Do Plumes Exist?

Gillian R. Foulger

Durham UniversityGEOL 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?

200 - 300 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

“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

QuickTime™ and aGIF decompressorare needed to see this picture.

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 10 - 30 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 seminars1. 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. Temperature5. Are the predictions of the plume hypothesis borne out by

observation? 2. Uplift6. 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?

http://www.mantleplumes.org/