Are the predictions of the plume hypothesis borne out by observation?

1.Temperature

Natalie Starkey

What is the plume hypothesis?• Proposed by Morgan (1971).

Hot upwellings of relatively primordial material which rise from the deep mantle originating from a ‘thermal boundary layer’ and feed ‘hotspots’.

• Thought to rise from the D’’ layer at the CMB.• Concept of a plume allows for the return flow of material to the

surface relative to subducted slabs.

Numerical simulation of mantle plumes.

Red=hot upwellings. Blue=cold downwellings

Keifer and Kellogg, 1998

How hot is the mantle?

• Opposing views on mantle temperatures

• Plume geologists estimate: Tp = 1280 +/- 20oC - McKenzie & Bickle (1988)

• Geophysical estimate:

Tp = 1400 +/- 200oC - Anderson & Bass (1984)

• >100oC difference between estimates

Measuring hotspot temperatures

• Petrology: studies of melt products of experimental phase equilibria

• Crustal Thickness: McKenzie & Bickle estimates

• Heat Flow: which is then to compared to data from thermal models of heat flow for cooling of oceanic crust in other regions.

• Bathymetry & Subsidence rates: Hot mantle = slower subsidence

• Seismic Velocities: slower velocities in hot mantle. Affected greatly by partial melt.

De Paolo and Manga, Science, 2003

P-wave velocities under Hawaii: red=slow, blue=fast. Red and yellow regions suggest anomalously high temperature.

How hot are hotspots?• At ‘normal’ mantle temperatures peridotite cannot produce melt

in the correct volumes or rate of that seen at hotspots/flood basalt regions, Cordery et al. (1997).

• High mantle temperatures are therefore required to produce the magmatism (Tp in excess of 1600oC).

•Griffiths and Campbell (1990) - cold head, hot tail. •Farnetani and Richards (1995) - hot head, cold tail.

• Fitton & Godard - Tp >1500oC for the Ontong Java Plateau

• Nisbet et al. (1993)- Tp = 1600oC for the Galapagos starting plume

• Gill et al. (1992) - Tp = 1550-1600oC for Iceland starting plume

These values lie within the geophysical range so may be regarded as ‘normal’ mantle temperatures!

• However, Thompson & Gibson (2000)- Tp = 1700oC for parts of the Tristan starting-plume head.

Starting-plume head temperatures

Present hotspot temperatures• Shen et al. (1998) and others

- Tp = 1450-1600oC for Hawaiian and Icelandic plumes- 150-300oC above global asthenospheric MORB- source

upper mantle.

• Korenaga & Kelemen (2000)- Tp for lavas of the North Atlantic Igneous Province and normal

mid-ocean ridge are very similar. - Variance of <70oC

• Ribe et al. (1995) - Temperature excesses of <70oC predicted for the Azores,

Galapagos and Iceland.

• No significant anomaly in heatflow for Hawaii as heatflow is found to be similar to that expected for lithosphere of the same age elsewhere.

• Petrological models predict pyrolite mantle.

• ‘Pyrolite melting’ models of Cordery et al. (1997) only produced melt when the hottest plume impacted the youngest (thinnest) lithosphere.

• Ambient mantle ~1300oC, excess magmatism at hotspots can be explained by localised hotter upwelling jets of material.

• Ambient mantle = 1400oC +/- 200oC then excess magmatism at hotspots is not caused by temperature excesses as observed temperatures lie within this range.

So … How is hotspot/flood basaltmagmatism explained?

Models

What is the source for hotspots?• Flood basalts have higher FeO for a given SiO2 than MORB.• Require a more basaltic source! Subducted ocean crust =

eclogitic.

•Eclogitic component lowers solidus - enhancing melt productivity. (More melt produced at lower temperatures).

•How is this melt entrained?

•Are plumes homogenous or hetereogeneous?

Summary• Disagreement over ambient mantle ‘normal’ temperature.

• 1300 +/- 20oC OR 1400 +/- 200oC.

• Disagreement over how to measure plume temperatures.

• Disagreement over source composition.

• Hotspots are ‘hot’ when compared to mantle of 1300oC but are not ‘hot’ when compared to 1400oC mantle.

• Which model of ‘normal’ mantle temperatures is correct?