the role of the core in mantle plumes graham smith university of durham dept. of earth sciences
DESCRIPTION
Why a CMB Origin? Size of some large igneous provinces requires an origin in the lower mantle or the CMB Mechanism of heat removal from the CMB area Large Igneous ProvincesEarth’s Heat BudgetEarth’s Geodynamo Modified from Condie (2003) CRETACEOUS SUPERCHRON 50-75% increase in rate of crustal production “Superplume” events correlate well with patterns of magnetic reversalTRANSCRIPT
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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?
Size of some large igneous provinces requires an origin in the lower mantle or the CMBMechanism of heat removal from the CMB area
Large Igneous ProvincesEarth’s Heat BudgetEarth’s Geodynamo
Modified from Condie (2003)
CRETACEOUS SUPERCHRON
50-75% increase in rate of crustal production
“Superplume” events correlate well with patterns of magnetic reversal
www.creaso.com
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Outer core enriched in Os by 300x mantle values
High 187Os/188Os and 186Os/188Os
OSMIUM“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 MantleFrom Brandon et al. 1999.
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OSMIUMCRUSTAL CONTAMINATION
e.g. Koolau lavas, Hawaii
High 187Os/188Os
Low 186Os/188Os
High d18O
High Sr Low Nd
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HELIUMCore enriched in primordial He during accretion High 3He/4He ratios
MORB = 8±1 RaModified from van Keken et al. (2002)
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FOZO: A common theme
Geochemical signatures all point to a common component
OIBs comprise a variety of different isotopic endmembers
Material rising from the CMB incorporates deep mantle material
But, FOZO is poor in Os
FOZO
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Ultra-Low Velocity Zones
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ImplicationsOsmium systematics suggest incorporation of outer core material at CMBPresence of recycled ancient crust suggests that slabs sink to CMB
Less degassed lower mantle separated from degassed upper mantle points to a double-layer mantle system.
Primordial 3He points strongly to a CMB origin for plumes
Courtesy of University of Hawaii, Institute for Astronomy
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