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

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.

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

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.

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

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

Are “hot spots” hot? What does “hot” mean? K is the minimum required for a plume How hot are “hot spots”?

Example: mantle potential temperature, Iceland

Can plumes explain the melt volumes observed?

Cordery et al. (1997) Modeling LIP volumes

“Hot spots” are not fixed

Hawaii relative to Atlantic “hot spots”

Seismology does not reliably detect them in the lower mantle

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

But what other theories are there?

Plate Tectonic Processes lithospheric extension mantle heterogeneity = variable magmatic fecundity

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

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

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

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

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

Other theories

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

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

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

EDGE convection e.g., Tristan

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

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?