Lecture 46

Report
The Earth II:
The Core; Mantle
Reservoirs
Lecture 46
Composition of the Core
• In the case of the Earth’s core, we have only two
types of constraints:
• Geophysical:
o density and seismic velocity derived from seismology and moment of
inertia. Also must generate a geomagnetic field.
o Density suggests a material 5-10% less dense than Fe.
• Cosmochemical:
o What materials of appropriate density are available in sufficient
abundance to constitute 1/3 the mass of the Earth?
o Iron meteorites provide a compositional model of the core.
o Again we turn to a chondritic model: we infer that siderophile elements
missing from the silicate Earth are in the core.
o For refractory siderophile elements, they should be in chondritic relative
proportions.
o For non-refractory siderophiles, the volatility trend provides a means of
estimating composition.
Volatility Trend
Composition of the Core
Understanding Core Formation
• Metal/silicate partition
coefficients depend on
pressure and oxygen
fugacity.
• Today, the core–mantle
boundary, is at 135 GPa
and 3000–4000 K.
• Experiments suggest
metal silicate
equilibration took place
at lower pressure (as in
planetesimals, there were
the building blocks of
Earth).
Mantle Geochemical
Reservoirs & Evolution
Mantle Reservoirs
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•
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We previously looked at the
composition of the silicate Earth
(BSE). This composition is also known
as ‘primitive mantle’ (mantle after
core segregation, but before crust
formation).
In reality, the mantle is processed,
heterogeneous, and no known
sample of mantle matches exactly
the ‘primitive mantle’ composition.
Isotope ratios of basalts (particularly
oceanic ones) provide views of the
time-integrated composition of
their sources.
o
•
Basalts are useful because they are common and
because their production involved larger regions
(>100 km3) of mantle. Elemental compositions are
changed during melting, but isotope ratios are not.
Isotope ratios shows a fundamental
two-fold division of basalts: MORB
and OIB.
Sr-Nd Mantle Array
Nd-Pb Isotope Systematics
MORB & the Depleted Mantle
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•
•
•
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Seafloor spreading creates 3 km2 new
area of ocean floor each year (an
equal area is subducted) and ~20 km3 of
mid-ocean ridge basalt (MORB) forms to
fill the gap. They are the most
voluminous magmas on the planet.
Compare to others, they have uniform
tholeiitic (richer in Si, poorer in alkalis
than alkali basalt) and are relatively
poor in compatible elements.
They have low 87Sr/86Sr and 206Pb/204Pb
and high εNd and εHf ratios implying low
time-integrated Rb/Sr, U/Pb, Nd/Sm, and
Hf/Lu ratios - that is low values of ratios of
more-to-less incompatible element
ratios.
They provide evidence of a voluminous
(incompatible element-) depleted
upper mantle (DUM) or DMM (depleted
MORB mantle).
The origin of the this DUM is most readily
explained by removal of an
(incompatible element-rich) melt that
has formed the continental crust.
How Much DUM?
•
•
•
•
•
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Suppose we consider the Earth as
consisting of three reservoirs:
o
o
o
Primitive mantle
Continental crust
Depleted Mantle
o
We don’t necessarily know the εNd of continental crust, but
we do know its Sm/Nd ratio and can guess at its age.
We write a series of mass balance
equations that allow us to solve for the
fractional mass of depleted mantle,
assuming we know the εNd and Nd
concentrations of the other 2 reservoirs
and their masses.
We can solve for the relative mass of
depleted mantle.
Likely answer is ~30% if BSE εNd = 0
(chondritic) but 40-100% if the Earth has
εNd = 3-7, consistent with collisional
erosion.
Bottom line: at a minimum, melt has been
extracted from a lot (~30%) or perhaps
most of the mantle to form the
continental crust.
If substantial volumes of primitive mantle
remain, we see little direct evidence of it.
OIB Reservoirs
• The OIB are
more diverse.
• They can be
divided into 4
main groups:
o
o
o
o
St. Helena (HIMU)
Kerguelen (EM I)
Society (EM II)
Hawaii (PREMA)
• This suggests
several distinct
(chemical)
evolutionary
pathways.
Primitive Mantle
• Convergence of OIB
arrays at Zindler & Hart’s
PREMA (prevalent
mantle) together with the
observations that the
highest 3He/4He ratios
occur in basalts with εNd
of 3-7 suggests primitive
mantle might be a
background component
of many OIB sources,
provided primitive mantle
has εNd of 3-7 as collisional
erosion (or the EER
hypothesis) predicts.
Mantle Plumes
•
•
•
Oceanic island volcanoes
(e.g., Hawaii, Iceland, Azores)
are widely (but not
universally) thought to be
products of mantle plumes columns of hot (but solid)
rock rising convectively from
the deep mantle (perhaps
from the core-mantle
boundary driven by heat
from the core).
Although still a bit
controversial, seismic
evidence is increasingly
consistent with this.
Thus OIB sample deep mantle
reservoirs (reservoir(s) could
be small: D’’ a candidate).
Evolution of OIB reservoirs
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•
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Many OIB have 87Sr/86Sr greater
and εNd lower than BSE. This
requires something other than
melt extraction.
Incompatible element pattern
consistent with melt
enrichment.
Although plumes come from
the deep mantle, incompatible
element patterns suggest
upper mantle processes (deep
mantle melts have very
different incompatible element
patterns).
Thus although they come from
the deep mantle, their
chemistry bears the signature of
upper mantle processing.
Slope on 207Pb/204Pb-206Pb/204Pb
plots suggest heterogeneity is
ancient, but not as old as the
Earth itself.
Extended rare earth or “spider”
diagram, in which the BSE-normalized
abundances are plots and elements
are ordered by incompatiblility.
Mantle Plumes from Ancient
Oceanic Crust
• Hofmann and White (1982)
proposed that the distinct
composition of OIB sources
(mantle plumes) comes
from oceanic crust
(+continent derived
sediment) subducted into
the deep mantle.
• This material is heated (by
the core) and eventually
rises to the surface as
mantle plumes (finally
melting in the upper most
100-200 km).

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