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

MELTexperiment

EPR

Mantle dehomogenising

• ridges• subduction zones• eclogitisation of

subducted crust

Eclogite is fusible

Pyrolite

Eclogite

Yaxley (2000)

A 30/70 eclogite-peridotite mixture can generate several

times as much melt as peridotite

Mantle dehomogenising

• metasomatism of oceanic and continental mantle lithosphere

• delamination of thickened lithosphere, including lower crust

• erosion of continental lithosphere during breakup

Pilet et al. (2005)

Cantal basalts model, Massif Central, France

Mantle dehomogenising

• metasomatism of oceanic and continental mantle lithosphere

• delamination of thickened lithosphere, including lower crust

• erosion of continental lithosphere during breakup

Schott et al. (2000)

QuickTime™ and aGIF decompressorare needed to see this picture.

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 ofthe 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 motionLourenço 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

Iceland

Ritsema & Montagner (2003)

T ~ 200˚C

T ~ 60˚C

Example:whole-mantle tomography:

Iceland

Ritsema et al. 1999

Please read our book:Plates, Plumes & Paradigms

Iceland: A plume from the core-mantle boundary?

Bijwaard & Spakman (1999)Hudson Bay plume?

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)