Download Isotopic Evolucon of the Earth (II)

Isotopic
Evolu-on
of
the
Earth
(II)
Early (> 4.4 Ga) differentiation of the Earth
Mantle
is
residue
a8er
extrac-on
of
other
reservoirs
Highly
incompa-ble
elements
enriched
in
CC,
depleted
in
upper
mantle
Mass
balance:
CC+UM+LB=BSE=chondrite
(non‐vola-le,
lithophile)
Later (< 4.4 Ga) differentiation of the Earth
Formation of oceanic crust by melting of upper mantle (creating
refractory residue)
Subduction and mixing of oceanic crust and sediment into mantle
Extraction of continental crust from mantle at subduction zones
P/D ratios fractionated  variable isotope compositions of mantle and crustal rocks  rates and timing of these processes
Con-nental
crust
forma-on
Highly
incompa-ble
elements
enriched
in
CC,
depleted
in
upper
mantle
Mass
balance:
CC+UM+LB=BSE=chondrite
(non‐vola-le,
lithophile)
Composi-on
of
upper
mantle
cannot
be
representa-ve
of
whole
mantle
Chemistry
of
the
mantle
‐
role
of
c.
crust
forma-on
CC
and
upper
mantle
have
very
different
Rb/Sr
(Rb
more
incompa-ble)
Since
much
of
the
con-nental
crust
older
than
2
Ga,
CC
and
upper
mantle
now
have
very
different
87Sr/86Sr
Chemistry
of
the
mantle
‐
role
of
c.
crust
forma-on
CC
and
upper
mantle
have
very
different
Sm/Nd
(Nd
more
incompa-ble)
Since
much
of
the
con-nental
crust
older
than
2
Ga,
CC
and
upper
mantle
now
have
very
different
143Nd/144Nd
Sm/Nd
ra-o
generally
not
frac-onated
by
crustal
processes
(metamorphism,
weathering,
etc)
Nd
model
age
gives
-me
of
extrac-on
of
crustal
sample
from
mantle
Shales
o8en
used
to
es-mate
‘mean
age’
of
crust
Mean
age
of
con-nental
crust
is
2.4‐2.6
Ga
Timing
of
con-nental
crust
forma-on
Growth
of
the
con-nental
crust
Condie
&
Aster
(2009)
Episodic
forma-on
of
the
con-nental
crust?
Compila-on
of
37839
zircon
analyses
Con-nental
crust
forma-on
at
subduc-on
zones?
Should
be
semi‐con-nuous
process
Although
possibly
faster
at
earlier
-mes
when
Earth
ho`er
Zircons
not
common
in
andesi-c/basal-c
subduc-on
zone
lavas
Zircon
peaks
probably
reflect
crustal
reworking
events
Is
depleted
upper
mantle
the
complement
to
the
CC?
We
can
es-mate
Sr
concentra-on
and
87Sr/86Sr
ra-o
of
both
reservoirs
From
mass
of
CC,
can
calculate
mass
of
mantle
from
which
CC
extracted
Result:
~30‐40%
mass
of
mantle
(~mass
of
upper
mantle
above
670
km)
Chemical
composi-on
of
the
deeper
mantle
The
shallow
upper
mantle
which
melts
to
form
MORB
at
MORs
cannot
be
representa-ve
of
the
whole
mantle
The
deeper
mantle
must
be
less
depleted
in
incompa-ble
trace
elements,
and
have
higher
87Sr/86Sr,
lower
143Nd/144Nd
than
upper
mantle
Since
4.56
Ga,
about
1‐2
whole
mantle
mass
has
been
processed
through
MOR
mel-ng
region
So
en-re
mantle
may
consist
of
oceanic
crust
+
mel-ng
residues,
OR
a
smaller
mass
of
mantle
(e.g.
upper
mantle)
may
have
been
several
-mes
In
la`er
case,
the
deeper
mantle
may
have
a
‘primi-ve’
composi-on
(not
affected
by
con-nental
or
oceanic
crust
forma-on/subduc-on)
Is
there
a
‘primi-ve’
reservoir
in
the
Earth?
Source
of
ocean
island
basalts
apparently
less
depleted
than
that
of
MORB
Mean
Sr,
Nd
composi-ons
similar
to
those
expected
for
chondri-c
reservoir
But:
large
sca`er,
and
trace
element
composi-ons
not
chondri-c
Therefore
OIB
do
not
come
from
a
‘primi-ve’
reservoir
in
the
mantle
Recycled
materials
in
the
Earth’s
mantle
The
Helium
–
heat
imbalance
Most
radiogenic
heat
produc-on
within
Earth
from
U,
Th
decay
Produc-on
of
heat
&
He
should
be
coupled
In
fact
only
5%
of
He
flux
predicted
from
radiogenic
heat
produc-on
in
the
mantle
is
degassed
at
MORs
?Boundary
layer
within
Earth
permeable
to
heat
but
not
He
The
‘missing’
argon
problem
If
K
content
Earth
known,
40Ar
produced
since
4.5
Ga
can
be
calculated
40Ar
produced
in
mantle
released
into
atmosphere
at
volcanoes
Only
~50%
40Ar
produced
since
4.5
Ga
is
located
in
the
atmosphere
Rest
may
be
in
isolated
lower
mantle?
The
lead
paradox
If
bulk
Earth
is
chondri-c,
average
composi-on
must
lie
on
Geochron
Most
accessible
Earth
reservoirs
have
Pb
isotope
composi-ons
which
lie
to
radiogenic
side
of
the
Geochron
?Hidden
reservoir
with
unradiogenic
Pb
(core,
or
core‐mantle
boundary)
Or
oceanic
basalts
do
not
faithfully
record
source
(mantle)
composi-on