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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. Evidence in support of a plume beneath Iceland. History of magmatism Uplift High temperatures Crustal structure Mantle structure. DISKO. FAROES & E GREENLAND.

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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.


Evidence in support of a plume beneath Iceland

  • History of magmatism

  • Uplift

  • High temperatures

  • Crustal structure

  • Mantle structure


DISKO

FAROES &

E GREENLAND

ODP

158

BRITISH

PROVINCE

1. History of magmatism

Jones (2005)

61-59 Ma

54 Ma


Formed over the last 54 Million years

Thick crust

1. History ofmagmatism:Iceland


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

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

Jones (2005)


3. High-temperatures

~ 100 K temperature anomaly for Iceland relative to MORB

Arndt (2005)


4. Crustal structure

Foulger et al. (2003)

Crustal structure from receiver functions


5. Mantle structure

Whole-mantle tomography: A plume from the core-mantle boundary.

Bijwaard & Spakman (1999)


The Iceland plume?

A slam dunk!


Let us look in detail, to find out more about what the Iceland plume is like.


Seismological studies of Iceland

Foulger et al. (2003)


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)


Crustal structure

The melting anomaly has always been centred on the

mid-Atlantic ridge


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)


Hudson Bay plume?

Whole-mantle tomography

Bijwaard & Spakman (1999)


Transition zone discontinuities

Predicted topography on the 410-km and 650-km discontinuities

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

Du et al. (2006)


Temperature

Can be investigated using:

  • Petrology

  • Seismology

  • Modeling bathymetry

  • Modeling vertical motion

  • Heat flow


Petrological temperature

~ 100 K temperature anomaly for Iceland relative to MORB

Arndt (2005)


?

Iceland?

Petrological temperature

Hawaii 1570˚

MORs 1280-1400˚

Gudfinnsson et al. (2003)


Temperature: Seismology

Vs

Vertical scale

x 10

DT ~ 200˚C

DT ~ 100˚C

Ritsema & Montagner (2003)

Vertical scale x 1

Iceland


Temperature: Iceland

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.


DISKO

FAROES &

E GREENLAND

ODP

158

BRITISH

PROVINCE

1. History of magmatism

Jones (2005)

61-59 Ma

54 Ma


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


An alternative model

Plate tectonic processes (“PLATE”)

  • Two elements:

    • Variable source fertility

    • Extensional stress

      A cool, shallow, top-driven model


Mid-ocean ridges (1/3 of all “hot spots”)

Many others intraplate extensional areas

PLATE: Lithospheric extension


PLATE: Variable mantle fertility

  • Possible sources:

    • recycling of subducted slabs in upper mantle

Peacock (2000)


PLATE: Variable mantle fertility

  • Possible sources:

    • delamination of continental lithosphere

Schott et al. (2000)


Pyrolite

Eclogite

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

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)


Iceland


Iceland: Extension

Jones (2005)

Iceland has been persistently centred on the mid-Atlantic ridge


Relationship to the Caledonian suture

Recycled Iapetus crust in source?

Can remelting of Iapetus slabs account for the excess melt, geochemistry & petrology?

Iceland: Mantle fertility

Closure of Iapetus


Melt fraction : Temperature

Yaxley (2000)

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


Iceland: REE patterns

Foulger et al. (2005)

Iceland REE can be modeled by extensive melting of subducted crust + small amount of alkali olivine basalt


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


Foulger et al. (2003)

Tectonics & crustal structure

Iceland is also a region of local, persistent tectonic instability


Iceland: Tectonic evolution

Foulger (in press)


Iceland: Tectonic evolution

Foulger (2002)


Crustal structure

The thickspot beneath Iceland may be a submerged oceanic microplate


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


Summary

  • Superficially, several observations are consistent with plume theory

  • Closer examination virtually never fulfills the predictions of plume theory


Summary

  • 2 approaches:

    • adapt plume theory to fit

    • accept that plume theory fails and boldly go where no man has gone before


Resources:

http://www.mantleplumes.org/


That’s all folks


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