Transcript nakai

Importance of tighter constraints on U
and Th abundances of the whole Earth
by Geo-neutrino determinations
Shun’ichi Nakai
ERI, The University of Tokyo
1
How can we estimate
the composition of the Earth?
The Earth is a differentiated planet. No sample records its average composition.
Building blocks of the Earth can be used for the purpose.
2
Evolution of parent bodies of meteorites
Dust of the solar nebula
Condensation and accretion
Small planetary bodies
Chondrites
melting by
1. heat produced by short-lived radioactive
isotopes such as 26Al
2. energy supplied with collisions.
differentiation
Achondrites Stony - iron meteorites Iron meteorites
mantle + crust
core
3
Solar photosphere
Chemical compositions of the Sun and CI chondrites
CI chondrite
4
Geochemists have believed in the chondrite model
that the composition of the Earth can be estimated
from that of chondrites.
Recent Nd isotope studies challenge this model.
What is the problem?
5
Decay systems
used for Earth and Planetary Sciences
Parent
Daughter
half life
decay constant
87Rb
87Sr
48.8 Byr
1.42 E-11yr-1
147Sm
143Nd
106.0 Byr
6.54 E-12yr-1
238U
206Pb
4.47 Byr
1.5521E-10yr-1
235U
207Pb
0.704 Byr
9.885E-10yr-1
232Th
208Pb
14.01 Byr
0.49475E-10yr-1
(Byr 109yr)
Sm-Nd decay is unique in that its isotopic evolution of the Earth’s mantle
can be estimated by that in chondrites.
6
Evolution of Nd isotope ratio
Parent(Daughter)
Decay constant
Half life
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
147Sm(143Nd)
6.54E-12 (yr-1)
106.0Byr Lugmair and Marti (1978)

143
Nd
144
 
Nd 
143
Nd
144
 
Nd 0 
147
Sm
144

147Sm T

Nd   e
 1


The evolution of 143Nd/144Nd depends on
(143Nd/144Nd)0 and 147Sm/144Nd.
7
Two important events in the early Earth
• Condensation and accretion
• Core formation
These two events could have changed parent / daughter (eg.
Sm/Nd) abundance ratios, which resulted in different evolution of the
Earth from that of chondrites.
8
unique property of Sm-Nd decay system
• Both parent and daughter elements are
refractory. No elemental fractionation occurred
during condensation of solid from solar nebula
gas.
9
volatile
Condensation temperature
U, Th, Sm,
Nd
refractory
Rb
Pb
Sr behaves
similarly to Ca.
10
How did two events change parent daughter ratios
of decay systems in silicate Earth?
Rb/Sr
U/Pb
Sm/Nd
Raw material
for the Earth
smaller than
chondritic
higher than
chondritic
chondritic
Condensation and
Accretion
decrease
increase
no change
Core Formation
Silicate Earth
Chondrite parent bodies were located more distant from the Sun
compared to the Earth.
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unique property of Sm-Nd decay system
• Both parent and daughter elements are
refractory. No elemental fractionation occurred
during condensation of solid from solar nebula
gas.
• Both parent and daughter elements are lithophile
elements. No elemental fractionation occurred
during core formation in the Earth
12
Elemental fractionation during core formation
Sm, Nd, Rb, Sr, U and Th are lithophile.
They prefer to stay in silicate part.
Pb is chalcophile and enters into the core.
13
How did two events change parent daughter ratios
of decay systems in silicate Earth?
Rb/Sr
U/Pb
Sm/Nd
Raw material
for the Earth
smaller than
chondritic
higher than
chondritic
chondritic
Condensation and
Accretion
decrease
increase
no change
Core Formation
no change
increase
no change
Silicate Earth
not chondritic
not chondritic
chondritic
Sm-Nd evolution of the Earth’s mantle can be estimated from
that of chondrites.
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Evolution of Nd isotope ratio
Parent(Daughter)
Decay constant
Half life
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
147Sm(143Nd)
6.54E-12(yr-1)
106.0Byr Lugmair and Marti (1978)

143
Nd
144
 
Nd 
143
Nd
144
 
Nd 0 
147
Sm
144

147Sm T

Nd   e
 1


The average 143Nd/144Nd of chondrites at present is 0.512638.
The average 147Sm/144Nd of chondrites at present is 0.1967.
The two parameters can describe the Nd isotope evolution of whole silicate Earth.
A reservoir which has the two parameters is called as “Chondritic Uniform
Reservoir (CHUR)”.
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Variation of Sm/Nd inchondrites
Differentiation within silicate Earth
- Partial melting changes Sm/Nd -
Partial melting of solid rock to form a felsic melt mad mafic residue
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Fractionations of elements
during partial melting of silicate rocks
• Incompatibility
Nd>Sm
• Changes of parent/daughter ratio
(Sm/Nd)melt< (Sm/Nd)source material < (Sm/Nd)residual solid
(143Nd/144Nd)=(143Nd/144Nd)0+
(147Sm/144Nd)×(e147Sm×t -1)
Faster evolution in residual solid (mantle)
18
Evolution of Nd isotopic ratios
in parts of silicate Earth
143Nd/144Nd
depleted mantle > CHUR > crust
time
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Variation of 143Nd/144Nd
0.5132
0.512638
> 0.511
MORB = present depleted mantle high Sm/Nd
Chondritic Uniform Reservoir =
Bulk Silicate Earth
old Continental crust
low Sm/Nd
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Average of chondrites
0.512638
Continental crust
143Nd/144Nd: ~ 0.511
87Sr/86Sr: ~ 0.73
Hofmann (1997)
21
Isotopes of Nd and Sm
30
25
Sm
20
→ 143Nd
146Sm → 142Nd
Two decay systems
147Sm
15
10
5
30
0
Nd
25
20
15
10
5
0
140
145
150
155
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Extinct nuclides
Parent
nuclides
Half life（Myr)
Daughter
nuclides
Abundances at the beginning of
the solar system
26Al
0.7
26Mg
26Al/27Al=5×10-5
53Mn
3.7
53Cr
53Mn/55Mn=6×10-6
129I
16
129Xe
129I/127I=1×10-4
182Hf
8.9
182W
182Hf/180Hf=1×10-4
146Sm
68
142Nd
146Sm/144Sm＝0.005-0.015
244Pu
82
Fission Xe
244Pu/238U＝0.004-0.007
These isotopes were present in the beginning of the Solar system.
Because of their short lives, the decayed away and now extinct.
The major input was limited at the time of the formation of the Solar system.
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Decay of radioactive nuclides
Half life
:6.8×107 year
147Sm :1.03×1011 year
146Sm
Proportion of remained radioactive isotope isotopes
1
decay away after 500 million years
since the beginning of the Solar system.
Sm
147
Sm
0.6
0.4
0.2
0
146Sm
146
0.8
0
0.5
1
1.5
2
2.5
Time since the beggining of the Solar system (10 9 years)
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24
142Nd/144Nd
146Sm-142Nd
1.14187
decay system
The slope of the line indicates the
(146Sm/144Sm)
at the time of meteorite formation.
1.14180



142
Nd
144
142
Nd
144
142
Nd
144

Nd 
Nd 
Nd present 


initial 
initial
146
Sm
144
146
Sm
147

Sm  
Nd 
147
Sm
144
Nd

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1.142
depleted part
enriched part chondrite
1.141
142
Nd/
144
Nd
1.141
142Nd/144Nd
Crust formed before 4.3Ga <
chondrites <
Residual mantle complementary to
the crust
1.14
1.14
0
500
1000
Time after the formation of the Solar system (Ma)
1500
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Thermal Ionization Mass Spectrometer (TIMS)
Thermo Fisher
27
Thermo Fisher
Repeated analyses on 142Nd/144Nd of a standard reagent show typical variations of 0.00001.
28
Errors are on five decimal place.
Different 142Nd/144Nd of terrestrial samples from chondrites
Boyet and Carlson, 2005
29
Rizo et al. (2012)
142Nd/144Nd
1.14186
1.14184
chondrites
20ppm difference between modern terrestrial samples (1.14186) and
chondrites (1.14184). Limited variation of modern samples suggests the Earth
has been homogenized by mantle convection through its history.
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1.142
depleted part
enriched part chondrite
1.141
142
Nd/
144
Nd
1.141
142Nd/144Nd
Crust formed before 4.3Ga <
chondrites <
Residual mantle complementary to
the crust
1.14
1.14
0
500
1000
Time after the formation of the Solar system (Ma)
1500
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Possible reasons that caused higher terrestrial 142Nd/144Nd
than chondrites
 The building block of the Earth has different composition from
chondrites. - It is difficult to test the hypothesis.
 We have not obtained samples from the part with lower Sm/Nd
than chondrites.
Material with lower Sm/Nd stayed around the core-manlte
boundary and no sample has risen to the surface.
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Formation of hidden reservoir
enriched in incompatible elements 1
Labrosse et al. 2007
Formation of hidden reservoir
enriched in incompatible elements 2
• Boyet and Carlson (2005) proposed that an incompatibleelement reservoir with low Sm/Nd formed early in Earth’s
history, sunk to the core – mantle boundary.
• No sample has been derived from the enriched reservoir
after its formation.
Possible reasons that caused higher terrestrial 142Nd/144Nd
than chondrites
 The building block of the Earth has different composition from
chondrites. - It is difficult to test the hypothesis.
 We have not obtained samples from the part with lower Sm/Nd
than chondrites.
Material with lower Sm/Nd stayed around the core-manlte
boundary and no sample has risen to the surface.
 The Earth lost a part with lower Sm/Nd than chondrites
A part with lower Sm/Nd stayed at the surface of the Earth in
the beginning, however it has ablated by heavy bombardment
of meteorites.
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Enriched reservoir has been lost to the space.
ablasion
It was likely that incompatible elements
were enriched in melt covering the surface
of the Earth.
The enriched layer with low Sm/Nd could
have been ablated by heavy bombardments
of meteorites resulting in high 142Nd/144Nd.
36
Campbell and O’Neill
(2012)
6% higher Sm/Nd ration can explain
higher 142Nd/144Nd of terrestrial samples
than chondrites.
In this case, U and Th depletion factors
reach half of the chondirite value.
37
Collisional erosion on the Mercury
Density of a planet usually depends
on its mass.
Mercury has abnormally high
density for its mass.
It is considered that rock part of the
planet was ablated by heavy
collisions.
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Tighter constraint from geo-neutrino analyses
could solve the problem.
1. Inaccessible deep part of the Earth may by enriched
in incompatible elements and has low Sm/Nd.
U and Th chondritic
2. The earth may have lost a part enriched in
incompatible elements in its early history.
U and Th non condritic
39
U and Th abundances
estimated from geo-nutrino observations.
U and Th abundances estimated
from geo-nutrino have still large
uncertainties.
Center value of the Kamland,
however, is consistent with the
erosion model.
The KamLAND collaboration, 2011
TW scales with U concentration; 10, 20, 30 TW ≈ 10, 20, 30 ppb U in BSE
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