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TESTING THE PLUME HYPOTHESIS

TESTING THE PLUME HYPOTHESIS. Ian Campbell The Australian National University. Testable Predictions of the Plume Hypothesis. New plumes consist of a large head followed by a small tail. Parana at 120 Ma. Characteristics of Flood Basalts. Equidimentional, typically 2000-2500 km

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TESTING THE PLUME HYPOTHESIS

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  1. TESTING THE PLUME HYPOTHESIS Ian Campbell The Australian National University

  2. Testable Predictions of the Plume Hypothesis • New plumes consist of a large head followed by a small tail

  3. Parana at 120 Ma

  4. 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

  5. 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

  6. 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

  7. Depth (km)

  8. Testable Predictions of the Plume Hypothesis • Plumes must originate from a hot boundary layer – the core-mantle boundary

  9. Seismic tomography (Montelli et al.)

  10. Testable Predictions of the Plume Hypothesis • Flatten plume heads should be 2,000 to 2,500 km in diameter

  11. Testable Predictions of the Plume Hypothesis • The hottest part of the head is at the centre and the temperature tapers towards the margin

  12. 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

  13. Both heads and tails should erupt high temperature picrites • Karroo • Deccan-Reunion • Parana • Emeishan • Caribbean • Hawaii • North Atlantic-Iceland

  14. Oahu Cross Section Basalts Picrites 0 50 100 km

  15. 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

  16. Iso-thickness contour of the Maokou Fm

  17. Biostratigraphic correlation of the Maokou Fm

  18. 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????

  19. 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

  20. Headless Plumes • A recent study by Farnetani of thermo-compositional 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)

  21. ISOSURFACE 140C Zoom on one plume

  22. The head-tail structure? No, only 'hot fingers'

  23. 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

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