TESTING THE PLUME HYPOTHESIS

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TESTING THE PLUME HYPOTHESIS

• Ian Campbell
• The Australian National University

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Testable Predictions of the Plume Hypothesis

• New plumes consist of a large head followed by a
  small tail

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Parana at 120 Ma
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Characteristics of Flood Basalts

• Equidimentional, typically 2000-2500 km
• Preceded by uplift
• Large volumes of magma
• Short eruption times, main phase 1 Myr
• Rapid contraction of volcanism to narrow chain of
  volcanoes to current position of plume

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Testable Predictions of the Plume Hypothesis

• Plume tails (upper mantle) should be about
  100-300 km across and have higher temperature
  that the adjacent mantle
• However, plume theory does not predict the
  temperature of plumes. This must be obtained
  from observation which suggests a temperature
  excess of 200 to 300 oC

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Diameter of Plume Tail

• Decreases with DT
• Increases with plume flux
• For DT 200-300 oC and buoyancy flux 104-105
  N/s, D 100-300 km

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Depth (km)
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Testable Predictions of the Plume Hypothesis

• Plumes must originate from a hot boundary layer
  the core-mantle boundary

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Seismic tomography (Montelli et al.)
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Testable Predictions of the Plume Hypothesis

• Flatten plume heads should be 2,000 to 2,500 km
  in diameter

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Testable Predictions of the Plume Hypothesis

• The hottest part of the head is at the centre and
  the temperature tapers towards the margin

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Testable Predictions of the Plume Hypothesis

• Both heads and tails should erupt high
  temperature picrites
• However picrites are dense magmas that often fail
  to reach the surface

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Both heads and tails should erupt high
temperature picrites

• Karroo
• Deccan-Reunion
• Parana
• Emeishan
• Caribbean
• Hawaii
• North Atlantic-Iceland

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Oahu Cross Section
Basalts
Picrites
0 50 100 km
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Testable Predictions of the Plume Hypothesis

• Flood volcanism should be preceded by 500 to 1000
  m of uplift
• Uplift should be dome shaped and be greatest at
  the centre, tapering towards the margins
• Plume hypothesis does not predict time-scale for
  uplift or volcanism both of which are controlled
  by the viscosity at the top of the upper mantle

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Iso-thickness contour of the Maokou Fm
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Biostratigraphic correlation of the Maokou Fm
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Other Examples of Uplift Preceding Volcanism

• Natkusiak, in northwest Canada
• 520 Ma Antrim River flood-basalt in the northwest
  of Western Australia
• Ethiopia
• North Atlantic Igneous Province
• Deccan Traps
• Siberian Traps????

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The plume hypothesis does not predict the
chemistry of plume basalts

• Plumes sample whatever is at the CMB at the time
• The expectation is that it will be mainly
  basalt-rich mantle because basalt is dense
  component in the mantle
• However observations show that mantle at CMB can
  be also depleted mantle

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Headless Plumes

• A recent study by Farnetani of thermo-compositiona
  l plumes suggests that the heads of weak plumes
  cannot penetrate the 670 km discontinuity
• However, the light component can separate from
  the dense component and form a new plume that
  originates from 670 km
• The new plume has a small head because it rises
  only 500 km (D 200 km)

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ISOSURFACE 140C
Zoom on one plume
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Testable Predictions of the Plume Hypothesis

• Picrites should be most abundant near the centre
  of the plume head (flood basalt) and less
  abundant towards the margin