1936 I. Lehmann
observed the weak arrivals of compressional waves in the gap
between 105° and 143°. They are interpreted as evidence for a
solid inner core."
William Lowrie: Fundamentals of Geophysics. Cambridge University
Press 1997, p.14.
THE STRUCTURE OF EARTH'S INTERIOR
1) Seismological studies have determined the structure of the
deep interior of the Earth based on seismic wave velocities. The
center of the Earth, found below 2900 km, comprises the core,
which is divided into two parts. The outer core is believed to be
composed of molten metallic iron alloyed with nickel and some
light elements such as oxygen, hydrogen, carbon, or sulfur. In
contrast, the inner core, which extends from 5155 km depth to the
Earth's center, is probably composed of solid iron-nickel metal
with significantly lesser amounts of light components.
2) The overlying mantle is divided into the upper and the lower
mantle by a zone of rapid increase in density and seismic wave
velocity at approximately 700 km depth. This and other
transitions are associated with a phase change in which silicate
crystals are transformed into a more dense structure. A
compositional change, such as higher ferrous iron content in the
lower mantle than the upper mantle, may also occur at the
boundary.
3) The upper mantle is divided into additional layers that are
categorized according to their rheology (how the materials behave
under stress), a property that is partly associated with the
theory of plate tectonics. A distinction is made between the
shallow, cool, uppermost part of the mantle, which together with
the overlying crust forms the "lithosphere", and the hot,
underlying material called the "asthenosphere".
4) At the higher temperatures that exist beneath the lithosphere,
the mantle is believed to slowly deform during the flow
associated with plate tectonics. The effective depth of the base
of the lithosphere differs somewhat, depending on the process
being considered. Three terms often in use are a) "thermal
lithosphere" to denote the region that behaves rigidly during
plate movements and that undergoes conductive cooling, b)
"petrological lithosphere" to denote the region where partial
melt is absent, and c) "mechanical lithosphere" to denote the
much thinner, elastic region that can support long-term loads
such as seamounts.
5) The asthenosphere represents the region between 100 km and 200
km depth that is more easily deformed than the rest of the
mantle, probably owing to the presence of small amounts of
partial melt. This region is sometimes called the "low-viscosity
zone" because it is easily deformed or the "low-velocity zone"
because it exhibits low seismic wave velocities. A phase
transition to more dense silicate structure occurs at a depth of
approximately 400 km. Consequently, some authors call the region
between 400 km and 700 km depth, which contains high-seismic-
velocity gradients, the "transition zone" and restrict the term
"upper mantle" to the region above 400 km. The term "mesosphere"
has been used to denote the lower mantle, the transition zone, or
the upper mantle beneath 200 km. In general, one must be careful
when reading the literature, as this terminology is not
standardized.
Adapted from: N.H. Sleep and K. Fujita: Principles of Geophysics.
Blackwell Science 1997, p.10.
INTRODUCTORY NOTES
1) Studies of seismic wave velocities have been central to our
understanding of the structure of the deep interior of the Earth.
The center of the Earth, located below 2900 kilometers, comprises
the "core", which is divided into two parts. The "outer core" is
believed to be composed of molten metallic iron alloyed with
nickel and some light elements such as oxygen, hydrogen, carbon,
or sulfur. In contrast, the "inner core", which extends from 5155
kilometers to the Earth's center, is probably composed of solid
iron-nickel metal with significantly lesser amounts of light
components. Overlying the core is the "mantle", divided into
"upper mantle" and "lower mantle" by a zone of rapid increase in
density and seismic wave velocity at approximately 700 kilometers
depth. The so-called "crust", the outermost solid layer of the
Earth, represents less than 1 percent of the Earth's volume and
varies in thickness from approximately 5 kilometers beneath the
oceans to approximately 60 kilometers beneath continental
mountain chains. The term "lithosphere" refers to the upper
(oceanic and continental) layer of the solid Earth, the
lithosphere comprising all crustal rocks and the brittle part of
the uppermost mantle. At the high temperatures that exist beneath
the lithosphere, the mantle is apparently slowly deforming, its
flow associated with "plate tectonics".
2) In general, the modern theory of plate tectonics provides a
framework for understanding the origin of continents, ocean
basins, and mountain ranges. In its presently accepted form, the
theory of plate tectonics divides the surface area of the Earth
into several "plates" that move relatively independently over the
surface of the planet. These plates consist of the crust and the
uppermost part of the mantle to a depth of approximately 100
kilometers: a plate is thus a segment of the lithosphere. Each
plate essentially behaves as a rigid solid shell, while the
material at greater depths in the Earth slowly flows under it.
Plates usually contain both oceanic and continental crust, with
the familiar continents passive passengers on the topmost parts
of their respective plates. In general, a plate may be as broad
as 10,000 kilometers (e.g., the Pacific plate) or as small as a
few thousand kilometers (e.g., the Philippines plate). There are
12 major plates (Antarctica, Africa, Eurasia, India, Australia,
Arabia, Philippines, North America, South America, Pacific,
Nazea, and Cocos) and several minor plates (e.g., Scotia,
Caribbean, Juan de Fuca). In general, convection within the
underlying less rigid "asthenosphere" causes the plates (and the
associated continents and crust) to move relative to each other.
3) Although the lateral movements of plates and plate-to-plate
contacts have received much attention, there is evidence that
vertical deformations of plates by underlying mantle dynamics are
of significance.
4) "Mantle plumes" are thin vertical conduits of molten rock
material from the core-mantle boundary to the crust.
5) In this context, the term "subduction" refers to the process
of underthrusting of the edge of a tectonic plate into the mantle
underlying an adjacent plate.
6). The term "hot spot" (also, hotspot) refers to a relatively
long-lasting center of surface volcanism and locally high heat
flow, and about 40 locations are now so labeled. Most hot spots
are in ocean basins, located at points where the lithosphere has
apparently upwelled, elevating the denser mantle material and
creating mass anomalies.
7) The term D" layer refers to an anomalous layer just above the
core-mantle boundary, the layer approximately 150 to 200
kilometers thick. In this layer, body-wave velocity gradients are
very small and may even be negative. Although part of the lower
mantle, the D" layer evidently serves as a boundary layer between
the mantle and core. The D" layer is distinguished from the D'
layer above it, with the D' layer characterized by smooth
velocity gradients and the absence of seismic discontinuities.
The alphabetical layers A to G are from a model of Earth's
interior proposed by K.E. Bullen in 1942, the mantle comprising
the B, C, and D layers.
8) Seismic waves are essentially of two types. When an earthquake
or an explosion occurs, energy is radiated out from the source,
with some waves ("surface waves") traveling along the surface,
while other waves travel through the Earth, and these waves are
called "body waves".
9) The term "prem model" refers to the "preliminary reference
Earth model" proposed in 1981 by A.M. Dziewonski and D.L.
Anderson, a model in which the distribution of body-wave
velocities in important layers of the Earth is represented by
cubic or quadratic polynomials of normalized radial distance.
THE LAYERED EARTH
"During the nineteenth century, the nature of the Earth's
interior was a matter of fierce and fascinating debate. All
theories were hampered by a lack of evidence -- the nature of
rocks deep below the surface was unknown. In 1906, Richard D.
Oldham observed that compressional seismic waves (P waves) slow
abruptly deep within the Earth and can penetrate no further. This
was strong evidence in favor of a liquid core. Three years later,
Andrija Mohorovicic noticed that the velocity of seismic waves
leaps from 7.2 to 8.0 km/s at around 60 km deep. He had
discovered the 'Moho' seismic discontinuity that marks the crust-
mantle boundary. In 1926, Beno Gutenberg obtained evidence for a
seismic discontinuity at the core-mantle boundary. This, the
Gutenberg discontinuity, was confirmed during the 1950s when
world-wide records of blasts from underground nuclear detonations
were scrutinized. Subsequent studies of the Earth's seismic
properties, using seismic waves propagated by earthquakes and by
controlled explosions to 'x-ray' the planet (a technique called
seismic tomography), have revealed a series of somewhat distinct
layers or concentric shells in the solid Earth. Each shell has
different chemical and physical properties..."
Richard John Huggett: Environmental Change. Routledge 1997, p.56.
ScienceWeek http://www.scienceweek.com
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2. STRUCTURE OF THE MANTLE
THE AGE OF THE EARTH
C.J. Allegre et al (Geochim & Cosmochim Lab Paris, FR) discuss
the age of the Earth, the authors making the following points:
1) Forty years ago, Patterson in his pioneering work on lead
isotopes defined the age of the Earth and of the meteorites with
4.55 Ga. The authors reconsider the question of the age of the
Earth and its relation to the age of the meteorites, and similar
to Patterson's approach, the authors use lead isotope
systematics. U-Pb investigations of three meteoritic objects on
which early thermal and/or chemical events generated large U/Pb
fractionations are outlined: refractory inclusions of the Allende
meteorite, phosphates in ordinary chondrites and basaltic
achondrites. All these samples contain lead with highly
radiogenic-compositions (Pb-206/Pb-204 > 150) and therefore their
Pb-207-Pb-206 age is almost independent of the isotopic
composition associated with the measured Pb-204 to within the
precision of a few million years. The Pb-Pb ages of the most
radiogenic compositions measured in Allende refractory inclusions
range from 4.568 to 4.565 Ga, the Pb-Pb ages of secondary
phosphates in equilibrated ordinary chondrites vary from 4.563 to
4.504 Ga, and basaltic achondrites show ages between 4.558 and
4.53 Ga.
2) These age determinations indicate precise time constraints for
the classical scenario concerning the formation of the first
planetary objects of the Solar System. Formation of the Allende
refractory inclusions occurs at 4566 Ma. Accretion of chondritic
bodies occurred at a maximum of 3 Ma later. Eight million years
after formation of the Allende inclusions, magmatic activity,
including partial melting, magma seg regation, and eruption
occurs on planetary bodies. During the next 200 Ma thermal
processing, shock perturbation, and heating takes place.
3) The meaning of an "age of the Earth" is evaluated in relation
to the major early processes, end of accretion, core formation,
and atmosphere extraction. A similar value for the age of the
Earth is found based on lead isotopes and I-Xe systematics; this
age is about 0.1 Ga younger than that of primitive meteorites.
Pb-Pb and I-Xe terrestrial ages are interpreted as mean ages of
core segregation and of atmosphere outgassing, respectively.
Within this framework, the "age of the Earth" corresponds to the
end of its accretion and to its early differentiation.
Geochim. Cosmochim. Acta 1995 59:1445
Related Background:
EARLY EVOLUTION OF THE EARTH AND MOON: NEW CONSTRAINTS FROM HF-W
ISOTOPE GEOCHEMISTRY
A. Halliday et al (University of Michigan, US) discuss the early
evolution of the Earth, the authors making the following points:
1) The W isotopic composition of the bulk silicate Earth (BSE) is
chondritic within current analytical uncertainties, indicating
that terrestrial core formation commenced more than 50 Myr after
the differentiation of the earliest planetesimals in the solar
system. This is consistent with U-Pb data and holds true whether
core formation is modeled as a single catastrophic event or as a
continuous process that started late, unless accretion was more
than about twice as slow as recently estimated. The chondritic W
isotopic composition of the BSE provides support for the
assumption that the overall lithophile/siderophile refractory
element ratio of the Earth is close to chondritic, but requires
mixing to remove early heterogeneities introduced by the
accretion of any planetesimals already segregated into silicate
and metallic portions with distinct W isotopic compositions. The
same applies to any later accreted material, such as the putative
Moon-forming giant impactor.
2) Many models of terrestrial accretion and core formation
involve a core that developed during the first 90% of accretion
history, generally considered to correspond to a time span
significantly shorter than that permitted by the W isotopic data.
These models are difficult to reconcile with the W isotopic data
unless the proto-Earth was re-homogenized by a major impactor, or
accretion took longer than currently estimated. The W isotopic
compositions of early lunar rocks provide the best hope of
determining which model of accretion and core formation is
correct for the Earth.
3) A conservative assessment of isotopic ages for lunar highlands
rocks, combined with the constraints from W isotopic data,
indicate that the onset of major terrestrial core segregation,
the formation of the Moon and the development of a lunar magma
ocean all took place within < 80 Myr at 4.47 +/- 0.04 Ga. Certain
isotopic ages for lunar rocks would be consistent with a more
restricted time window of 4.50 +/- 0.01 Ga. Potassium and Cr
isotopic data indicate early volatile depletion of the material
from which the Earth and Moon formed and constrain models of pre-
core Pb isotopic evolution. The various estimates for the Pb
isotopic composition of the BSE seem best explained by strong
U/Pb fractionation accompanying terrestrial core formation. Using
the 4.47 +/- 0.04 Ga age of the core, the second stage Pb
isotopic evolution reproduces reasonable estimates for the
present day BSE Pb isotopic composition if the second stage U-
238/Pb-204 (mu) is in the range of 8.9 +/- 0.5. The "lead
paradox" is entirely predictable from the 4.47 +/- 0.04 Ga age of
the core.
4) The similarities between the late ages of the Earth's core,
the Moon and the degassing of Xe from the terrestrial mantle are
consistent with an accretion history which is more protracted
than currently modeled. Alternatively, late impacts may have
triggered all of these events. If a single late impact is invoked
as an explanation, the Moon must have been derived primarily from
the silicate portion of the impactor. Otherwise, the Hf-W data
may define the age of a core that formed as a result of another
impact, shortly prior to that which formed the Moon.
Earth & Planetary Sci. Lett. 1996 142:75
Related Background:
THE EARTH'S MANTLE
G.R. Helffrich and B.J. Wood (Tokyo Institute of Technology, JP)
discuss Earth's mantle, the authors making the following points:
1) The Earth's mantle comprises 82% of its volume and 65% of its
mass. It constitutes virtually all of the silicate part of the
Earth, extending from the base of the crust (0.6% of Earth's
silicate mass) to the top of the metallic core. When the core
segregated from the silicate and gas of the proto-Earth, it
incorporated high concentrations of the siderophile elements,
leaving lithophile elements in the silicate mantle. Thus, the
current composition of the mantle has core formation imprinted on
it -- as pronounced depletions in, for example, Fe, Ni, S, W, Pt,
Au and Pb relative to the chondritic meteorites(1,2), which are
used to constrain the composition of the whole Earth. Owing to
the fractionation of lithophile radioactive parent isotopes such
as U-238 and Hf-182 from their siderophile daughters Pb-206 and
W-182, core formation can be dated as the time at which the
evolution of the isotopic compositions of Pb and W diverged from
the meteorite trend. The result (about 4.5 Gyr ago) corresponds
to 50–100 Myr after the formation of the oldest meteorite bodies
in the Solar System(3,4).
2) Mantle rocks that occur occasionally at the surface, either as
tectonic fragments (kilometer scale) or as inclusions in
explosive eruptives (centimeter scale) are predominantly
peridotites. One of the main questions facing the Earth sciences
is whether a peridotite composition, representative of the upper
150 km of the mantle, can also be assumed to represent the
remainder of the mantle down to 2900 km depth. Many geochemical
data suggest not. For example, the current heat flux at the
Earth's surface is about 44 TW (44 10^(12) W), most of which can
be reasonably attributed to radioactive decay of K, U and Th in
the mantle(5). The upper-mantle source region of mid-ocean ridge
basalt (MORB) is depleted in these elements, however, and only
produces 2 to 6 TW. The simplest explanation of the shortfall is
that there is a lower layer enriched in the heat-producing
elements that is only sporadically involved in the production of
surface rocks. Similarly, the flux of the radioactive products
Ar-40 (from K) and He-4 (from U, Th) into the atmosphere should
be predictable from the surface heat-flow and the approximate
abundances of K, U and Th in the bulk Earth. In fact, about 50%
of the Ar-40 produced over the age of the Earth is missing from
the atmosphere, while the flux of He-4 from the oceans is only 5%
of that predicted from oceanic heat flow. These observations
again lead naturally to the concept of a region in the deep
Earth, rich in the products of radioactive decay, which exchanges
heat but little mass with the convecting upper mantle.
3) In summary: Seismological images of the Earth's mantle reveal
three distinct changes in velocity structure, at depths of 410,
660 and 2700 km. The first two are best explained by mineral
phase transformations, whereas the third -- the D" layer --
probably reflects a change in chemical composition and thermal
structure. Tomographic images of cold slabs in the lower mantle,
the displacements of the 410-km and 660-km discontinuities around
subduction zones, and the occurrence of small-scale
heterogeneities in the lower mantle all indicate that subducted
material penetrates the deep mantle, implying whole-mantle
convection. In contrast, geochemical analyses of the basaltic
products of mantle melting are frequently used to infer that
mantle convection is layered, with the deeper mantle largely
isolated from the upper mantle. The authors demonstrate that
geochemical, seismological and heat-flow data are all consistent
with whole-mantle convection provided that the observed
heterogeneities are remnants of recycled oceanic and continental
crust that make up about 16 and 0.3 per cent, respectively, of
mantle volume.
References (abridged):
1. McDonough, W. F & Sun, S.-S. The composition of the Earth.
Chem. Geol. 120, 223-253 (1995).
2. Newsom, H. E. in Global Earth Physics: Handbook of Physical
Constants Vol. 1 (ed. Ahrens, T. J.) 159-189 (Reference Shelf
Series, American Geophysical Union, Washington DC, 1995).
3. Allègre, C. J., Manhès, G. & Gopel, C. The age of the Earth.
Geochim. Cosmochim. Acta 59, 1445-1456 (1995).
4. Halliday, A., Rehkämper, M., Lee, D.-C. & Yi, W. Early
evolution of the Earth and Moon: New constraints from Hf-W
isotope geochemistry. Earth Planet. Sci. Lett. 142, 75-89 (1996).
5. Kellogg, L. H., Hager, B. H. & van der Hilst, R. D.
Compositional stratification in the deep mantle. Science 283,
1881-1884 (1999).
Nature 2001 412:501
Related Background:
THE COMPOSITION OF THE EARTH
W.F. Mcdonough and S.S. Sun (Australian National University, AU)
discuss the composition of the Earth, the authors making the
following points:
1) Compositional models of the Earth are critically dependent on
three main sources of information: the seismic profile of the
Earth and its interpretation, comparisons between primitive
meteorites and the solar nebula composition, and chemical and
petrological models of peridotite-basalt melting relationships.
Whereas a family of compositional models for the Earth are
permissible based on these methods, the model that is most
consistent with the seismological and geodynamic structure of the
Earth comprises an upper and lower mantle of similar composition,
an Fe-Ni core having between 5% and 15% of a low-atomic-weight
element, and a mantle which, when compared to CI carbonaceous
chondrites, is depleted in Mg and Si relative to the refractory
lithophile elements.
2) The absolute and relative abundances of the refractory
elements in carbonaceous, ordinary, and enstatite chondritic
meteorites are compared. The bulk composition of an average CI
carbonaceous chondrite is defined from previous compilations and
from the refractory element compositions of different groups of
chondrites. The absolute uncertainties in their refractory
element compositions are evaluated by comparing ratios of these
elements. These data are then used to evaluate existing models of
the composition of the Silicate Earth.
3) The systematic behavior of major and trace elements during
differentiation of the mantle is used to constrain the Silicate
Earth composition, Seemingly fertile peridotites have experienced
a previous melting event that must be accounted for when
developing these models, The approach taken by the authors avoids
unnecessary assumptions inherent in several existing models, and
results in an internally consistent Silicate Earth composition
having chondritic proportions of the refractory lithophile
elements similar to 2.75 times that in CI carbonaceous
chondrites. Element ratios in peridotites, komatiites, basalts
and various crustal rocks are used to assess the abundances of
both non-lithophile and non-refractory elements in the Silicate
Earth. The authors suggest these data provide insights into the
accretion processes of the Earth, the chemical evolution of the
Earth's mantle, the effect of core formation, and indicate
negligible exchange between the core and mantle throughout the
geologic record (the last 3.5 Ga).
4) The composition of the Earth's core is poorly constrained
beyond its major constituents (i.e. an Fe-Ni alloy). Density
contrasts between the inner and outer core boundary are used to
suggest the presence (similar to 10 to 15%) of a light element or
a combination of elements (e,g,, O, S, Si) in the outer core. The
core is the dominant repository of siderophile elements in the
Earth, The limits of our understanding of the core's composition
(including the light-element component) depend on models of core
formation and the class of chondritic meteorites we have chosen
when constructing models of the bulk Earth's composition.
5) The Earth has a bulk Fe/Al ratio similar to 20 +/- 2,
established by assuming that the Earth's budget of Al is stored
entirely within the Silicate Earth and Fe is partitioned between
the Silicate Earth (similar to 14%) and the core (similar to
86%). Chondritic meteorites display a range of Fe/Al ratios, with
many having a value close to 20. A comparison of the bulk
composition of the Earth and chondritic meteorites reveals both
similarities and differences, with the Earth being more strongly
depleted in the more volatile elements. There is no group of
meteorites that has a bulk composition matching that of the
Earth's.
Chem. Geol. 1995 120:223
Related Background:
DETERMINING THE COMPOSITION OF THE EARTH
M.J. Drake and K. Righter (University of Arizona, US) discuss the
composition of the Earth, the authors making the following
points:
1) Most workers assume implicitly that the Earth is made of some
sort of extant primitive material delivered to the Earth as
meteorites and probably originating in the asteroid belt. Indeed,
much has been learned from meteorites about the materials present
and processes occurring in the accretion disk as the planets
grew. However, confusion is caused by the convenient reference of
terrestrial rock abundances to CI chondrites or just
"chondrites", leading to an unintended perception that the Earth
must be made of such materials.
2) The formation of the Earth has been debated in terms of
heterogeneous accretion versus homogeneous accretion, with the
former holding sway for the last 25 years or so. Heterogeneous
accretion, as most prominently championed by Wänke(1), envisioned
the material accreting to the Earth changing in composition and
oxidation state with time. Driven by the "stair-step"
distribution of siderophile (metal-seeking) elements in the
terrestrial upper mantle, Wänke(1) suggested that the first 80%
to 90% of the Earth accreted from very reducing materials. All
elements except the refractory lithophile elements such as Sc and
the rare earth elements (REE) would be quantitatively extracted
into the core, and the mantle would be devoid of Fe2+. The next
20% to 10% or so of material accreting to the Earth would be more
oxidizing, and all but the highly siderophile elements (Ir, Os,
Au and so on) would remain stranded in the mantle. The highly
siderophile elements were again quantitatively extracted into the
core. The last roughly 1% added (the "late veneer"[2]) was so
oxidizing that metallic Fe did not exist (note that CI
chondrites(3) and the Tagish Lake meteorite(4) are the only
chondrites containing no metal(3), and all siderophile elements
delivered by the "late veneer" were forced to remain in the
mantle, where they were very efficiently homogenized at the hand-
specimen (centimeter) scale on a global basis. The stair-step
pattern of siderophile elements is thus explained. The authors
note that the term "late veneer" is unfortunate, as the last
dregs of material accreted to Earth are well mixed into at least
the upper mantle, rather than veneering the surface. The term is,
however, entrenched in the literature.
3) In summary: A long-standing question in the planetary sciences
asks what the Earth is made of. For historical reasons, volatile-
depleted primitive materials similar to current chondritic
meteorites were long considered to provide the "building blocks"
of the terrestrial planets. But material from the Earth, Mars,
comets and various meteorites have Mg/Si and Al/Si ratios,
oxygen-isotope ratios, osmium-isotope ratios and D/H, Ar/H2O and
Kr/Xe ratios such that no primitive material similar to the
Earth's mantle is currently represented in our meteorite
collections. The "building blocks" of the Earth must instead be
composed of unsampled "Earth chondrite? or "Earth achondrite".(5)
References (abridged):
1. Wänke, H. Constitution of terrestrial planets. Phil. Trans. R.
Soc. Lond. 303, 287-302 (1981).
2. Chou, C.-L. Fractionation of siderophile elements in the
Earth's upper mantle. Proc. Lunar Planet. Sci. Conf. 9, 219-230
(1978).
3. Brearley, A. J. & Jones, R. H. in Planetary Materials. Reviews
in Mineralogy Vol. 36 (ed. Papike, J. J.) 3-1-3-398 (The
Mineralogical Society of America, Washington DC, 1998).
4. Brown, P. G. et al. The fall, recovery, orbit, and composition
of the Tagish Lake meteorite: a new type of carbonaceous
chondrite. Science 290, 320-325 (2000).
5. Drake, M. J. Accretion and primary differentiation of the
Earth: a personal journey. Geochim. Cosmochim. Acta 64, 2363-2370
(2000).
Nature 2002 416:39
Related Background Brief:
A SHEAR-VELOCITY MODEL OF THE MANTLE. The authors present a new
model of shear velocity structure in the mantle which is designed
to fit data-sets of absolute and differential body-wave travel
times, surface-wave phase velocities over a broad range of
frequencies, and structure coefficients of modes of free
oscillation, The model is parameterized laterally by spherical
harmonics (truncated at degree 16) and by 30 natural cubic B-
splines in radius. The best model features large-amplitude
structure (up to +/-6% anomalies in shear velocity) in the
topmost 400 km of the mantle and in the lowermost 500 km (up to
+/-2.5%) with most of the power in the low harmonics (l < 6). The
middle of the mantle is characterized by low-amplitude anomalies
with a much whiter spectrum. The models generally show no
distinctive changes in structure (in either shape or amplitude)
at the 660 km discontinuity, supporting the idea that an
endothermic phase transition is not a barrier to large-scale flow
in the mantle. G. Masters et al: Phil. Trans. Roy. Soc. London
Ser.A 1996 354:1385.
ScienceWeek http://www.scienceweek.com
Related Background:
BODY-WAVE VS. NORMAL-MODE REFERENCE EARTH MODELS
J.P. Montagner and B.L. Kennett (Australian National University,
AU) discuss reference Earth models, the authors making the
following points:
1) Reference earth models can be retrieved from either body waves
or normal-mode eigenperiods. However, there is a large
discrepancy between different reference earth models, which
arises partly from the type of data set used in their
construction and partly from differences in parametrization.
Reference models derived from body-wave observations do not give
access to density, attenuation factor, and radial anisotropy.
Conversely, reference models derived from normal modes cannot
provide the correct locations for the depth of seismic
discontinuities, nor the associated velocity jump. Eigenperiods
derived from reference models constructed using body-wave data
together with classifical attenuation models differ significantly
from the observed eigenperiods.
2) The body-wave and normal-mode approaches can be reconciled.
The V-p and V-s velocities given by body-wave models are
considered as constraints, and an inversion is performed for
parameters that cannot be extracted from body waves in the
context of a radially anisotropic model, i.e., the density rho,
the quality factor Q(mu), and the anisotropy parameters zeta, phi
and eta. The influence of anelasticity is very large, although
insufficient by itself to reconcile the two types of model.
However, by including in the inversion procedure the density and
the three anisotropic parameters, body-wave models can be brought
into complete agreement with eigenperiod data.
3) The authors report that a number of reference models derived
from body waves were tested and used as starting models: iasp91,
sp6, and two new models ak303 and ak135. A number of robust
features can be extracted from the inversions based on these
different models. The quality factor Q(mu) is found to be much
larger in the lower mantle than in previous models (e.g. prem).
Anisotropy, in the form of transverse isotropy with a vertical
symmetry axis, is significant in the whole upper mantle, but very
small in the lower mantle except in the lower transition zone
(between the 660 km discontinuity and 1000 km depth) and in the
D"-layer. Compared with the prem model there is an increase of
density in the D"-layer and a decrease in the lower transition
zone. The attenuation estimates have been derived using velocity
dispersion information, but are in agreement with available
direct measurements of normal-mode attenuation. Such attenuation
data are still of limited quality, and the present results
emphasize the need for improved attenuation measurements.
Geophys. J. Internat. 1996 125:229
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3. MANTLE DYNAMICS
ON MANTLE FLOW
P.G. Silver and W. E. Holt (Carnegie Institution of Washington,
US) discuss mantle flow, the authors making the following points:
1) The authors suggest it is surprising that after more than
three decades into the plate tectonic revolution, we have so
little direct observation of the mantle flow field that
accompanies tectonic plate motion. The most straightforward
measure of mantle flow is provided by the trajectory of subducted
slabs whose seismicity and high seismic velocities provide
tracers of the flow. Yet even in subduction zone environments
there is evidence for complex three-dimensional flow both above
and below the slab (1-4). Far from slabs, even less information
is available to delineate the mantle flow field. Various
approaches have been used to predict this flow field
theoretically. One approach (5) calculates the mantle flow field
that would result if the motions of the plates are imposed as
boundary conditions, in addition to considering the trenches and
ridges as sources and sinks of mass. This flow field is dominated
by plate-entrained flow and a corresponding counterflow.
2) More recently, several groups have calculated the
instantaneous field arising from density anomalies in the mantle
inferred from either seismic tomography or the history of
subduction. The plates are again taken as boundary conditions on
this flow field, and a plate velocity is chosen such that the
integrated torque on each plate vanishes. Both approaches
adequately predict plate velocities, although the accompanying
mantle flow fields and driving forces are different. The major
difference in these approaches has to do with the role of density
anomalies that are not directly attached to currently subducting
plates, but are either inferred from global seismic tomography or
from the long-term history of subduction. One way of testing
these models is to measure the flow field beneath a plate that is
not attached to a slab, but that has a mantle density anomaly
beneath it and therefore different mantle flow fields predicted
by the various models. The North American plate has these
characteristics.
3) The authors report they have combined observations of surface
deformation and upper mantle seismic anisotropy to estimate the
horizontal mantle flow field for western North America. They
report that the mantle velocity is 5.5 ± 1.5 centimeters per year
due east in a hot spot reference frame, nearly opposite to the
direction of North American plate motion (west-southwest). The
flow is only weakly coupled to the motion of the surface plate,
producing a small drag force. This flow field is probably due to
heterogeneity in mantle density associated with the former
Farallon oceanic plate beneath North America.
References (abridged):
1. R. M. Russo and P. G. Silver, Science 263, 1105 (1994)
2. J. Polet, et al., J. Geophys. Res. 105, 6287 (2000)
3. V. Peyton, et al., Geophys. Res. Lett. 28, 379 (2001)
4. G. P. Smith, et al., Science 292, 713 (2001)
5. C. G. Chase, Geophys. J. R. Astron. Soc. 56, 1 (1979)
Science 2002 295:1054
Related Background:
MODELS OF MANTLE CONVECTION
Don L. Anderson (California Institute of Technology, US)
discusses mantle convection, the author making the following
points:
1) There are two competing models for mantle convection. In the
first model, the mantle is stratified into two or more separate
convecting regions. In the second model, the whole mantle
convects as a single unit. Recent progress in plate tectonics,
seismology, solid-state physics, and mantle convection is
providing strong support for the stratified convection model. The
results may also help explain how plate tectonics relate to
mantle convection: upper mantle convection may be driven by plate
tectonics, whereas the deep mantle may convect in a completely
different style.
2) Evidence for whole mantle convection comes primarily from
seismology, and involves high-velocity seismic anomalies that
appear to be slabs traversing the mantle. The evidence for
occasional slab penetration below 650 kilometers is usually
considered sufficient evidence for whole mantle convection. Whole
mantle convection is also the reigning paradigm among geodynamic
modelers because of the seismic evidence and the similarity
between the geoid (the surface of constant gravitational
potential that would represent the sea surface if the oceans were
not in motion) and deep mantle seismic tomography (which works
much like medical x-ray tomography except that seismic velocities
are imaged). Whole mantle convection simulations are also easier
to do.
3) Arguments for stratified convection are more complex and more
difficult to understand. Pressure suppresses the effect of
temperature on density, making it more difficult for the deep
mantle to convect. Pressure also suppresses the effect of
temperature on seismic velocities, which are used by
seismologists to map temperature variations. Ab initio
calculations of mantle minerals indicate that subtle differences
in seismic gradients and velocities may be compositional: even
small changes in chemistry can stratify mantle convection.
Science 2001 293:2106
Related Background:
ON SEISMIC TOMOGRAPHY AND MANTLE DYNAMICS
In general, the term "tomography" refers to a representation in
cross-section in which neighboring 2-dimensional cross-sections
are combined to provide a 3-dimensional model. The use of
computer-aided tomography (CAT) in medical diagnosis is well-
known as a non-invasive method of examining internal organs for
abnormal regions. X-rays or ultrasonic waves are absorbed
unequally be different materials, and computer-aided tomography
consists of studying the attenuation of x-rays or ultrasonic
waves that pass through the body in distinctly controlled planar
sections. The technique of "seismic tomography" uses the same
principles, with the difference that the travel-times of the
signals, rather than their attenuation, are observed. Thus, the
technique of seismic tomography may be described as the 3-
dimensional modeling of the velocity distribution of seismic
waves in the Earth. In general, the technique requires powerful
computational facilities and sophisticated programming.
T. Tanimoto and T. Ley (2 installations, US) present a review of
current research on mantle dynamics and seismic tomography, the
authors making the following points:
1) The authors point out that the advent of the theory of plate
tectonics approximately 30 years ago established that most near-
surface geological phenomena such as earthquakes, volcanoes, and
mountain belts can be understood in the context of a unifying
model of interacting surface plates. However, our understanding
of this system has largely been limited to detailed kinematics of
plate motions, leaving the nature of the driving motions in the
interior as a puzzle. Questions such as what is the configuration
of convection, and how are surface tectonics controlled by
internal processes, have long been raised, but a lack of tools
and a lack of evidence prevented evaluation of various
hypotheses. Thus, most views regarding mantle dynamics remained
highly speculative until recently. Seismic tomography, which
emerged in the early 1980s, has provided a major probe of the
dynamical system of which plates are just the surface veneer.
2) The primary question concerning mantle dynamics is whether
mantle convection occurs in mantle-wide convective cells or
whether it involves a layered system, with separate flow regimes
in the upper mantle (i.e., above 650 kilometers) and lower
mantle. One of the most exciting results from work during the
last 5 years is the verification of deep penetration of former
oceanic lithosphere into the lower mantle. Tomography shows
thickened tabular extensions of subducted material to depths as
great as 2000 kilometers directly below deep subduction zones
where earthquakes occur in oceanic slabs down to approximately
650-kilometer depth. Thus, strictly layered mantle convection can
now be ruled out with good confidence.
3) In summary, seismic tomography has resulted in breakthrough
advances in the last two decades, revealing fundamental
geodynamical processes throughout the Earth's mantle and core.
Convective circulation of the entire mantle is taking place, with
subducted oceanic lithosphere sinking into the lower mantle,
overcoming the resistance to penetration provided by the phase
boundary near 650-kilometer depth that separates the upper and
lower mantle. The boundary layer at the base of the mantle has
been revealed to have complex structure, involving local
stratification, extensive structural anisotropy, and massive
regions of partial melt. The Earth's high *Rayleigh number
convective regime is now recognized to be much more interesting
and complex than suggested by textbook cartoons, and continued
advances in seismic tomography, geodynamic modeling, and high-
pressure-high-temperature mineral physics will be needed to fully
quantify the complex dynamics of our planet's interior.
Proc. Nat. Acad. Sci. 2000 97:12409
Notes:
... ... *Rayleigh number: The Rayleigh number is a dimensionless
parameter used in the theory of fluid dynamics. In general, the
Rayleigh number provides a determination of when convection is
initiated in a fluid.
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4. PLATE DYNAMICS IN THE MANTLE
SEISMIC ANISOTROPY: TRACING PLATE DYNAMICS IN THE MANTLE
J. Park and V. Levin (Yale University, US) discuss plate
dynamics, the authors making the following points:
1) The rocks of Earth's upper mantle deform in the slow
convective flows of our planet's hot interior. Deformation
preferentially aligns mineral crystals within rock (1,2). This
alignment induces elastic anisotropy, which affects how fast
seismic waves propagate in different directions. In principle,
measurement of wavespeed directionality and related seismic
properties can image deformation in the rocks of Earth's mantle
and may illuminate the mantle's role in plate motion. Seismic
indicators of anisotropy are often subtle and can have
conflicting interpretations. A simple image of Earth's present-
day mantle flow is complicated by several factors, including
textures produced in past deformation episodes, total strain,
shear style, temperature, and volatile content, all of which
influence microscopic processes within deforming mineral crystals
(3,4).
2) Four basic seismic wave behaviors have been used to identify
seismic anisotropy: direction-dependent travel time anomalies,
shear-wave birefringence, surface-wave scattering, and direction-
dependent conversion of compressional (P) to shear (S) waves.
Each approach has advantages and disadvantages.
3) Anisotropy is found on several scales. Many rocks are
anisotropic on the scale of individual minerals (3,4), or have a
preferred alignment of microcracks or pore spaces. Larger scale
geologic textures can also produce seismic anisotropy, as when
different rock types are interlayered on scales much smaller than
the wavelengths of seismic P and S waves (>500 m for typical
earthquake signals). Crack-induced and fine-layering anisotropy
are most prevalent in the uppermost crust, but the effects of
macroscopic rock texture may be important in some of the mantle
near the base of the crust. A sequence of veins from the ascent
of basalt or fluids through the lithosphere can also produce
seismic anisotropy.
4) In summary: Elastic anisotropy is present where the speed of a
seismic wave depends on its direction. In Earth's mantle, elastic
anisotropy is induced by minerals that are preferentially
oriented in a directional flow or deformation. Earthquakes
generate two seismic wave types: compressional (P) and shear (S)
waves, whose coupling in anisotropic rocks leads to scattering,
birefringence, and waves with hybrid polarizations. This varied
behavior is helping geophysicists explore rock textures within
Earth's mantle and crust, map present-day upper-mantle
convection, and study the formation of lithospheric plates and
the accretion of continents in Earth history.(5)
References (abridged):
1. V. Babuska, M. Cara, Seismic Anisotropy in the Earth (Kluwer
Academic, Dordrecht, Netherlands, 1991).
2. M. Kumazawa and O. L. Anderson, J. Geophys. Res. 74, 5961
(1969).
3. A. Nicolas, F. Boudier, A. M. Boulier, Am. J. Sci. 10, 853
(1973).
4. N. I. Christensen, Geophys. J. R. Astron. Soc. 76, 89 (1984).
5. N. I. Christensen, Geophys. J. R. Astron. Soc. 76, 89 (1984).
Science 2002 296:485
Related Background:
ON SEISMIC ANISOTROPY AND MANTLE DEFORMATION
M.K. Savage (Victoria University Wellington, NZ) discusses mantle
deformation, the author making the following points:
1) Shear wave splitting measurements now allow us to examine
deformation in the lithosphere and upper asthenosphere with
lateral resolution < 50 km. In an anisotropic medium, one
component of a shear wave travels faster than the orthogonal
component. The difference in speed causes the waves to separate;
this phenomenon is called "shear wave splitting". The
polarization of the fast component and the time delay between the
components provide simple measurements to characterize the
anisotropy. Strain aligns highly anisotropic olivine crystals in
the mantle, which is the most likely cause of splitting measured
from records of distant earthquakes. The seismic community is in
the fundamental stages of determining the relations between
strain and anisotropy, measuring anisotropy around the world, and
determining how much is formed by past and present lithospheric
deformation and how much is formed by crustal-asthenospheric
sources.
2) The mantle appears isotropic between 600 km depth and the D"
layer at the top of the core-mantle boundary. Shear wave
anisotropy of up to 4% is ubiquitous in the upper 200 km of the
crust and mantle. Evidence for stronger and deeper anisotropy is
less common. Anisotropy in the transition zone between 400 and
600 km and in the D" layer may be patchy. Transcurrent
deformation at plate boundaries appears to be one of the best
mechanisms for causing splitting on nearly vertically traveling
waves by aligning foliation planes and the fast axes of olivine
within the lithosphere parallel to the boundary and in the most
favorable orientation for splitting. Similar deformation may also
contribute to anisotropy observed at convergent margins.
3) Shear wave splitting data are challenging conventional beliefs
about mantle flow. Simple models of asthenosphere diverging at
spreading centers and flowing downward beneath subduction zones
appear to be only part of the story, with significant components
of flow parallel to ridges and trenches. Parallelism between fast
polarizations of waves passing through the deep mantle beneath
cratons and surficial geological strain indicators has been used
to suggest that the mantle at depths of several hundred
kilometers beneath the cratons may have been stable since the
initial deformation in the Archean. New paths of investigation
include testing a wider range of anisotropic symmetry systems and
more complicated models by examining variations in splitting as a
function of earthquake arrival angle and distance and by
numerical modeling of waveforms and of proposed deformation
scenarios.
Revs. Geophys. 1999 37:65
Related Background:
BOUNDARY LAYERS AND SEISMIC ANISOTROPY IN EARTH'S MANTLE
J.P. Montagner (CNRS, FR) discusses seismic anisotropy, the
author making the following points:
1) During the last 30 years, considerable evidence of seismic
anisotropy has accumulated, demonstrating that it is present at
all scales, but not in all depth ranges. The author details which
conditions are necessary to detect large-scale seismic
anisotropy. Firstly, minerals must display a strong anisotropy at
the microscopic scale, and/or the medium must be finely layered.
Secondly, the relative orientations of symmetry axes in the
different crystals must not counteract in destroying the
intrinsic anisotropy of each mineral, and there must be efficient
mechanisms of orientation of minerals and aggregates. Finally,
the strain field must be coherent at large scale in order to
preserve long wavelength anisotropy. Part of shallow anisotropy
can be related to the past strain field (frozen-in anisotropy);
however the deep anisotropy is due to the present strain held.
All these conditions are fulfilled only in boundary layers of
convective mantle.
2) The author reviews the seismic data sets which provide insight
into the location at depth of large-scale anisotropy from the D"-
layer up to the lithosphere. In addition to the well-documented
seismic anisotropy in the lithosphere and asthenosphere, there is
new evidence of seismic anisotropy in the upper (400-660 km) and
lower (660-900 km) transition zones and in the D"-layer.
Nonetheless the bulk of the lower mantle seems close to isotropy.
If we assume the hypothesis that seismic anisotropy is associated
with boundary layers in convective systems, these observations
strongly suggest that the transition zone is a boundary layer
which makes the passage of matter between the upper and the lower
mantle difficult. However, this general statement does not rule
out flow circulation between the upper and lower mantles.
Pure and Appl. Geophys. 1998 151:223
Related Background:
ON CRUSTAL DEFORMATIONS AND MANTLE DYNAMICS
Michael Gurnis (California Institute of Technology, US) presents
a review of current research on mantle dynamics and crustal
deformations, the author making the following points:
1) Recent discoveries have produced a vivid and dynamic picture
of the motions of the mantle, and researchers are beginning to
understand that these motions shape the surface of the Earth in
many ways. Such motions help to drive the horizontal movement of
tectonic plates, but they also lift and lower the continents.
Enigmatic dips and swells have occurred over continent-size
swaths of the Earth's surface several times in the past. Southern
Africa, for example, has been lifted approximately 1000 feet over
the past 20 million years, and the highest peaks of a sunken
continent today form the islands of Indonesia. The causes of
these vertical motions apparently lie deep within the interior of
the planet and involve mantle dynamics. Perhaps the most
intriguing discovery is that motion in the deep mantle lags
behind the horizontal movement of tectonic plates, so that
positions of ancient plate boundaries can be related to the way
the surface of the Earth is shaped many millions of years later.
2) The author suggests that our ability to view the dynamics of
mantle convection and plate tectonics will rapidly expand as new
ways of observing the mantle and techniques for simulation its
motions are introduced. When mantle convection changes, the
gravitational field of the Earth changes, so that tracking
variations in Earth's gravitational field is an important
research objective. The author concludes: "Plans to make
unprecedented images and measurements of the mantle in the coming
decade, together with the use of ever more powerful
supercomputers, foretell an exceptionally bright future for
deciphering the dynamics of the Earth's interior."
Scientific American March 2001
Related Background Brief:
LATTICE PREFERRED ORIENTATION OF OLIVINE AGGREGATES DEFORMED IN
SIMPLE SHEAR. Regions of the Earth's upper mantle show
significant seismic anisotropy due to the preferred
crystallographic orientation ('lattice preferred orientation')
adopted by its constituent minerals in response to deformation.
Seismic anisotropies thus provide clues to the flow and/or stress
patterns in the upper mantle, but the use of lattice preferred
orientation to infer such properties from seismic data has been
hampered by the lack of experimental studies relating changes in
crystallographic orientation to simple-shear deformation. (Simple
shear is probably the dominant mode of deformation in the upper
mantle, whereas most previous experiments have focused on the
effects of uniaxial compression.) The authors describe the
results of simple-shear deformation experiments on olivine
aggregates, conducted at high temperatures and pressures (similar
to 1,500 K and 300 MPa). For large strains (up to 150%), the
experiments reproduce the lattice preferred orientation observed
in highly deformed upper-mantle rocks, in which the olivine [100]
axes lie nearly parallel to the bow direction. But for relatively
small strains, the preferred orientation is rotated with respect
to the flow direction, indicating that seismic anisotropy should
also be sensitive to the sense of shear is the upper mantle. S.Q.
Zhang and S. Karato: Nature 1995 375:774.
Related Background Brief:
WATER-INDUCED FABRIC TRANSITIONS IN OLIVINE. The interpretation
of seismic anisotropy in Earth's upper mantle has traditionally
been based on the fabrics (lattice-preferred orientation) of
relatively water-poor olivine. The authors demonstrate that when
a large amount of water is added to olivine, the relation between
flow geometry and seismic anisotropy undergoes marked changes.
Some of the puzzling observations of seismic anisotropy in the
upper mantle, including the anomalous anisotropy in the central
Pacific and the complicated anisotropy in subduction zones, can
be attributed to the enrichment of water in these regions. H.
Jung and S-I Karato: Science 2001 293:1460.
Related Background Brief:
EMPLACEMENT OF MANTLE PERIDOTITE IN THE LOWER CONTINENTAL-CRUST,
IVREA-VERBANO ZONE, NORTHWEST ITALY. The Ivrea-Verbano zone has
been cited repeatedly as an exposed pre-Alpine crust-mantle
transition on the basis, in part, of the presence of lenses of
mantle peridotite and a possible petrologic Moho. A 10-km-thick
mafic complex has been described as an intrusion at the contact
between the mantle and a thick crustal sequence of metamorphosed
wacke and pelitic rocks. The authors report that the significance
of these features is now challenged by new mapping that
demonstrates that mantle peridotites in the southern Ivrea-
Verbano zone were lenses tectonically interfingered with
metasedimentary rocks prior to intrusion of the gabbroic complex.
Although it remains possible that the Ivrea-Verbano section
exposes part of a complex crust-mantle interface within which
crustal and mantle rocks were interleaved, the association of
peridotite and metasedimentary rocks is also consistent with
assembly in an accretionary wedge. In either case, present
exposures are from an unknown distance above the pre-Alpine
contiguous mantle and reference to the section as a complete
crust-mantle transition could be misleading. J.E. Quick et al:
Geology 1995 23:739.
Related Background Brief:
CRUSTAL ANISOTROPY IN THE URAL MOUNTAINS FOREDEEP FROM
TELESEISMIC RECEIVER FUNCTIONS. Radial and transverse teleseismic
receiver functions (RFs) at GSN station ARU, in central Eurasia,
display variation in back-azimuth psi consistent with a 1-D
anisotropic crustal structure. In a broad psi range, the
transverse RFs possess a strong phase at similar to 5-sec delay
relative to direct P, with a polarity reversal at psi similar to
50 degrees. The radial RFs peak at the transverse-RF polarity
reversal for this converted phase. The first motion of the
transverse RFs varies with psi also, reversing polarity at psi
similar to 345 degrees. The azimuthal variation can be modeled by
a 5-layer velocity profile with substantial (15%) seismic
anisotropy in both the lowermost crust and a low-velocity surface
layer. Assuming hexagonal symmetry, the lowermost crust has a
tilted "slow" symmetry axis, i.e., an oblate phase velocity
surface. The strike of the axis is oblique to the north-south
Urals trend, but deviates < 20 degrees from the mantle fast-axis
inferred from SKS splitting. The magnitude and tilt of the
model's anisotropy suggests that fine layering and/or aligned
cracks augment mineral-orientation anisotropy near the top and
bottom of the crust. V. Levin and J. Park: Geophys. Res. Lett.
1997 24:1283.
Related Background Brief:
A KINEMATIC MODEL FOR RECRYSTALLIZATION AND TEXTURE DEVELOPMENT
IN OLIVINE POLYCRYSTALS. The interpretation of seismic anisotropy
in the mantle requires a knowledge of the relationship between
the lattice preferred orientation (LPO) of crystals and the
convective flow field. In order better to understand this link,
the authors present a model for the evolution of LPO in olivine
aggregates that deform by both intracrystalline slip and dynamic
recrystallization. Dynamic recrystallization depends on the
dislocation density of the grains, which is a function of the
applied local stress. Grains with a large density of dislocations
lower their bulk strain energy by nucleating strain-free sub-
grains at a rate proportional to a dimensionless nucleation
parameter lambda*. Grains with high energy are then invaded by
grains with low energy by grain-boundary migration, at a rate
proportional to a dimensionless grain-boundary mobility M*. The
value of lambda* is constrained by observed LPO patterns in
experimentally deformed olivine aggregates, and M* is constrained
by the temporal evolution of the strength of the LPO. For M* =
125 +/- 75 and M* > 3, the model predictions agree well with the
experimental results. Numerical calculations of LPO using the
model are significantly faster than those based on viscoplastic
self-consistent or equilibrium-based theories, making the model
especially suitable for applications for complex convective
flows. E. Kaminski and N.M. Ribe: Earth & Planetary Sci. Lett.
2001 189:253.
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5. MID-MANTLE DYNAMICS
MID-MANTLE DEFORMATION INFERRED FROM SEISMIC ANISOTROPY
J. Wookey et al (University of Leeds, UK) discuss mid-mantle
deformation, the authors making the following points:
1) Seismic anisotropy in the upper 200 km of the Earth's mantle
is primarily attributed to the preferred alignment of olivine
crystals which have deformed by dislocation creep(5). The origin
of anisotropy at greater depths is more speculative, but there is
evidence for anisotropy in the transition zone in some regions,
but not in others. In an effort to reconcile discrepancies in
global velocity models derived from body-wave travel times and
normal-mode observations, Montagner and Kennett(1996) allowed
both anisotropy and attenuation in a joint inversion of these
data sets. Their final model shows significant levels of
anisotropy in the uppermost and lowermost mantle, but also in the
vicinity of the 660-km discontinuity (hereafter referred to as
the "660"). This motivated an investigation of mid-mantle
anisotropy on a regional scale. The authors investigate shear-
wave splitting in deep-focus events that image a region below the
Australian plate.
2) Stations in Australia are ideal for investigating near-source
anisotropy, as studies have shown that they exhibit very little,
if any, receiver-side shear-wave splitting. For example, 52 SKS-
wave measurements (i.e., measurements of waves that pass through
the core) with good azimuthal coverage at the station CAN show
that shear waves that are traveling nearly vertically are not
split while crossing the Australian lithosphere beneath this
station. In contrast, the authors find that deep-focus events
from the Tonga–Kermadec and New Hebrides subduction zones show
very large degrees of shear-wave splitting at this and four other
Australian stations, suggesting anisotropy deeper in the mantle,
away from the receiver.
3) In summary: With time, convective processes in the Earth's
mantle will tend to align crystals, grains and inclusions. This
mantle fabric is detectable seismologically, as it produces an
anisotropy in material properties -- in particular, a directional
dependence in seismic-wave velocity. This alignment is enhanced
at the boundaries of the mantle where there are rapid changes in
the direction and magnitude of mantle flow(1), and therefore most
observations of anisotropy are confined to the uppermost mantle
or lithosphere(2,3) and the lowermost-mantle analogue of the
lithosphere, the D" region(4). The authors present evidence from
shear-wave splitting measurements for mid-mantle anisotropy in
the vicinity of the 660-km discontinuity, the boundary between
the upper and lower mantle. Deep-focus earthquakes in the
Tonga–Kermadec and New Hebrides subduction zones recorded at
Australian seismograph stations record some of the largest values
of shear-wave splitting hitherto reported. The authors suggest
the results indicate that, at least locally, there may exist a
mid-mantle boundary layer, which could indicate the impediment of
flow between the upper and lower mantle in this region.
References (abridged):
1. Montagner, J.-P. Where can seismic anisotropy be detected in
the Earth's mantle? In boundary layers .... Pure Appl. Geophys.
151, 223-256 (1998).
2. Silver, P. G. Seismic anisotropy beneath the continents:
Probing the depths of geology. Annu. Rev. Earth Planet Sci. 24,
385-432 (1996).
3. Savage, M. K. Seismic anisotropy and mantle deformation: What
have we learned from shear wave splitting? Rev. Geophys. 37, 65-
106 (1999).
4. Kendall, J.-M. & Silver, P. G. in The Core-Mantle Boundary
Region (eds Gurnis, M., Wysession, M., Knittle, E. & Buffett, B.)
97-118 (Geodynamics series 28, American Geophysical Union,
Washington DC, 1998).
5. Karato, S. & Wu, P. Rheology of the upper mantle: A synthesis.
Science 260, 771-778 (1993).
Nature 2002 415:777
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6. DEEP-MANTLE DYNAMICS
ON SOLID-STATE CONVECTION OF EARTH'S LOWER MANTLE
A.K. McNamara et al (University of Michigan, US) discuss Earth's
mantle, the authors making the following points:
1) Seismological observations reveal highly anisotropic patches
at the bottom of the Earth's lower mantle, whereas the bulk of
the mantle has been observed to be largely isotropic (1-4). These
patches have been interpreted to correspond to areas where
subduction has taken place in the past or to areas where mantle
plumes are upwelling, but the underlying cause for the anisotropy
is unknown — both shape-preferred orientation of elastically
heterogeneous materials (5) and lattice-preferred orientation of
a homogeneous material have been proposed. Both of these
mechanisms imply that large-strain deformation occurs within the
anisotropic regions, but the geodynamic implications of the
mechanisms differ. Shape-preferred orientation would imply the
presence of large elastic (and hence chemical) heterogeneity
whereas lattice-preferred orientation requires deformation at
high stresses.
2) The authors report that on the basis of numerical modeling
incorporating mineral physics of elasticity and development of
lattice-preferred orientation, that slab deformation in the deep
lower mantle can account for the presence of strong anisotropy in
the circum-Pacific region. In this model — where development of
the mineral fabric (the alignment of mineral grains) is caused
solely by solid-state deformation of chemically homogeneous
mantle material — anisotropy is caused by large-strain
deformation at high stresses, due to the collision of subducted
slabs with the core–mantle boundary.
References (abridged):
1. Lay, T., Williams, Q. & Garnero, E. J. The core-mantle
boundary layer and deep Earth dynamics. Nature 392, 461-468
(1998).
2. Lay, T., Williams, Q., Garnero, E. J., Kellogg, L. &
Wysession, M. E. in The Core-Mantle Boundary (eds Gurnis, M.,
Wysession, M. E., Knittle, E. & Buffett, B. A.) 299-318
(Geodynamics Series Vol. 28, Am. Geophys. Union, Washington DC,
1998).
3. Kendall, J. M. in Earth's Deep Interior: Mineral Physics and
Tomography from the Atomic to the Global Scale (eds Karato, S.,
Forte, A. M., Liebermann, R. C., Masters, G. & Stixrude, L.) 133-
159 (Geophysics Monograph 117, Am. Geophys. Union, Washington DC,
2000).
4. Ritsema, J. Evidence for shear velocity anisotropy in the
lowermost mantle beneath the Indian Ocean. Geophys. Res. Lett.
27, 1041-1044 (2000).
5. Kendall, J. M. & Silver, P. G. Constraints from seismic
anisotropy on the nature of the lowermost mantle. Nature 381,
409-412 (1996).
Nature 2002 416:310
Related Background Brief:
HETEROGENEITY OF THE LOWERMOST MANTLE. Strong heterogeneity at a
variety of scale lengths has been imaged in the lowermost mantle
using different forward and inverse methods. Coherent patterns in
differential travel times of waves that sample the base of the
mantle, such as diffracted shear waves (Sdiff) and compressional
waves (Pdiff), are readily apparent, and are compared with
results from tomographic studies. Travel time and waveform
modeling studies have demonstrated the presence of intense
lateral variations in a variety of mapped features, such as a
regionally detected high velocity D" layer, ultralow velocity
zones, D" anisotropy, strong scattering and heterogeneity. Such
short-wavelength variations currently preclude confident mapping
of D" structure at smaller scales. Issues of seismic resolution
and uncertainties are emphasized here, as well as the limitations
of one-dimensional modeling/averaging in highly heterogeneous
environments. E.J. Garnero: Annu Rev Earth & Planetary Sci 2000
28:509.
Related Background Brief:
THE FATE OF SUBDUCTED BASALTIC CRUST IN THE EARTH'S LOWER MANTLE.
The subduction of oceanic lithosphere into the Earth's deep
interior is thought to drive convection and create chemical
heterogeneity in the mantle. The oceanic lithosphere as a whole,
however, might not subduct uniformly: the fate of basaltic crust
may differ from that of the underlying peridotite layer because
of differences in chemistry, density and melting temperature. It
has been suggested that subducted basaltic crust may in fact
become buoyant at the mantle's 660-km discontinuity, remaining
buoyant to depths of at least 800 km, and therefore might be
gravitationally trapped at this boundary to form a garnetite
layer. The authors report the phase relations and melting
temperatures of natural mid-ocean ridge basalt at pressures up to
64 GPa (corresponding to similar to 1500 km depth). The authors
find that the former basaltic crust is no longer buoyant when it
transforms to a perovskitite lithology at about 720 km depth, and
that this transition boundary has a positive pressure-temperature
slope, in contrast to the negative slope of the transition
boundary in peridotite. The authors therefore predict that
basaltic crust with perovskitite lithology would gravitationally
sink into the deep mantle. The melting data of the authors
suggest that, at the base of the lower mantle, the former
basaltic crust would be partially molten if temperatures there
were to exceed 4000 kelvins. K. Hirose et al: Nature 1999 397:53.
Related Background:
ON THE ORIGIN OF SEISMIC ANISOTROPY IN THE D" LAYER
S. Karato (University of Minnesota, US) discusses the D" layer,
the author making the following points:
1) Physical mechanisms of seismic anisotropy in the D" layer are
examined by the author based on seismological and mineral physics
observations. The results of body-wave seismology on the fine
structure of the D" layer and of mineral physics studies on the
elastic constants and the lattice preferred orientation in lower
mantle minerals as well as the shape preferred orientation of
melt pockets are taken into account. Evidence of large but depth
(pressure)-dependent elastic anisotropy of lower mantle minerals,
particularly (Mg,Fe)O, and of tilted shape preferred orientation
of sheared partial melts is summarized. The author demonstrates
that both shape preferred orientation of partial melts (or iron-
rich secondary phases) and lattice preferred orientation of
minerals with well-documented slip systems are difficult to
reconcile with seismological observations. However, lattice
preferred orientation of highly anisotropic mineral, (Mg,Fe)O, is
consistent with most of the seismic observations if the dominant
glide plane under the D" layer conditions is {100} rather than
{110} as observed at lower pressures. Such a change in glide
plane in MgO (or (Mg,Fe)O) is likely to occur as a result of
pressure-induced change in elastic anisotropy and/or in the
nature of chemical bonding and possibly due to high
temperatures).
2) Both solid state and partial melt mechanisms of anisotropy
imply that the V-SH > V-SV (V-SV > V-SH) polarization anisotropy
means horizontal (vertical) flow. In the solid-state mechanism,
significant V-SH > V-SV in the D" layer beneath the circum-
Pacific (Alaska and the Caribbean) implies horizontal shear at
high stress caused presumably by the collision of subducting
materials with the core-mantle boundary. Highly variable
anisotropy beneath the central-Pacific can be attributed to
solid-state fabrics caused by a complicated three-dimensional
flow presumably related to the upwelling of plumes, but
anisotropy in this region could also be attributed to the shape
preferred orientation of melt pockets, the presence of which is
suggested by very low average velocities.
Earth Planets and Space 1998 50:1019
SEISMIC ANISOTROPY IN THE DEEP MANTLE BOUNDARY LAYERS AND THE
GEOMETRY OF MANTLE CONVECTION
S.I. Karato (University of Minnesota, US) discusses deep mantle
boundary layers, the author making the following points:
1) The author reports an attempt to explore the geodynamical
significance of seismic anisotropy in the deep mantle on the
basis of mineral physics. The mineral physics observations used
include the effects of deformation mechanisms on lattice and
shape preferred orientation, the effects of pressure on elastic
anisotropy and the nature of lattice preferred orientation in
deep mantle minerals in a dislocation creep regime. Many of these
issues are still poorly constrained, but a review of recent
results shows that it is possible to interpret deep mantle
seismic anisotropy in a unified fashion based on solid state
processes without invoking partial melting. The key notions are
(a) the likely regional variation in the magnitude of anisotropy
as deformation mechanisms change from dislocation to diffusion
creep (or superplasticity) associated with a change in the stress
level and/or grain-size in the convecting mantle with a high
Rayleigh number, and (b) the change in elastic anisotropy with
pressure in major mantle minerals, particularly in (Mg, Fe)O.
2) The results provide the following constraints on the style of
mantle convection: (a) the SH > SV anisotropy in the bottom
transition zone and the SV > SH anisotropy in the top lower
mantle can be attributed to anisotropy structures (lattice-
preferred orientation and/or laminated structures) caused by the
horizontal flow in this depth range, suggesting the presence of a
mid-mantle boundary layer due to (partially) layered convection;
(b) the observation of no significant seismic anisotropy in the
deep mantle near subduction zones implies that deformation
associated with subducting slabs is due mostly to diffusion creep
(or superplasticity) and therefore slabs are weak in the deep
mantle and hence easily deformed when encountered with resistance
forces; and (c) the SH > SV anisotropy in the cold thick portions
of the D" layer is likely to be due to horizontally aligned shape
preferred orientation in perovskite plus magnesiowustite
aggregates formed by strong horizontal shear motion in the recent
past.
Pure & Appl. Geophys. 1998 151:565
Related Background Brief:
SOME MINERAL PHYSICS CONSTRAINTS ON THE RHEOLOGY AND GEOTHERMAL
STRUCTURE OF EARTH'S LOWER MANTLE. The authors explore the
implications of recent mineral physics measurements of diffusion
coefficients and melting temperatures of lower mantle materials
on the rheological and geothermal structure of Earth's lower
mantle. The authors demonstrate that MgSiO3 perovskite is
significantly stronger than MgO periclase and therefore the
rheology of the lower mantle depends strongly on the geometry of
a weaker phase, periclase. The authors calculate viscosities of
the lower mantle for two cases: (1) where periclase occurs as
isolated grains and (2) where periclase occurs as continuous
films, using mineral physics data and models of two-phase
rheology. The authors find that the effective viscosity for the
former is similar to 10-1000 times higher than the latter. The
authors therefore suggest that the rheology of the lower mantle
is structure- and hence strain-dependent, leading to weakening at
large strains due to the formation of continuous films of
periclase. Overall depth variation of viscosity depends not only
on the pressure dependence of creep but also on the geothermal
gradient. Both MgSiO3 perovskite and periclase have relatively
small activation energies, and therefore the depth variation of
viscosity is rather small, even for a nearly adiabatic
temperature gradient. However, the geothermal gradients
consistent with the geodynamical inference of nearly depth-
independent viscosity are sensitive to the pressure dependence of
viscosity, which is only poorly understood. A superadiabatic
gradient of up to approximately 0.6 K/km is also consistent with
mineral physics and geodynamical observations. D. Yamazaki and S.
Karato: Amer. Mineralogist 2001 86:385.
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7. MANTLE PLUMES
ON MANTLE PLUMES AND MOUNTAINS
J.B. Murphy et al (4 authors at 3 installation, CA US) present a
review of current studies concerning the relation between mantle
plumes and mountain building (orogeny; orogenesis), the authors
making the following points:
1) Theories of mountain building were revolutionized in the 1960s
by plate tectonic theory. The horizontal motions of rigid plates
of material above a pliable mantle helped explain the creation
and destruction of oceans, the generation of mountain belts and
sedimentary basins, the distribution of volcanic and earthquake
activity, and the locations of ore, oil, and gas deposits. But
plate-tectonic theorists have had difficulty accounting for many
of the geological details of southwestern North America,
including the uplifting of the Rocky Mountains, the extent of the
Basin and Range region of Nevada, Utah, and Arizona, and the
extensive volcanic deposits of the Columbia Plateau.
2) Established methods of mountain building all depend either
directly or indirectly on subduction zones. Three methods are
currently recognized:
... a) A subduction zone may lead directly to mountains formed by
ascending *magma and heat, as is the case in the Andes.
... b) The subduction process may also transport
*microcontinental fragments to the continental margin, where they
accrete as "*terranes", a process that has added significantly to
the North American West Coast over the past 400 million years.
... c) When an ocean is consumed by colliding continents, as in
the example of India and Asia, spectacular mountain building can
result.
3) Mantle plumes produce island chains such as the Hawaiian
Islands. The plume remains relatively stationary while the
oceanic plate moves over it. Plumes are thought to rise all the
way from the core-mantle boundary, 2,900 kilometers below the
Earth's surface, in relatively narrow columns. On reaching the
base of the lithosphere, a plume spreads out, underplating a
large area of lithosphere, causing it to heat and be bowed
upward.
4) Hot spots, the surface manifestations of mantle plumes, are
widely distributed around the Earth, although the exact number is
controversial. Hot spots are essentially stationary relative to
the faster-moving plates. No modern ocean could be consumed at a
subduction zone without a plate margin encountering a hot spot,
so the interaction between subduction zones and hot spots must be
common throughout geological time.
5) Most of the mountain building activity on the western margin
of North America over the past 300 million years represents
episodes of *magmatism and deformation associated with
microcontinent collisions. The Sonoma, Nevada, and Sevier
mountain-building events are examples. The Rocky Mountain
(Laramide) orogeny, however, is distinctive because it is
characterized by a lack of magmatism coupled with widespread
deformation in the continental interior.
6) The authors propose that an additional (fourth) method of
mountain building has been largely overlooked and may help
explain not only the Laramide Orogeny but also other unusual
geological features of the southwestern US. Their model involves
the interplay of the horizontal motions of traditional
subduction-related mountain-building processes with vertical
plumes of hot mantle ascending from thousands of kilometers below
the Earth's surface. The authors suggest that "together these
mechanisms may offer a convincing explanation for what long has
been a geologically puzzling part of the world and may lead to
better understanding of mountain building worldwide."
American Scientist 1999 87:146
Notes:
... *magma: In general, any molten mass of rock.
... *microcontinental fragments: In general, any fragment or
remnant of continental crust up to approximately the size of
Madagascar (Malagasy).
... *terranes: (terrains) In general, a terrane is any region of
crust with well-defined margins which differs significantly in
apparent tectonic evolution from neighboring regions.
... *magmatism: In general, the development and movement of magma
within the Earth.
Related Background:
THE INFLUENCE OF A CHEMICAL BOUNDARY LAYER ON THE FIXITY, SPACING
AND LIFETIME OF MANTLE PLUMES
A.M. Jellinek and M. Manga (University of California Berkeley,
US) discuss mantle plumes, the authors making the following
points:
1) Convective motions driven by core cooling have a structure
that is three-dimensional and time-dependent. Consequently, the
dynamics of the interaction between this flow and an underlying
dense layer is complex. Because of the computational challenge of
resolving small (kilometer) length scales while tracking
viscosity and density interfaces, numerical simulations for
conditions appropriate to mantle convection are typically limited
to two dimensions, though three-dimensional simulations are
currently being performed. Laboratory experiments are thus often
used to study thermochemical convection, and the experiments
presented by the authors extend previous investigations to the
situation in which the dense layer is thin and has a low
viscosity. Here we need to distinguish "plumes" from "thermals":
the authors use "plume" to describe buoyant upwellings (or
downwellings) that extend continuously from the hot (or cold)
boundary layer, and "thermal" to indicate a discrete buoyant
blob. Under conditions of thermal equilibrium, more-viscous cold
fluid descends in narrow sheet-like plumes, whereas lower-
viscosity hot fluid ascends mostly in thermals.
2) In summary: Seismological observations provide evidence that
the lowermost mantle contains superposed thermal and
compositional boundary layers(1) that are laterally
heterogeneous(2,3). Whereas the thermal boundary layer forms as a
consequence of the heat flux from the Earth's outer core, the
origin of an (intrinsically dense) chemical boundary layer
remains uncertain(4). Observed zones of "ultra-low" seismic
velocity(5) suggest that this dense layer may contain metals or
partial melt, and thus it is reasonable to expect the dense layer
to have a relatively low viscosity. Also, it is thought that
instabilities in the thermal boundary layer could lead to the
intermittent formation and rise of mantle plumes. Flow into
ascending plumes can deform the dense layer, leading, in turn, to
its gradual entrainment. The authors use analog experiments to
demonstrate that the presence of a dense layer at the bottom of
the mantle induces lateral variations in temperature and
viscosity that in turn determine the location and dynamics of
mantle plumes. A dense layer causes mantle plumes to become
spatially fixed, and the entrainment of low-viscosity fluid
enables plumes to persist within the Earth for hundreds of
millions of years.
References (abridged):
1. Lay, T., Williams, Q. & Garnero, E. J. The core-mantle
boundary layer and deep Earth dynamics. Nature 392, 461-468
(1998)
2. Wysession, M. E. Large-scale structure at the core-mantle
boundary from diffracted waves. Nature 382, 244-248 (1996)
3. Castle, J. C., Creager, K. C., Winchester, J. P. & van der
Hilst, R. D. Shear wave speeds at the base of the mantle. J.
Geophys. Res. 105, 21543-21557 (2000)
4. Gurnis, M., Wysession, M. E., Knittle, E. & Buffett, B. A.
(eds) The Core-Mantle Region Monograph 28 (American Geophysical
Union, Washington DC, 1998)
5. Garnero, E. J. Heterogeneity of the lowermost mantle. Annu.
Rev. Earth Planet. Sci. 28, 509-537 (2000)
Nature 2002 418:760
Related Background:
SUPERPLUMES FROM THE CORE-MANTLE BOUNDARY TO THE LITHOSPHERE:
IMPLICATIONS FOR HEAT FLUX
B. Romanowicz and Y. Gung (University of California Berkeley, US)
discuss superplumes, the authors making the following points:
1) Global seismic tomography aims to improve our understanding of
mantle dynamics by providing constraints on three-dimensional
(3D) temperature and composition with the use of elastic
velocities as proxies. Much progress has been made in recent
years in resolving increasingly finer details in the 3D
distribution of elastic velocities from the inversion of seismic
phase and travel time data (1-3). In particular, regions of
faster-than-average velocity, associated with subduction around
the Pacific rim, have revealed a variety of behaviors of
lithospheric slabs in the transition zone, some stagnant around
the 670-km discontinuity, whereas others penetrate into the lower
mantle to depths in excess of 1500 km (4). These results agree
with geodynamic models in which the cold and dense down-going
slabs play a driving role in global mantle circulation heated
primarily from within (5).
2) The detailed morphology and role of upwellings, as manifested
by two prominent zones of lower than average velocity in the
lowermost mantle and commonly referred to as "superplumes", is
less clear. Their location, under the south-central Pacific and
under Africa, correlates with the global distribution of hot
spots, as well as two major geoid highs. Recent tomographic S
wave velocity models suggest that the superplumes rise high above
the core-mantle boundary (CMB) (3), and joint seismic-geodynamic
studies imply that they may be active upwellings. However, finer
scale resolution is still lacking.
3) In summary: Three-dimensional modeling of upper-mantle
anelastic structure reveals that thermal upwellings associated
with the two superplumes, imaged by seismic elastic tomography at
the base of the mantle, persist through the upper-mantle
transition zone and are deflected horizontally beneath the
lithosphere. This explains the unique transverse shear wave
isotropy in the central Pacific. The authors infer that the two
superplumes may play a major and stable role in supplying heat
and horizontal flow to the low-viscosity asthenospheric channel,
lubricating plate motions and feeding hot spots. The authors
suggest that more heat may be carried through the core-mantle
boundary than is accounted for by hot spot fluxes alone.
References (abridged):
1. G. Masters, S. Johnson, G. Laske, B. Bolton, Philos. Trans. R.
Soc. London Ser. A 354, 1385 (1996).
2. Y. J. Gu, A. M. Dziewonski, W.-J. Su, G. Ekstroem, J. Geophys.
Res. 106, 11169 (2001).
3. C. Mégnin and B. Romanowicz, Geophys. J. Int. 143, 709 (2000).
4. Y. Fukao, S. Widiyantoro, M. Obayashi, Rev. Geophys. 39, 291
(2001).
5. D. Bercovici, Y. Ricard, M. A. Richards, Geophys. Monogr. Am.
Geophys. Union 121 (American Geophysical Union, Washington, DC,
2000), p. 5.
Science 2002 296:513
Related Background Brief:
ADVECTION OF PLUMES IN MANTLE FLOW: IMPLICATIONS FOR HOTSPOT
MOTION, MANTLE VISCOSITY AND PLUME DISTRIBUTION. Because of their
slow relative motion, hotspots, mainly in the Pacific, are often
used as a reference frame for defining plate motions. A coherent
motion of all Pacific hotspots relative to the deep mantle may,
however, bias the hotspot reference frame. Numerical results on
the advection of plumes, which are thought to cause the hotspots
on the Earth's surface, in a large-scale mantle flow field are
therefore presented by the authors. Bringing the results into
agreement with observations also leads to conclusions regarding
the viscosity structure of the Earth's mantle, as well as the
sources and distribution of plumes. The abrupt change in
direction of the Hawaiian-Emperor chain implies an upper-mantle
viscosity under the Pacific similar to 1.5 x 10^(20) Pa or less.
Slow relative motion of hotspots requires high lower-mantle
viscosity, unless hotspots are located at large-scale stationary
upwellings that are currently unresolved by seismic tomography.
For the preferred model, the authors obtain coherent motion of
Pacific hotspots in a reference frame of no net rotation, as well
as coherent motion relative to African hotspots, caused by return
flow antiparallel to plate motion. Advection and regional
differences in life expectancy can to a large part explain the
distribution of plumes in relation to ridges, subduction zones
(present and past) and seismic anomalies. Plume conduits are
substantially tilted in the lower mantle. The surface motion of
hotspots is often smaller than the advection rate of plume
conduits in the lowermost mantle. B. Steinberger and R.J.
O'Connell: Geophys. J. Internat. 1998 132:412.
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