Download Lecture 45 - Earth and Atmospheric Sciences

The Earth I:
The Mantle
Lecture 45
Structure of the Earth
h
(km)
Atmosphere
Hydrosphere
V
1012
km3
Mean
ρ
kg/m3
Mass
1024 kg
Mass
%
0.000005
0.00009
3.80
0.00137
1026
0.00141
0.024
Crust
17
0.0087
2750
0.024
0.4
Mantle
2883
0.899
4476
4.018
67.3
Core
3471
0.177
10915
1.932
32.3
Whole
Earth
6371
1.083
5515
5.974
100.00
The Mantle
• Most of the mass of the Earth
consists of its mantle, so we’ll
begin with that.
• We have three kinds of
constraints on its composition:
o Geophysical: including seismic
velocities, density, and moment of
inertia (the latter constrains density
distribution)
o Cosmochemical: the Earth must have a
composition that can plausibly be
derived from the nebular raw material
(i.e., chondritic)
o Geochemical: direct analyses of
mantle samples (xenoliiths, alpine
peridotites); Indirect: basalts produced
by melting of the mantle.
• Experimental mineral physics
helps us translate composition
into mineralogy and physical
properties.
•
The composition that meets
these constraints is peridotite
o
•
Xenoliths in basalts are
dominantly peridotite,
(specifically lherzolite)
o
•
by definition a rock composed of 50% or
more of olivine
Many are harzburgitic, an Ol-Opx-rock
which can be derived from by melting
out a basaltic component.
Peridotite has appropriate
seismic velocities and
densities and can be derived
from an approximately
chondritic composition by
remove of metal (which forms
the core).
Peridotite
Phase Changes
In the upper mantle, the aluminous
phase changes from plagioclase to
spinel to garnet
Silicates go from tetrahedral to
octahedral structure at 660 km.
Lower Mantle
•
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The transition to octahedral
structure results in a sharp
increase in seismic velocity at
660 km.
In the deepest mantle,
another significant phase
change occurs, which may
partly account for the seismic
properties of the lowermost
~200 km, called D’’.
At present, seismic data are
consistent with a radially
uniform mantle composition
with variation in seismic
velocity resulting from selfcompression and phase
changes.
o
Compositional zonation, however, cannot
be entirely ruled out.
In the lower mantle, the dominant
mineral is Mg-perovskite: MgSiO3
with the structure of CaTiO3. Next
most abundant mineral is Mg,FeO
(magnesiowüstite or ferripericlase)
Estimating Earth’s Composition
• A goal of geochemistry
since at least Goldschmidt
and F. W. Clark has been to
determine the composition
of the Earth.
• Since the mantle is most of
the Earth, this effort focuses
on the compositions of
peridotites.
o
These, however, have been
variously modified from a ‘primitive
mantle’ composition, by addition or
removal of melts.
• Hart & Zindler estimated
Mg/Si and Al/Si ratios by
taking the intersection of
the peridotite trend with the
chondrite trend.
Estimating Earth’s Composition
• Palme and O’Neill (2003)
began by adopting an
FeO concentration of
8.1% (the least modified
peridotites have a
MgO/(MgO+FeO) molar
ratio (Mg#) of 0.89. Using
that they calculated an
MgO concentration for
the mantle of 36.8%.
• They tend deduced
CaO, Al2O3, and SiO2
from the intersection of
the peridotite trend with
MgO = 36.8%.
Composition of the Silicate Earth
Estimating Trace Element Concentrations
• Lithophile elements with 50% condensation temperatures
above that of Mg (1340K) are generally present in constant
proportions in chondritic meteorites.
o
Among major elements, the refractory lithophile elements (RLE’s) include Ca, Al, and
Ti.
• Lithophile trace element concentrations can be estimated
from the chondritic ratios of these elements (e.g., Nd/Al).
• Non-refractory and non-lithophile element concentrations can
be estimated from their ratios to RLE’s or from isotope ratios.
o
o
For example, the K/U ratio in both the mantle and crust is about 13,000. Knowing U,
we can estimate K. Also, the 40Ar in the atmosphere provides a minimum estimate of
K in the Earth as 120 ppm.
The Rb/Sr ratio can be estimated from 87Sr/86Sr ratios and the intersection of the
mantle array with εNd = 0 (assuming the Sm/Nd ratio of the Earth is chondritic).
• Using this approach, concentrations of most elements in the
periodic table can be build up.
o
Estimating ‘atmophile’ element concentrations such as H, N, C, and the rare gases is
more problematic, although for some of these elements, most of the inventory is in
the atmosphere.
• Following convention, we’ll call this composition Bulk Silicate
Earth (BSE) or Primitive Mantle.
The
142Nd
Conundrum
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The non-chondritic 142Nd/144Nd of
the Earth presents a challenge to
the conventional approach to
estimating Earth’s composition
based on the chondritic RLE
assumption.
The Earth’s 142Nd/144Nd ratio is
~0.2 epsilon units (20 ppm) higher
than chondritic. Given the short
half-life of 146Sm (68 Ma), this
implies a Sm/Nd ~6% greater
than chondritic (only ~3%
variation among all chondrites).
This opens the possibility that the
RLE’s are not present in the Earth
in chondritic proportions.
o
•
Sm and Nd are extremely similar chemically. If
their ratio varies this much, other ratios might
vary even more.
Three possible explanations have
been proposed.
Nebular Isotopic Heterogeneity
•
•
•
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Small variations in the isotopic
compositions of chondrites
have been observed and
can be related to incomplete
mixing of s- and r+p process
nuclides.
142Nd is s-process, 144Nd is sand r-process. 146Sm is p-only.
Incomplete mixing of red
giant or supernova debris
could cause either or both
the 142Nd/144Nd or 146Sm/144Nd
ratio to vary.
But Earth plots of the mixing
line. Also, recent studies with
complete sample dissolutions
suggests chondrites are
homogeneous.
Early Enriched Reservoir
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Boyet and Carlson suggested
that as the Earth’s magma
ocean cooled, an
incompatible elementenriched crust formed - the
Early Enriched Reservoir analogous to lunar highlands,
but not plag enriched). This
eventually became unstable,
sank the the base of the
mantle, where it has
remained ever since.
The rest of the Earth, including
the crust and all recognized
mantle reservoirs have
differentiated from the
complimentary Early
Depleted Reservoir.
EER, a.k.a. D’’
Collisional Erosion
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•
Caro et al. (2008) as well as
others, have suggested that as
planets evolve from the
planetesimal size to planet size,
collisions are energetic enough
to partially melt the bodies,
producing a magma ocean
that crystallizes an incompatible
element-enriched crust
(essentially the same idea as
Boyet and Carlson).
Collisions are also energetic
enough to intermittently blast
away a fraction of this crust and
the incompatible elements in it.
At the end of the process, the
planet is depleted in
incompatible trace elements (is
not a significant effect for major
elements like Si).
Do Geoneutrinos have the
Answer?
•
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Still debated, but the
consequences are important.
Among the highly incompatible
elements are K, U, and Th radioactive elements that
produce the energy to drive
geologic processes.
In the collisional erosion
hypothesis, K, U, and Th could be
~40% lower than the ‘standard
model’. In the EER hypothesis, this
amount of K, U, and Th is
concentrated in the deep mantle.
One way to try to resolve this is to
measure the amount of neutrinos
being produced by radioactive
decay.
o
A very small fraction of these neutrinos induce
the so-called inverse beta reaction, creating a
neutron and positron from a proton. The
positron can then be detected when it is
annihilated..
Results so far: Ambiguous