Post on 01-Jan-2016
description
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”?
Plate Tectonic Processes
• lithospheric extension
• mantle heterogeneity
= variable magmatic fecundity
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
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
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?