Do Plumes Exist?

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Do Plumes Exist?
Durham University GEOL 4061 Frontiers of Earth
Science

• Gillian R. Foulger

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What is a plume?

• A plume is a bottom-heated convective upwelling
  that rises through its own thermal buoyancy.
• Plumes almost certainly must rise from a thermal
  boundary layer, i.e., from material that lies
  just above a hot body.

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1971 Plumes were invented to explain

• excess volcanism
• hot spots fixed relative to one-another
• linear island chains

Morgan (1971)
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Later the plume-head, plume-tail model developed

• Griffiths Campbell (1990)
• Plumes created by injecting syrup/water mix (to
  be less dense) into the tank.

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Problems

• There is little evidence that hot spots are hot
• Some have very small melt volumes
• They are not fixed relative to one-another
• Many chains not time-progressive
• Seismology does not reliably detect them in the
  lower mantle

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An unfalsifiable hypothesis

• However, study of melting anomaly origins has
  not progressed because of plume belief

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Are hot spots hot?

• What does hot mean?
• 200 - 300 K is the minimum required for a plume
• How hot are hot spots?

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Example mantle potential temperature, Iceland
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Can plumes explain the melt volumes observed?
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Modeling LIP volumes
Cordery et al. (1997)
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Hot spots are not fixed
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Hot spots are not fixed

• Hawaii relative to Atlantic hot spots

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Seismology does not reliably detect them in the
lower mantle
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Examplewhole-mantle tomography Iceland
Ritsema et al. 1999
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But what other theories are there?
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Plate Tectonic Processes

• lithospheric extension
• mantle heterogeneity
• variable magmatic fecundity

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PTP Lithospheric extension

• Intraplate deformation
• Mid-ocean ridges (1/3 of all hot spots)

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PTP Mantle heterogeneity

• Possible sources
• recycling of subducted slabs in upper mantle

Peacock (2000)
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PTP Mantle heterogeneity

• Possible sources
• delamination of continental lithosphere

Bertram Schott et al. (2000)
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Melt fraction Temperature
A 30/70 eclogite-peridotite mixture can generate
several times as much melt as peridotite
Yaxley (2000)
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PTP model Iceland

• Geochemistry indicates recycled Iapetus crust in
  source
• Eclogite more fertile than peridotite
• Geochemistry melt volume could come from
  recycled Iapetus slabs

Closure of Iapetus
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Other theories
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Plate-boundary junctions

• Extensional stresses occur at RT and RRR
  intersections and can permit volcanism
• e.g., Amsterdam/St. Paul, Easter

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Meteorite impacts

• Recent modeling suggests that meteorites 10 - 30
  km in diameter could form LIPs
• e.g., Bushveldt, Ontong Java

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Lithospheric delamination

• Overthickening of the crust causes
  eclogitisation, delamination and triggers LIP
  volcanism
• e.g., Siberian Traps

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EDGE convection
e.g., Tristan
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Current problems

• Origin of excess melt
• source consistent with geochemistry
• energy budget to melt large volumes must either
• accumulate melt over long period of time and
  retain in the mantle, or
• melt very rapidly - a melt-as-erupted basis
• Hawaii

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Student seminars

1. What is a plume?
2. Are plumes predicted by realistic convection
   experiments and numerical simulations?
3. What is the origin of ocean island basalt (OIB)?
4. Are the predictions of the plume hypothesis borne
   out by observation? 1. Temperature
5. Are the predictions of the plume hypothesis borne
   out by observation? 2. Uplift
6. What is the origin of high 3He/4He?
7. Have plumes been detected seismologically?
8. What alternatives are there to the plume
   hypothesis?
9. Can the plume hypothesis be tested, and if so
   how?
10. How can the Plate Tectonic Processes theory be
    tested?

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http//www.mantleplumes.org/