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Do Plumes Exist?

Durham University GEOL 4061 Frontiers of Earth Science. Do Plumes Exist?. Gillian R. Foulger. What is a plume?. A plume is a bottom-heated convective upwelling that rises through its own thermal buoyancy.

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Do Plumes Exist?

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  1. Durham University GEOL 4061 Frontiers of Earth Science Do Plumes Exist? Gillian R. Foulger

  2. 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.

  3. 1971: Plumes were invented to explain: • excess volcanism • “hot spots” fixed relative to one-another • linear island chains Morgan (1971)

  4. 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.

  5. 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

  6. An unfalsifiable hypothesis However, study of melting anomaly origins has not progressed because of “plume belief”

  7. Are “hot spots” hot? What does “hot” mean? 200 - 300 K is the minimum required for a plume How hot are “hot spots”?

  8. Example: mantle potential temperature, Iceland

  9. Can plumes explain the melt volumes observed?

  10. Modeling LIP volumes Cordery et al. (1997)

  11. “Hot spots” are not fixed

  12. “Hot spots” are not fixed Hawaii relative to Atlantic “hot spots”

  13. Seismology does not reliably detect them in the lower mantle

  14. Example:whole-mantle tomography: Iceland Ritsema et al. 1999

  15. But what other theories are there?

  16. Plate Tectonic Processes • lithospheric extension • mantle heterogeneity = variable magmatic fecundity

  17. PTP: Lithospheric extension • Intraplate deformation • Mid-ocean ridges (1/3 of all “hot spots”)

  18. PTP: Mantle heterogeneity • Possible sources: • recycling of subducted slabs in upper mantle Peacock (2000)

  19. PTP: Mantle heterogeneity • Possible sources: • delamination of continental lithosphere Bertram Schott et al. (2000)

  20. Melt fraction : Temperature A 30/70 eclogite-peridotite mixture can generate several times as much melt as peridotite Yaxley (2000)

  21. 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

  22. Other theories

  23. Plate-boundary junctions Extensional stresses occur at RT and RRR intersections and can permit volcanism e.g., Amsterdam/St. Paul, Easter

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

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

  26. EDGE convection e.g., Tristan

  27. 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

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

  29. http://www.mantleplumes.org/

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