No Plume Beneath Iceland
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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. History of magmatism
2. Uplift
3. High temperatures
4. Crustal structure
5. Mantle structure
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1. History of magmatism
ODP158
DISKO
BRITISHPROVINCE
FAROES &E GREENLAND
61-59 Ma 54 MaJones (2005)
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1. History ofmagmatism:Iceland
• Formed over the last 54 Million years
• Thick crust
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2. Uplift
0-200 m0 - 200 m
500-800 m
400-900 m 420-620 m
180-425 m
0-100 m
380-590 m
Jones (2005)
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2. Uplift
0-200 m0 - 200 m
500-800 m
400-900 m 420-620 m
180-425 m
0-100 m
380-590 m
• Uplift rapid• Approached
1 km in some places
Jones (2005)
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3. High-temperatures
~ 100 K temperature anomaly for Iceland relative to MORBArndt (2005)
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4. Crustal structure
Crustal structure from receiver functionsFoulger et al. (2003)
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5. Mantle structure
Bijwaard & Spakman (1999)
Whole-mantle tomography: A plume from the core-mantle boundary.
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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
Foulger et al. (2003)
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Crustal structure
• Variations in crustal thickness should be parallel to spreading direction
• Crust should be thickest in the west, behind the plume
Foulger et al. (2003)
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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- & higher-mode Rayleigh-wave phase velocities
– normal-mode frequencies
• Probably best spherical harmonic model for the transition zone & mid-mantle
Ritsema et al. (1999)
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Whole-mantle tomography
Bijwaard & Spakman (1999)
Hudson Bay plume?
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Transition zone discontinuities
Predicted topography on the 410-km and
650-km discontinuitiesDu et al. (2006)
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Transition zone discontinuities
• 410 warps down by 15 km
• 650 flat
• No evidence for anomalous structure or physical conditions at 650 km beneath Iceland
Du et al. (2006)
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Temperature
Can be investigated using:
• Petrology • Seismology• Modeling bathymetry• Modeling vertical motion• Heat flow
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Petrological temperature
~ 100 K temperature anomaly for Iceland relative to MORBArndt (2005)
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6810121416
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4246505458
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MgO (wt%)
MgO (wt%)
MgO (wt%)
MgO (wt%)
MORBIcelandic basalt glassesReykjanes Peninsulaand TheistareykirKistufell (Breddam 2002)Puna Ridge(Clague et al. 1995)Gudfinnsson et al. (2003)
Hawaii 1570˚
MORs 1280-1400˚
6810121416
46810121416
4246505458
46810121416
8101214161820
46810121416
68101214161820
46810121416
MgO (wt%)
MgO (wt%)
MgO (wt%)
MgO (wt%)
MORBIcelandic basalt glassesReykjanes Peninsulaand TheistareykirKistufell (Breddam 2002)Puna Ridge(Clague et al. 1995)Petrological temperature
Iceland??
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Temperature: Seismology
Iceland
Ritsema & Montagner (2003)
T ~ 200˚C
T ~ 100˚C
Vertical scalex 10
Vertical scale x 1
Vs
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Temperature: Iceland
Foulger et al. (2005)
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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
ODP158
DISKO
BRITISHPROVINCE
FAROES &E GREENLAND
61-59 Ma 54 MaJones (2005)
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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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• Mid-ocean ridges (1/3 of all “hot spots”)
• Many others intraplate extensional areas
PLATE: Lithospheric extension
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29 Peacock (2000)
PLATE: Variable mantle fertility
• Possible sources:– recycling of subducted slabs in upper mantle
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QuickTime™ and aGIF decompressorare needed to see this picture.
Schott et al. (2000)
PLATE: Variable mantle fertility
• Possible sources:– delamination of continental lithosphere
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31Cordery et al. (1997)
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
Pyrolite
Eclogite
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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)
Iceland has been persistently centred on the mid-Atlantic ridge
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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?
Closure of
Iapetus
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Melt fraction : Temperature
A 30/70 eclogite-peridotite mixture can generate several times as much melt as peridotite
Yaxley (2000)
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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 & O18 suggest recycling of subducted, aged oceanic crust, ± sub-arc magmatism, ± sediments
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Iceland: REE patterns
Iceland REE can be modeled by extensive melting of subducted crust + small amount of alkali olivine basalt
Foulger et al. (2005)
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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
Foulger et al. (2003)
Iceland is also a region of local, persistent tectonic instability
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Iceland: Tectonic evolution
Foulger (in press)
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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 low-velocity 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