The%20Role%20of%20the%20Core%20in%20Mantle%20Plumes

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The Role of the Core in Mantle Plumes

• Graham Smith
• University of Durham
• Dept. of Earth Sciences

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How is the Core Involved?
Source of
HEAT
Origin of plumes at the Core- Mantle Boundary
(CMB)?
from Ritsema et al. 1999.
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Why a CMB Origin?
Large Igneous Provinces
Earths Heat Budget
Earths Geodynamo
Superplume events correlate well with patterns
of magnetic reversal
Size of some large igneous provinces requires an
origin in the lower mantle or the CMB
Mechanism of heat removal from the CMB area
50-75 increase in rate of crustal production
CRETACEOUS SUPERCHRON
Modified from Condie (2003)
www.creaso.com
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OSMIUM
Outer core enriched in Os by 300x mantle values
High 187Os/188Os and 186Os/188Os
Plume-related basalts carry this signal
Addition of 1wt. outer core material
BUT, High Os ratios can also be generated in the
crust
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Why Not the Upper Mantle?
PUM Primitive Upper Mantle
From Brandon et al. 1999.
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OSMIUM
CRUSTAL CONTAMINATION
e.g. Koolau lavas, Hawaii
High 187Os/188Os
Low 186Os/188Os High d18O High Sr Low Nd
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HELIUM
Core enriched in primordial He during accretion
High 3He/4He ratios
MORB 81 Ra
Modified from van Keken et al. (2002)
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FOZO A common theme
OIBs comprise a variety of different isotopic
endmembers
Geochemical signatures all point to a common
component
Material rising from the CMB incorporates deep
mantle material
FOZO
But, FOZO is poor in Os
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Ultra-Low Velocity Zones
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Implications
Osmium systematics suggest incorporation of outer
core material at CMB
Presence of recycled ancient crust suggests that
slabs sink to CMB
Primordial 3He points strongly to a CMB origin
for plumes
Less degassed lower mantle separated from
degassed upper mantle points to a double-layer
mantle system.
Courtesy of University of Hawaii, Institute for
Astronomy
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References
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(2003) Plate Tectonics and Crustal Evolution, 4th
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(2001). Nature 412 527-29. Gurnis et al. (1998).
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