Download Differentiation of the Earth

Differentiation of the Eath
Sujoy Mukhopadhyay
Harvard University
CIDER 2008
What we will cover in this lecture
„
„
Short lived nuclides
Differentiation of the Earth (Core, crust, and
atmospheric formation)
Planet Formation in its simplest form
The great temperature differences between the hot inner regions and the
cool outer regions of the nebula determined what kinds of condensates
were available to form planets.
Near Mercury’s orbit, metal started to condense. Moving outwards to Venus
and Earth, more rock condensed
Only beyond the frost line, which lay between the present-day orbits of
Mars and Jupiter, were temperatures low enough for hydrogen compounds
to condense into ices.
4567.2±0.6 Ma (Amelin et al., 2002)
What is the age of the Earth
Chondrules -> 4564.7±0.6 Ma (Amelin et al., 2002)
Eucrites (achondrites) ~4560 Ma
So did the Earth also form within a few (ten) million years after the start of the solar
system?
Extinct radionuclides and early Earth history
(182Hf Æ 182W, 146Sm Æ 142Nd, 129I Æ 129Xe
Age equation for a long-lived radioactive element
 Pr 
 Dr 
 Dr 
 
+   (e λt − 1)
=  
 D s today  D s initial  D s today
Measurable quantities
For an extinct radionuclide we have
 Dr 
 Dr 
 Pr 
 
=  
+  
 D s today  D s initial  D s initial
But Pr does not exist anymore…..
 Dr 
 Dr 
 Pr 
 Ps 
 
 
=  
+  
 D s today  D s initial  Ps initial  D s 
Pr is radioactive parent
Dr is daughter produced from
radioactive decay of Pr
Ds is stable isotope of Dr and
not produced by radioactive
decay
Ps stable isotope of Pr
λ is the decay constant
Extinct radionuclides: 182Hf Æ 182W
 Dr

 Ds



182
W
183
W





 today
182

 Dr 
 Pr 


= 
+  
 today  D s  initial  Ps  initial
W
183
W



= 
 today 
182
W
183
W



+ 
 initial 
t2



W
183
W


 initial
182
Hf
180
Hf





 initial



180
Hf
183
W



 182 Hf   182 Hf 
 180  =  180  exp[−λ∆t ]
 Hf  2  Hf 1
∆t = t 2 − t1
slope=(Pr/Ps)=182Hf/180Hf
t1
182
 Ps

 Ds



180
Hf
183
W



Hafnium –Tungsten (182Hf-182W) systematics; t1/2 = 9 M.y
Hf is highly lithophile, retained in the silicate portion of the Earth.
W is moderately siderophile, most enters core, some retained in the
silicate mantle.
So ideal for tracking core formation
Silicate mantle
Core
Growth
Evolution due to decay of 182Hf
t1/2 = 9 m.y
182W/183W
182W*
0
Silicate Mantle
z
Bulk Earth
Core
Time (Ma)
60
So what do the data look like?
Compared to primitive
meteorites, Earth’s mantle
has an excess of 182W. Core
formation must have
happened when 182Hf was
still alive.
Mean time of core formation
was 11 Myr and coreformation was completed
within 30 Myr
Moon forming impact at ~30
Myr (Yin et al 2002; Jacobsen
Kleine et al. 2002, Yin et al. 2002
2005)
ε is the deviation of a sample from
a reference or standards in parts per 10,000
Yin et al., 2002
Mean time of core formation was 11 Myr and core-formation was completed
within 30 Myr
Moon forming impact at ~30 Myr (Yin et al 2002; Jacobsen 2005)
Was there equilibration between metal and silicate
during core formation?
Nimmo and Agnor, 2006
Nimmo and Agnor, 2006
Nimmo and Agnor, 2006
Without significant metal-silicate equilibration during accretion
silicate Earth would end up too radiogenic in ε182W
Has the final word on the
moon forming impact been
said?
Recent Hf-W data from the moon
Touboul et al. (2007)
Lithophile – silicate loving
Siderophile – iron loving
Chalcophile – sulfur loving
Atmophile – species prefer gas/
fluid phase
Ratio of these elements are
chondritic Æ used to argue
for a late veneer after core
formation
Elemental abundance versus 50% condensation temperature (Wood et al., 2006)
Relative to volatility trend, some elements are grossly depleted in silicate portion of
the earth
Samarium –Neodymium (146Sm-142Nd) systematics; t1/2 = 103 M.y
Both Sm and Nd are lithophile. Both are incompatible. However Nd is
more incompatible than Sm. So ideal for tracking crust-mantle
differentiation.
Crust
Silicate mantle
The early differentiation of the Earth’s mantle:
evidence from 142Nd
146Sm
Æ
147Sm Æ
142Nd;
t1/2= 103 My.
143Nd; t
1/2 = 109 Gy
Early crust-mantle differentiation; ~30
Myr after start of the solar system.
Boyet and Carlson (2005)
Boyet and Carlson (2005) attributed the 142Nd difference due to
formation of an early enriched layer (at ~ 30 Myr) that
subsequently sank back into the mantle; this hidden layer is not
sampled today at either mid ocean ridge volcanism or ocean island
volcanism.
The enriched layer would be enriched in heat producing radioactive
elements
But are there other interpretations?
But what we one assumes that the different stellar
components were not well mixed in the solar system?
Is it possible that because of incomplete mixing of r- and
s- process components Earth and meteorites had different
starting 142Nd/144Nd values?
Solar Nebula
Earth
Meteorite Parent
Bodies
(142Nd/144Nd)0,a
(142Nd/144Nd)0,b
To test this idea one needs to find another heavy element having isotopes produced
by both the r- and s-process but not having any significant contribution from
radioactive decay.
Barium Æ isotopes are not affected by radioactive
decay
Chondrites have an
excess of r-process
produced Ba-isotopes
Chondrite Ba Normalized to Earth Ba
0.7
0.6
0.5
εBa
0.4
Allende
Murchison
Bruderheim
Guarena
Grady
0.3
Normalizing
isotopes
0.2
0.1
0.0
-0.1
133
s
r,s
s
r,s
r,s
134
135
136
137
138
Ba Mass Ratio (i/136)
Ranen and Jacobsen, 2006
139
The Ba isotopic
differences can only
be formed by
incomplete mixing of
stellar components
between the Earth
and Meteorite parent
bodies
(i.e. decay of
radioisotopes cannot
contribute to these
variations)
So is the Earth chondritic? (Ranen 2008)
The results for Ba predict a negative 142Nd effect in chondrites
compared to Earth as observed Æ so did the Earth really undergo
an early episode of crustal differentiation?
How constant is 146Sm/144Sm?
Ranen (2008)
The effect of in-homogeneous
146Sm in the solar system
(Ranen 2008)
Evidence of Early Crust
{3.7-3.8 Ga Isua metasediments
Derived from a mantle source that
has already been depleted in rare
earth elements
(Caro et al.,2003)
MORB source as the complement to crust formation
Continental crust
Mid-ocean ridge basalt
The trace element pattern of mid-ocean ridge basalts can be modeled as
resulting from melting a source previously depleted of incompatible elements
to form the continental crust.
The isotopic composition of MORBs can be modeled to
generate growth curves for continental crust
Using 143Nd/144Nd to constrain continental
crust growth
Rapid crustal growth in the first 1 Gy of Earth history
Areal extent of Archean continental crust is ~14%
Jacobsen 1988
The formation of the atmosphere
Pu-I-Xe systematics
129I Æ 129Xe; t =16My
1/2
244Pu Æ 136Xe; t
136Xe also produced from U fission
=
80My;
1/2
Kunz et al., 1998
I/(Pu+U) ratio determines the slope of the line
Fundamental observation:
MORB data differ from
atmosphere, and show
mixing between air and a
component with excesses
of both 129Xe (from 129I
decay) and fissiogenic Xe
isotopes (from Pu and/or
U decay)
Noble gases in the atmosphere of
terrestrial planets
1. Massive depletion of
volatiles from Earth
2. Abundance pattern
looks like carbonaceous
meteorites
3. No evidence that
present atmosphere
is a remnant of a
primary atmosphere
Solar N/Ne ~1; terrestrial N/Ne ~86,000 =>most of the nitrogen
delivered in condensed form.
Secondary Atmosphere: Degassing of the
Earth’s mantle
A reducing atmosphere
(H2, CO, CH4, NH3 small
amount of CO2)
Or a neutral atmosphere
(CO2, H2O, N2)
- depends if degassing happens
before core formation is over
So when did the mantle degass?
Pu-I-Xe systematics
129I Æ 129Xe; t =16 My
1/2
244Pu Æ 136Xe; t
136Xe also from U fission
=
80
My;
1/2
Excess 129Xe in the mantle
has been used to calculate
mantle degassing age.
(e.g., Staudacher and
Allegre, 1982)
Kunz et al., 1998
Mean age of degassing 15-30 Myr; prior to the end of core formation
Æ Early atmosphere was likely to have been reducing
But the isotopes do not add up……
Atmosphere and mantle have
different composition
OIBs
MORBs
Earth started off with a
solar composition; maybe
mixture of nebular
and planetary gases
Primitive
meteorites
Xenon in the atmosphere
Atmospheres on Earth and Martian have preferentially lost
the lighter isotopes.
An example of what we have done so far: Xe in the mantle
Pu-I-Xe systematics
129I Æ 129Xe; t =16 My
1/2
244Pu Æ 136Xe; t
136Xe also from U fission
1/2 = 80 My;
Our knowledge of xenon to a large
extent is based on a single rock
(known as popping rock)
from a mid-ocean ridge.
Difficult measurements
and we need to be able to look
through the veil of atmospheric
contamination
Kunz et al., 1998
Mid Ocean Ridge (MORB) Xenon
Atlantic
Indian
Pacific
Air
Air-MORB
Popping
rockmixing line
(Kunz et al., 1998)
MORBs
The upper mantle seems
to be homogeneous in
it’s composition.
Xenon in Ocean Island Basalts
Source
Iceland
(Trieloff et al, 2000)
2.6
2.5
136Xe/130Xe
136Xe/130Xe
2.7
Iceland slope is about 10% steeper
than MORBs.
2.4
2.3
2.2
129Xe/136Xe
6.4
6.8
7.2
7.6
8.0
129Xe/130Xe
Data from our Lab
Iceland sample
So….?
Air-MORB
Air-MORB
mixing line
2.6
136Xe/130Xe
OIBS and MORBs have the same trend.
2.7
2.5
2.4
136Xe/130Xe
2.1
2.3
2.2
2.1
6.4
129Xe/136Xe
6.8
7.2
129Xe/130Xe
7.6
8.0
What we have learnt have so far
„
We do not have a clear and coherent story on the early history
of the atmosphere Æ inventories established early but
uncertainties beyond this.
„
Mantle degassing by itself does not generate the secondary
atmosphere; if the noble gas signature is not related to a late
veneer massive amounts of H2 loss is required to explain the
isotopic signature in the atmosphere Æ very early change in the
redox state of our planet.
„
Have to be open to the idea that atmosphere and mantle could
have evolved with different I/Xe ratios and unfortunately,
mantle 129Xe* excess may not yield meaningful degassing age.