Are the predictions of the plume hypothesis borne out by observation? 1.Temperature
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Are the predictions of the plume hypothesis borne out by observation?
1.Temperature
Natalie Starkey
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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
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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
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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.
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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.
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• 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
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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.
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• 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
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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?
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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?