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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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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˚

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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)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

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

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