Transcript No plume under Iceland

No Plume Beneath Iceland
talk given at the Colorado School of Mines, 2nd March 2006
Gillian R. Foulger
Durham University, U.K.
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Evidence in support of a plume
beneath Iceland
1.
2.
3.
4.
5.
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History of magmatism
Uplift
High temperatures
Crustal structure
Mantle structure
1. History of magmatism
DISKO
FAROES &
E GREENLAND
ODP
158
BRITISH
PROVINCE
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61-59 Ma
54 Ma
Jones (2005)
1. History of
magmatism:
Iceland
• Formed over the last
54 Million years
• Thick crust
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2. Uplift
400-900 m
420-620 m
380-590 m
180-425 m
0-100 m
500-800 m
0 - 200 m
0-200 m
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Jones (2005)
2. Uplift
• Uplift rapid
• Approached
1 km in some
places
400-900 m
420-620 m
380-590 m
180-425 m
0-100 m
500-800 m
0 - 200 m
0-200 m
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Jones (2005)
3. High-temperatures
Arndt (2005)
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~ 100 K temperature
anomaly for Iceland
relative to MORB
4. Crustal structure
Foulger et al. (2003)
Crustal structure from receiver functions
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5. Mantle structure
Whole-mantle
tomography:
A plume from
the core-mantle
boundary.
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Bijwaard & Spakman (1999)
The Iceland plume?
A slam dunk!
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Let us look in detail, to find out
more about what the Iceland
plume is like.
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Seismological studies of Iceland
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Foulger et al. (2003)
Crustal structure
Foulger et al. (2003)
• Variations in crustal thickness should be parallel to
spreading direction
• Crust should be thickest in the west, behind the plume
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Crustal
structure
The melting anomaly
has always been
centred on the
mid-Atlantic ridge
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Iceland: Mantle tomography
• Over 2,000,000 data
– S-wave arrival times (S,
SS, SSS, ScS & SKS)
– fundamental- & highermode Rayleigh-wave
phase velocities
– normal-mode frequencies
• Probably best spherical
harmonic model for the
transition zone & midmantle
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Ritsema et al. (1999)
Whole-mantle tomography
Hudson Bay plume?
Bijwaard & Spakman (1999)
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Transition zone discontinuities
Predicted topography
on the 410-km and
650-km discontinuities
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Du et al. (2006)
Transition zone discontinuities
• 410 warps down by
15 km
• 650 flat
• No evidence for
anomalous structure
or physical
conditions at 650 km
beneath Iceland
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Du et al. (2006)
Temperature
Can be investigated using:
•
•
•
•
•
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Petrology
Seismology
Modeling bathymetry
Modeling vertical motion
Heat flow
Petrological temperature
Arndt (2005)
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~ 100 K temperature
anomaly for Iceland
relative to MORB
Petrological temperature
Hawaii 1570˚
?
Iceland?
MORs 1280-1400˚
Gudfinnsson et al. (2003)
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Temperature: Seismology
Vs
Vertical
scale
x 10
DT ~ 200˚C
DT ~ 100˚C
Ritsema & Montagner (2003)
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Iceland
Vertical scale x 1
Temperature: Iceland
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Foulger et al. (2005)
Uplift: Magnitude & Duration
• 61 Ma uplift associated with British igneous
activity variable, low amplitude (few 100
m) & localised.
• 54 Ma uplift associated with igneous
activity distant from proposed plume, high
amplitude (up to 1 km) & widespread.
• Time between onset and peak uplift for both
igneous phases probably << 1 Myr.
• Uplift history complex & not satisfactorily
explained by any single published model.
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1. History of magmatism
DISKO
FAROES &
E GREENLAND
ODP
158
BRITISH
PROVINCE
25
61-59 Ma
54 Ma
Jones (2005)
Summary
• Variations in crustal thickness inconsistent with
plume predictions
• Mantle anomaly confined to upper mantle
• No reliable evidence for plume-like
temperatures
• Uplift history complex and not well explained
• Distribution of magmatism inconsistent with
plume predictions
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An alternative model
Plate tectonic processes (“PLATE”)
• Two elements:
– Variable source fertility
– Extensional stress
A cool, shallow, top-driven model
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PLATE: Lithospheric extension
• Mid-ocean ridges
(1/3 of all “hot
spots”)
• Many others intraplate
extensional areas
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PLATE: Variable mantle fertility
• Possible sources:
– recycling of subducted slabs in upper mantle
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Peacock (2000)
PLATE: Variable mantle fertility
• Possible sources:
– delamination of continental lithosphere
QuickTime™ and a GIF decompressor are needed to see this picture.
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Schott et al. (2000)
The liquidus & solidus of subducted
crust are lower than peridotite
• Subducted crust
transforms to eclogite
at depth
• Eclogite is extensively
molten at the peridotite
solidus
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Pyrolite
Eclogite
Cordery et al. (1997)
Geochemistry of “hot spot” lavas
• Can be modeled as
fractional melting of
MORB
• Ocean Island Basalt
(OIB) comes from
recycled near-surface
materials e.g.,
subducted oceanic
crust
Hofmann & White (1982)
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Iceland
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Iceland: Extension
Jones (2005)
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Iceland has been persistently centred on the
mid-Atlantic ridge
Iceland: Mantle fertility
• Relationship to the
Caledonian suture
• Recycled Iapetus crust
in source?
• Can remelting of
Iapetus slabs account
for the excess melt,
geochemistry &
petrology?
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Closure
of
Iapetus
Melt fraction : Temperature
Yaxley (2000)
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A 30/70 eclogite-peridotite mixture can generate
several times as much melt as peridotite
Geochemical evidence for crustal
recycling
• Recent papers: Korenaga & Keleman (2000);
Breddam (2002); Chauvel & Hemond (2000)
• Estimated primary mantle melt from Iceland, E &
SE Greenland shows source mantle enriched in
Fe; Mg# is as low as 0.87
• Heterogeneity suggests MORB mantle also
involved
• Sr-Nd-Hf-Pb isotopes & dO18 suggest recycling of
subducted, aged oceanic crust, ± sub-arc
magmatism, ± sediments
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Iceland: REE patterns
Foulger et al. (2005)
Iceland REE can be modeled by extensive melting of
subducted crust + small amount of alkali olivine basalt
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The alternative hypothesis is...
• Iceland is a “normal” part of the MAR
where excess melt is produced from
remelting Iapetus slabs
• However, the amount of melt produced by
isentropic upwelling of eclogite cannot at
present be calculated
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Tectonics & crustal structure
Iceland is also a region of local, persistent
tectonic instability
Foulger et al. (2003)
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Iceland: Tectonic evolution
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Foulger (in press)
Iceland: Tectonic evolution
Foulger (2002)
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Crustal structure
The thickspot beneath Iceland may
be a submerged oceanic microplate
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Iceland: The mantle anomaly
• Can be explained by 0.1% partial melt
– a more fusible mantle composition
– CO2 fluxing
• Could simply be a place where the lowvelocity zone is thicker
Iceland
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Summary
1. Superficially, several
observations are consistent
with plume theory
2. Closer examination virtually
never fulfills the predictions of
plume theory
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Summary
3. 2 approaches:
1. adapt plume theory to fit
2. accept that plume theory fails and boldly go
where no man has gone before
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Resources:
http://www.mantleplumes.org/
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That’s all folks
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