Transcript What is the role of the core in plumes?

The Role of the Core in
Mantle Plumes
Graham Smith
University of Durham
Dept. of Earth Sciences
How is the Core Involved?
Source of HEAT
Origin of plumes at the CoreMantle Boundary (CMB)?
from Ritsema et al. 1999.
Why a CMB Origin?
Large
Earth’s
Earth’sIgneous
Geodynamo
Heat Budget
Provinces
“Superplume”
events
correlate
well requires
Size
of some large
igneous
provinces
with
patterns
oflower
magnetic
reversal
Mechanism
removal
from
theCMB
CMB area
an
origin
in of
theheat
mantle
or the
50-75% increase in rate
of crustal production
CRETACEOUS
SUPERCHRON
Modified from Condie (2003)
www.creaso.com
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
Why Not the Upper Mantle?
PUM = Primitive Upper Mantle
From Brandon et al. 1999.
OSMIUM
CRUSTAL CONTAMINATION
e.g. Koolau lavas, Hawaii
High 187Os/188Os
Low 186Os/188Os
High d18O
High Sr Low Nd
HELIUM
Core enriched in primordial He during accretion
Modified from van Keken et al. (2002)
High 3He/4He ratios
MORB = 8±1 Ra
FOZO: A common theme
OIBs comprise a
variety of different
isotopic endmembers
Geochemical
signatures all point to a
common component
FOZO
Material rising from the
CMB incorporates deep
mantle material
But, FOZO is poor in Os
Ultra-Low Velocity Zones
Implications
Osmium systematics suggest incorporation of outer core material at CMB
Presence
of recycled
ancient
crust to
suggests
that slabs
sink to CMB
3He points
Primordial
strongly
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
References
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