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Penrose Conference
Plume IV:
Beyond the Plume Hypothesis
Tests of the plume paradigm and alternatives
Scientific report:
GSA Today, 14, 26-28,
2004. |
For
a PDF version, click here.
Can “hot spots” –
regions of anomalous volcanism – be explained
by mechanisms other than deep thermal upwellings?
This question was the subject of a week-long debate
at the Penrose Conference, Plume IV: Beyond the Plume
Hypothesis, held August 25-29, 2003 in Hveragerdi, Iceland.
Over 60 geologists, geophysicists, and geochemists from
12 nations gathered to brainstorm the fundamental evidence
that constrains the origins of volcanism. Seismology,
convection modeling, heat flow, geothermometry, petrology,
geochemistry, radiometric dating, kinematics, morphology
and basic geology were all considered. This meeting
was the most significant gathering ever held of scientists
working on alternative mechanisms for anomalous volcanic
regions. More such meetings will follow.
Seismology
presented included wave propagation, tomography, anisotropy,
and a summary of what seismology can and cannot tell
us. Contrary to general assumptions, seismic wave speeds
beneath Hawaii are relatively high (Julian).
Whole-mantle tomography favors strong impedence of flow
across the 660-km discontinuity and a relatively homogenous
middle mantle (Dziewonski). Tomographic evidence
for slab penetration to the core-mantle boundary is
sparse and detailed scrutiny reveals problems with these
interpretations of the most likely images. Anisotropy
can reveal the orientation of flow beneath volcanic
regions (Montagner). Credible images of low-velocity
bodies traversing the entire mantle as predicted for
hot upwellings from the core-mantle boundary are lacking.
Low-velocity bodies extending down to the transition
zone have been found beneath a few volcanic areas but
similar bodies also underlie long-extinct volcanic areas
such as Brazil and the Ontong-Java plateau (Foulger).
The interpretation of high- and low-velocity anomalies
is ambiguous. Melting, mineralogy, composition, anisotropy
and temperature variations can all explain seismic wave-speed
anomalies, with temperature having a relatively small
effect (Anderson). A serious problem in many
regions is scarcity of ray-path data resulting from
the uneven distribution of earthquakes and seismic stations,
especially in the Pacific ocean (Dziewonski).
This situation could improve if large numbers of ocean-bottom
seismometers are deployed (Montagner).
Modeling can explore whether convection
within the Earth is layered or whole-mantle and whether
thermal plumes are likely to form (Ihinger, King).
Early, simple investigations involving tanks of liquids
or numerical models have been influential in shaping
popular concepts of plumes. However, Earth is radically
different from a tank of liquid, and is not reliably
modeled using mathematical approximations such as ignoring
the temperature and pressure dependence of rheology,
thermal conductivity and the coefficient of thermal
expansion (Anderson). The most sophisticated
lower-mantle models to date predict few, vast, sluggish
upwellings in the lower mantle, not many localised plumes,
and these models still neglect critical factors such
as temperature-dependent viscosity (King).
The Earth is probably chemically layered as suggested,
for example, by dynamic topography, different grain
sizes in the upper and lower mantles and seismic velocities
in the transition zone (Anderson, Hofmeister).
It is generally assumed that “hot spots”
are hot. They are hot in the sense that volcanic activity
occurs at the surface, but the important point is the
relative potential temperature
of the mantle beneath. Three sessions
focused this critical issue. Marine heat flow measurements
provide little evidence for enhanced heat flow around
“hot spots”. Two spectacular examples of
this are Hawaii and Iceland where heat flow measurements
provide no evidence for elevated sub-lithospheric temperatures.
Such findings are typical of “hot spots”
(C. Stein).
Experimental studies consistently indicate
low solidus temperatures for mantle materials containing
likely amounts of water or carbon and suggest that the
seismic low-velocity zone beneath oceans may result
from incipient melt since volatiles lower the solidus
temperature in adiabatically ascending mantle material
(Green, Presnall). Thus, either mantle composition
or potential temperature is mapped by the low-velocity
zone. Subduction re-introduces materials more fusible
than peridotite such as recycled ocean crust into the
mantle (Green). The existence or otherwise
of picritic magmas in both MOR and “hot spot”
locations is a critical issue, as the temperatures of
primitive magmas are the most direct evidence of mantle
potential temperatures (Green, Presnall). Parental
picritic magmas were proposed in both Hawaiian and MOR
settings (N-MORB and E-MORB) leading to the inference
of similar mantle potential temperatures (Green).
Liquidus spinels and harzburgite-residue trends for
Hawaiian picrites indicate a more refractory (in major
elements) but refertilized (incompatible elements) source,
and more refractory residue, than MORB. The role of
picrites in MOR settings was challenged and parental
MOR magmas comprising olivine tholeiites at ~150°C
below the potential temperature of Hawaiian picrites
was advocated (Fitton, Gudfinnsson, Presnall).
“Hot spots” on or adjacent to ridges were
considered to be formed at low potential temperatures
comparable to those along “normal” ridges
(Presnall).
Continental
breakup and collision – the start
and end games of plate tectonics – de-homogenize
the shallow Earth. Continental collision traps slabs
in and beneath sutured subcontinental lithosphere, where
they provide a source of magma both during collision
and subsequent continental breakup (Chalot-Prat,
Cigolini, Peccerillo,Finn, Flower, Barry, Fekiacova,
Wilson). Young, thin slabs may be neutrally buoyant
in the shallow mantle. The great melt volume of the
North Atlantic Volcanic Province and Iceland may result
from mantle fertilised with subducted Iapteus crust
(Foulger). Intense magmatism at the onset of
continental breakup may be a result rather than a cause
of breakup (Anderson). The Central Atlantic
Magmatic Province (CAMP) reflects stress patterns related
to Pangaean rifting, and requires a widespread mantle
source and extraction of melt from different depths
(Mangas, McHone). It is best explained by distributed,
shallow, small-scale convection. The intense volcanism
associated with the breakup of the South Atlantic persisted
at some locations e.g., the Walvis and Rio Grande ridges,
where recent, detailed gravity data suggest control
by variable lithospheric stress. This may arise from
large-scale internal deformation and stress redistribution
within the African plate, resulting from changes in
its plate-tectonic boundary conditions (Wilson).
Transform zones (Beutel) and also normal ridges
(Bonatti, Chalot-Prat) are important both in
controlling stress and facilitating mantle melting and
the ascent of this melt to the surface.
Extraterrestrial
volcanism can potentially increase our
understanding of volcanism on Earth. Venus has no plate
tectonics, and its heat budget and thermal history are
poorly constrained (Smrekar, Stofan). Volcanism
may have been localized in part by distributed rifting
(Jurdy). The large (> 2,000 km) rimmed circular
structures may also be impact craters dating from the
main accretion of the planet, before 3.9 Ga (Hamilton).
Geochemistry,
including Sr-, Nd-, Pb-, and He- isotopes, cannot be
used to estimate a depth of origin or volume of mantle
sources. Connections have been made between He isotopes
and FOZO or C in oceanic island isotopic systematics
but there is no requirement that FOZO and other enriched
components (EM1, EM2, HIMU) must come from the deep
mantle (Natland, Peccerillo, Smith). Enriched
basalts are widely distributed over thousands of seamounts,
and are not confined to tops of islands in linear chains.
Plume sources cannot explain them all. Os isotopes have
been interpreted to indicate a core component but they
could reflect instead a metamorphic or pyroxenitic component
(Anderson, Smith, Walker). W-Hf isotopic systematics
may soon offer a means of testing whether a signal from
the core-mantle boundary ever reaches Earth's surface
(Anderson).
Field associations suggest a heterogeneous
upper mantle over which ridges migrate
(Bonatti, Dick, Natland). The upper mantle
appears to contain variably fertile to barren lherzolite
and harzburgite, on scales varying from hand-specimen-sized
domains to subducted slabs of partly altered, eclogitized
ocean crust (Natland). Also present is abyssal
peridotite, pyroxenite produced by reaction between
peridotite and migrating melts, and former terrigenous
and marine sediment carrying quartz, water, and carbonate.
Blocks of continental lithosphere, isolated during continental
rifting, lie in the middle of oceans. Subcontinental
mantle may be incorporated in basaltic magma generated
in new ocean basins (Natland, Smith). The scale
of heterogeneity produced at spreading ridges is re-introduced
into the mantle at subduction zones, with sedimentary
components added. Isotopic variability testifies that
melting domains beneath islands and ridges cannot be
represented by a single mantle lithology, but that source
materials that formerly interacted with the atmosphere
and hydrosphere are present.
Melting models based on homogeneous
peridotite that vary only the extent of partial melting
and mantle temperature are too simple. Iceland, for
example, is compositionally anomalous in so many respects
that an unusually fertile mantle source, perhaps including
substantial eclogite that was originally ocean crust,
may be required and be able to explain both the geochemistry
and the crustal thickness there (Foulger, Natland).
The interpretation of the Icelandic seismic crust in
terms of melt thickness is still unclear (Björnsson,
Foulger). The scarcity of evidence for excessive
mantle potential temperatures (Green, Gudfinnsson,
Presnall, C. Stein) encourages consideration of
source variability (Natland). Models that involve
fertile, fusible patches and lithospheric stress may
provide viable alternatives to localized high temperature
for many volcanic regions.
The ideal of truly rigid tectonic plates
breaks down in reality. Plates
move coherently but are not completely rigid,
and cracks and volcanic chains can form (Anderson,
Natland, Smith, Winterer). The Pacific plate may
be affected by changes in subduction geometry which
change plate-wide stress.
Can such stresses produce propagating lithospheric fractures
along which basaltic magma erupts? Mesozoic Pacific
volcanoes are widely scattered and do not form linear
chains. As continental collisions accompanied the close
of Tethys in the Eocene (Flower), the western
Pacific plate became largely bordered with subduction
zones and much subsequent seamount and island volcanism
on the plate has been linear, age progressive, and parallel.
The Emperor-Hawaiian bend does not record a substantial
change in direction of the Pacific plate, as may be
seen from the continuity of spreading patterns and transform
zones of the same age (Hamilton) but may represent
a change in the orientation of stress imposed on the
plate from its edges. Scattered Pacific magmatism could
be related to shear heating generated by the so-called
westward drift of the lithosphere (Doglioni).
Various aspects of volcanism in the
Pacific appear to require new models. Linear
chains, especially where not time-progressive,
suggest tears in the plate or eruption along pre-existing
structures (Geist, Harpp, O'Connor). The most
credible non-thermal model for the Emperor-Hawaiian
chain is a propagating crack related to stress in the
plate. Much plateau formation was associated with triple
junctions (Smith). Finite-element modeling
of ridge-transform junctions reveals an intrinsic pattern
of extensional stress that encourages volcanism (Beutel).
In the south-west Pacific, the small magma volumes,
lack of rifting and uplift, and broad areal distribution
could be explained by episodic plate reorganization
and recycling of metasomatized lithosphere from subducted
slabs (Finn).
The Ontong Java plateau, Shatsky Rise,
Kerguelen plateau, Deccan Traps and Bushveld complex
are Large Igneous Provinces
(LIPs) that represent volumes of magma so huge and eruption
rates so rapid that they are difficult to explain by
any process. The uniformity and major element geochemistry
of Ontong Java plateau basalts was cited as supporting
a plume origin and ruling out the involvement of eclogite
(Fitton). The lack of evidence for precursory
uplift, and the existence of magnetic stripes across
the plateau remain unsolved problems in this model,
as is the lack of evidence for fertility in models based
on source heterogeneity. Sublithospheric ponding of
magmas prior to eruption of LIPs may be required to
explain the huge volumes, eruption rates and compositional
homogeneity (Anderson, Foulger).
The Shatsky Rise is shown by magnetic
stripes to have formed at a migrating triple junction
(Sager). The Kerguelen plateau contains a continental
crustal component and had a long duration of emplacement
compared with some other LIPs (Pringle). A
model presented for the Deccan Traps proposed a source
in recycled eclogite trapped in ancient sutures. Evidence
for high temperatures is lacking in the petrology and
there was no precursory uplift (Sheth), features
that are shared by the continental Columbia River Basalts,
which formed in a back-arc environment (Christiansen).
A multiple bolide impact
origin for the Bushveld Complex is supported by evidence
for ultra-high temperature debris flows and intense
deformation there (Elston). The absence of
LIPs at the ends of many linear volcanic chains e.g.,
the Emperor chain, and the lack of linear chains emanating
from many LIPs, e.g., the Ontong Java plateau, brings
into question the traditional “plume head –
plume tail” model. The classic example of a plume
head-tail, the Deccan Traps – Chagos – Laccadive
Ridge – Reunion Island, was questioned, and the
Chagos – Laccadive Ridge ascribed to melting and
melt focusing along a southward propagating fracture
in the Indian plate (Sheth).
An exciting diversity of ideas and
concepts was presented at the conference, along with
a healthy infusion of skepticism and challenges. In
volcanic regions where evidence for a thermal origin
is lacking, source fertility, volatiles, recycling of
subducted slabs or continental lithosphere, intraplate
deformation along faults, rifts and sutures, stress
variations and bolide impacts are promising avenues
to consider. These models require re-evaluation of other
aspects of our planet, such as the interpretation of
seismic anomalies, convection, the longevity of shallow
heterogeneities, the importance of lithospheric stress
and structure, the origin of geochemical tracers, the
fate of subducted slabs and the melt-retention capabilities
of the mantle. Critical data such as seismic measurements
from the oceans, heat flow, radiometric dates and petrological
laboratory data are required. Methodologies such as
geothermometry, and thermodynamic modeling of mantle
convection are still too primitive to answer the critical
questions. Much work remains to be done.
This Penrose conference brought together
for the first time scientists who still seek to understand
the fundamental origins of volcanic regions. The full
range of ambient ideas in this embryonic field was laid
out, brainstormed, criticised and challenged. The problems,
needs and tasks ahead were brought into focus. We are
at the start of a long and exciting journey.
Participants were:
Don Anderson, Tiffany Barry, Erin Beutel, Axel Bjornsson,
Enrico Bonatti, Francoise Chalot-Prat, Richard Chamberlin,
Bob Christiansen, Corrado Cigolini, Marc Davies, Henry
Dick, Carlo Doglioni, Adam Dziewonski, Wolfgang Elston,
Zuzana Fekiacova, Carol Finn, Godfrey Fitton, Martin
Flower, Gillian Foulger, Bjarni Gautason, Dennis Geist,
David Green, Gudmundur Gudfinnsson, Giuseppe Guzzetta,
Warren Hamilton, Karen Harpp, Anne Hofmeister, Dorthe
Holm, Fredrik Holm, Gregory Huffman, Phillip Ihinger,
Sveinn Jakobsson, Leonard Johnson, Bruce Julian, Donna
Jurdy, Scott King, Vlad Manea, Marina Manea, Jose Mangas,
Greg McHone, Jean-Paul Montagner, James Natland, John
O'Connor, Angelo Peccerillo, Emma Perez-Chacon, Brian
Pope, Malcolm Pringle, Dean Presnall, Will Sager, Hetu
Sheth, Olgeir Sigmarsson, Alan Smith, Suzanne Smrekar,
Carol Stein, Seth Stein, Ellen Stofan, Richard Walker,
Phil Wannamaker, Dayanthie Weeraratne, Marjorie Wilson,
Jerry Winterer and Don Wright.
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