Download Mantle plumes do not exist

The Generation of Melting
Anomalies by Plate Tectonic
Processes
Gillian R. Foulger
University of Durham
.... based on ideas developed over several years by a
working group that includes ....
Don Anderson, Warren Hamilton, Jerry Winterer,
Jim Natland, Dean Presnall, Peter Vogt, Anders
Meibom, Hetu Sheth, Seth Stein, Mike O’Hara &
Alan Smith .... etc
for more see www.mantleplumes.org
Two key elements:
1. Variations in lithosphere stress
2. Mantle inhomogeneity
•
•
Simply put
Stress governs location of volcanism
Fusibility governs volume of magma
Mantle dehomogenising
• ridges
MELT
experiment
EPR
Mantle dehomogenising
• ridges
• subduction zones
• eclogitisation of
subducted crust
Eclogite is fusible
Pyrolite
A 30/70 eclogite-peridotite
mixture can generate several
times as much melt as
peridotite
Eclogite
Yaxley (2000)
Mantle dehomogenising
• metasomatism of
oceanic and
continental mantle
lithosphere
• delamination of
thickened lithosphere,
including lower crust
• erosion of continental
lithosphere during
breakup
Cantal basalts model, Massif Central,
France
Pilet et al. (2005)
Mantle dehomogenising
• metasomatism of
oceanic and
continental mantle
lithosphere
• delamination of
thickened lithosphere,
including lower crust
• erosion of continental
lithosphere during
breakup
QuickTime™ and a GIF decompressor are needed to see this picture.
Schott et al. (2000)
Mantle dehomogenising
• metasomatism of
oceanic and
continental mantle
lithosphere
• delamination of
thickened lithosphere,
including lower crust
• erosion of continental
lithosphere during
breakup
Mantle dehomogenising
adapted from Meibom & Anderson (2003)
Variations in stress
• Lithosphere cooling
• Spatial and temporal
variations in plate
boundary type &
tectonics
• Variations in
lithosphere strength
Variations in stress
• Lithosphere cooling
• Spatial and temporal
variations in plate
boundary type &
tectonics
• Variations in
lithosphere strength
From Natland, 2004
Variations in stress
• Lithosphere cooling
• Spatial and temporal
variations in plate
boundary type &
tectonics
• Variations in
lithosphere strength
adapted from Lundin & Doré (2005)
Proposal
“Hot spot” volcanism occurs where
– stress is extensional
– mantle is highly fusible
Examples
“Hot spots” on MORs
• 1/3 of all “hot spots” are on or near MORs
East African Rift
• Afar
• Other EAR “hotspots”?
Basin & Range Province
• Broad, intraplate
extensional
region
• Associated with
subduction of
“Farallon slab”
• Widespread
volcanism
NAVP & Iceland
• Formed when
continent rifted along
Iapetus suture
• Diverse data suggest
not hot
• Recycled Iapetus crust
can explain
geochemistry & melt
volume
Closure of
the Iapetus
Azores
–
–
–
–
Kinematic models
EQ focal mechanisms
bathymetry
suggest Azores branch is:
– oblique, ultra-slow
spreading (3-4 mm/yr)
– diffuse plate boundary
– dextral differential
shear motion
Lourenço et al. (1998)
Time-progressive volcanism
Predictions
• Melt volumes can be explained by
lithosphere extension + source fertility
• Vertical motions related to shallow tectonic
processes
• Upper mantle is inhomogeneous
• Migration of volcanism = migration of
locus of extension
Predictions
• Seismic tomography anomalies indicate
composition and melt, not only temperature
• “Hot spot” lavas not required to be hot
• Geochemistry can be explained by
inhomogeneities in the shallow mantle
That’s all folks!
Example: mantle potential
temperature, Iceland
Temperature (Tp) from petrology
• mid-ocean ridges: ~1280 - 1400˚C
• Iceland:
~1280 - 1460˚C
• Hawaii:
~1560˚C
The only place on Earth hot enough for a weak
upper-mantle plume is Hawaii
Seismology does not reliably
detect them in the lower mantle
Vertical exaggeration x 10
DT ~ 200˚C
DT ~ 60˚C
Iceland
Ritsema & Montagner (2003)
Example:
whole-mantle
tomography:
Iceland
Ritsema et al. 1999
Please read our book:
Plates, Plumes & Paradigms
Iceland: A plume from the
core-mantle boundary?
Hudson Bay plume?
Bijwaard & Spakman (1999)
Resolution of the “whole-mantle plume”
The data used by
Bijwaard &
Spakman have no
resolution in the
lower mantle
(courtesy of Karason & van
der Hilst)
From Foulger et al. (2001)