Plumes, hotspots and

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Plumes, hotspots and hot fingers The nature of
convective instabilities in the Earths mantle
Marjorie Wilson School of Earth Sciences Leeds
University LEEDS LS2 9JT, UK
MPRG
Warsaw (5) 6-9 October 2003
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The original Plume Concept Jason Morgan (1972)
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The Nature of Mantle convection
Issues for debate

• Whole mantle versus two -layer convection
• Super-plumes
• Mantle hot fingers
• Seismic tomography
• Numerical modelling
• Geoid anomalies

Is red i.e low velocity hot ?
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Why do we need to have mantle plumes?
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What is the nature of the controversy?

• Do mantle plumes exist?
• Do all volcanoes not associated with plate
  boundaries require a deep mantle plume.?

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• How does the Earths mantle convect?
• Single layer or multi-layer?

• Do mantle plumes exist as a discrete form
• of mantle convection?

• If plumes exist what is their excess potential
• temperature?
• Do they have a distinct geochemical fingerprint?

• Can we tell the difference between upper and
• lower mantle plumes?

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The nature of mantle convection
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East European Platform
LATE DEVONIAN PLUME CLUSTER Frasnian-Fammenian
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Is there mass /heat exchange with the Lower
Mantle?
Or an oversimplification?

• Seismic tomography
• He isotopes
• Sr-Nd-Pb-Hf isotopes

Louise Kellogs Stealth Layer model
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Has the style of mantle convection changed
through time?
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MANTLE COMPOSITION

• Heterogeneities introduced by plate subduction
  plus delamination of continental lithospheric
  roots
• Redox state of the mantle?
• Role of CO2 v H2O in mantle melting
• Efficiency of melt extraction

• HIMU, EMI, EM II, DM, FOZO
• Plum pudding v layer cake?
• Scale lengths of heterogeneities
• Does recycled oceanic crust remain distinct for
  how long?

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COMPOSITIONAL HETEROGENEITY IN THE MANTLE
143Nd/144Nd
He isotopes
What should we read into this?
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Scale lengths of mantle convection
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SUPERSWELLS

• Regions of the lower mantle beneath the African
  Pacific plates characterised by low seismic wave
  velocities are inferred to be broad, hot
  upwellings secondary,weaker plumes are inferred
  to form in the upper mantle above these
  superswells.

Courtillot et al. 2003, EPSL
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Mantle hot fingers
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Local seismic tomography experiments have
identified upper mantle plumes.hot fingers
UK
EUROPE
Ritter et al (2001)
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Links between hot fingers mantle dynamics
Alps
Etna
Eifel Massif Central plumes identified by local
seismic tomography experiments
Granet et al. (1995) EPSL
EAR
Ritter et al. (2001)
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Why does the mantle melt?
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Thermal gradients in the mantle

• Nature of the asthenosphere
• Where does the
• adiabatic part of the geotherm start?
• Where are the main thermal boundary layers in the
  Earth?
• What can heat flow tell us?
• Mantle Potential Temperature

Related Issues Which mantle solidus do we
use? Wet ? CO2? Dry?
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Evidence for a deep mantle origin for some
hotspots

• mid-ocean ridges migrate over hotspots without
  changing the hotspot track
• The high magma production rate at Hawaii requires
  an upwelling velocity of 50cm/year i.e. 10 x the
  average plate velocity

• This implies that the hotspot source is deeper
  than 200 km
• The Hawaiian upwelling must, therefore, be
  distinct from the flow associated with plate
  motions

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• The upwelling mantle under Hawaii must be 200-300
  K hotter than the surrounding mantle to result in
  relatively high degree melting below 80 km thick
  lithosphere
• The chemistry isotopic composition of many
  hotspot lavas indicate that they sample regions
  of the mantle distinct from the source of MORB
• Numerical simulations of mantle convection
  produce plume-like structures that can account
  for many of the geophysical observations

• The upwelling mantle must, therefore, come from a
  thermal boundary layer in the mantle e.g. the
  core-mantle boundary
• High 3He/4He suggests the involvement of a
  primitive mantle source component
• (? FOZO)
• E.g. the rate of magma productiontopographic
  gravity anomalies

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Criteria for recognising plumes

• The presence of a hotspot track and an associated
  flood basalt province
• A large buoyancy flux (the product of the volume
  flux through the plume and the density difference
  between the plume and its surroundings)
• A high 3He/4He ratio
• A monotonic age progression of seamounts and
  volcanic islands along the hotspot track

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Possible lower mantle plumes
Courtillot et al. (2003) EPSL 205
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Large Igneous Provinces -LIPS
CFBs Oceanic Plateaus

• Might they represent a periodic/different mode of
  mantle convection?
• Where is the main magma source (i.e lithosphere
  or the asthenosphere)
• How much mantle is processed during the melting
  event?
• Mantle lithosphere is a non-renewable source
• Are LIPS really short-lived?

Etendeka province, Namibia ca 135 Ma Photograph
Sally Gibson
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Causes of Continental Break-up

• Are mantle plumes
• needed to break-up super-continents?
• Thermal insulation
• of super-continents

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Penrose ConferenceHveragerdi, Iceland - 25-29
August 2003

• Plume IV
• Beyond the plume Hypothesis

Convenors Gillian Foulger, Don Anderson Jim
Natland Web site http//www.mantleplumes.org
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Aims of the meeting

• To test the plume paradigm and to evaluate
  alternative hypotheses
• To define what we mean by the term plume
• To question our knowledge and assumptions about
  the nature of mantle convection and the
  composition of the Earths mantle
• To evaluate the driving forces of plate tectonics

Emphasis on objective evaluation of data a
multi-disciplinary approach
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PLUME IV Did we reach a consensus?

• Probably not..BUT
• General agreement that the Earths mantle is
  compositionally heterogeneous, on a variety of
  length scales, due to plate recycling
• Far-field stresses and lithospheric architecture
  are important in magma generation processes
• Numerical and analogue modelling may have led us
  astray
• Seismic Tomography images should be interpreted
  with caution (RED does not always mean HOT)
• Geochemists, geologists and geophysicists must
  work together
• Geological data are IMPORTANT - they give a time
  perspective