Transcript Frontiers Powerpoint slides - Durham University Community

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.
• 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:
• excess
volcanism
• “hot spots”
fixed relative to
one-another
• linear island
chains
Morgan (1971)
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?
Modeling LIP volumes
Cordery et al. (1997)
“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 a GIF decompressor are 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 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?
http://www.mantleplumes.org/