Transcript D" as the chemical complement to a non-chondritic

Signatures of Early Earth Differentiation in the
Deep Mantle?
Richard W. Carlson
Department of Terrestrial Magnetism
Carnegie Institution of Washington
COMPRES, June 15, 2011
Continental Crust Formation has Caused
Chemical Differentiation of the Mantle
1000
Element Concentration Normalized by BSE Concentrations
Sample/Bulk-Silicate-Earth
Mass Fraction
100
Continental Crust (Rudnick and Gao, TOG, 2004)
0.45%
10
30-70%
Cont. Crust plus MORB Source (40% of mantle)
1
0.1
MORB Source (Workman and Hart, 2005)
0.01
Rb Ba Th U Nb La Ce Pb Sr Nd Hf Sm Eu Gd Er Yb Lu
70-30%
Elements in Order of Decreasing Incompatibility
During Melting in the Upper Mantle
WHEN DID THIS SEPARATION OCCUR?
Sm-Nd model ages for MORB = 200 - 2000 Ma
Pb-Pb model age for oceanic basalts ~1800 Ma
“Average” continental crust Sm-Nd model age ~2000 Ma
LLSVPs:
A Remnant of
Early
Differentiation or
Modern
Subduction?
Garnero and McNamara, 2008
Some Meteorites are Compositionally Similar to the Sun.
These Serve as a Starting Point for Estimating Bulk Earth Composition,
but how well is the Chondrite Model Matched by Real Earth Rocks?
SolarComposition Normalized to CI Chondrites
102
N
101
C
In?
100
For most elements, CI
chondrites provide a good
approximation of solar
composition
10-1
10-2
Li
10-3
Be N NeAl S K Ti MnNi Ga SeRb ZrRhCdSbXeLaNdGdHoYbTaOsAuPb U
B O Na Si Cl Ca VFeCuGe Br SrNbPd In TeCsCeSmTb ErLu W Ir Hg Bi
Li C F Mg P Ar Sc CrCoZnAs Kr Y RuAg Sn I Ba PrEuDyTmHfRe Pt Tl Th
Elements in Order of Atomic Mass
Solar and CI compositions from Palme and O’Neill, Treatise on Geochemistry, 2003
The Bulk Earth is NOT CI Chondritic: Volatile Depletion is a
Characteristic of Many Solar System Objects, Including Earth
From McDonough
TOG, 2003
CI-normalized terrestrial volatile element abundances decrease with decreasing
condensation temperature. Same pattern, though less extreme, is seen in “primitive”
meteorites. Volatile depletion of Earth may be a “pre-accretion” phenomena
Dating Early Earth Differentiation
Actively-used short-lived radioactive isotopes
Parent Isotope
Atom %
Half-life (Ma)
Daughter
Isotope
26Al
0.005
0.73
26Mg
60Fe
3.7 x 10-7
1.5
60Ni
53Mn
0.00063
3.7
53Cr
107Pd
0.0015
6.5
107Ag
182Hf
0.0037
9
182W
129I
0.011
15.7
129Xe
244Pu
244Pu/238U
80
Fission Xe
103
142Nd
=
0.0068
146Sm
0.026
Condensation – Volatile Loss: Al-Mg, Mn-Cr, Pd-Ag, I-Xe
Metal – Silicate Separation: Fe-Ni, Pd-Ag, Hf-W
Silicate Differentiation: Al-Mg, Fe-Ni, Mn-Cr, Hf-W, Sm-Nd
The Bulk Earth is NOT CI Chondritic: Volatile Depletion is a
Characteristic of Many Solar System Objects, Including Earth
From McDonough
TOG, 2003
CI-normalized terrestrial volatile element abundances decrease with decreasing
condensation temperature. Same pattern, though less extreme, is seen in “primitive”
meteorites. Volatile depletion of Earth may be a “pre-accretion” phenomena
Earth Formed Volatile Depleted
Chondrite Mn/Cr variation correlates with 53Cr/52Cr.
Earth has a lower 53Cr/52Cr than almost all chondrites. Mn more volatile than Cr.
Earth’s volatile depletion occurred while 53Mn was alive (t1/2 = 3.7 Ma)
Earth
From Qin et al., GCA 2010
Earth’s Mantle is Depleted in Siderophile Elements
Palme and O’Neil, TOG, 2003
Reconciling Mn-Cr, Pd-Ag, and Hf-W Constraints on the
Timescale of Earth Volatile-Depletion and Core Formation
26 Myr accretion of volatile-poor
material (86% of Earth mass)
4% CI added at 26 Myr
(Adds another 9% of Earth Mass)
Schonbachler et al.,
Science 2010
Refractory Lithophile Elements SHOULD be Present in the BSE in
Chondritic Relative Abundances, but Often They are Not
“Fertile” mantle xenoliths
(from Palme and O’Neill, TOG, 2004, after Jagoutz et al., 1979)
146,147Sm-142,143Nd
Systematics
Short-lived chronometer:
146Sm
142Nd
0.008
1.14190
1.14180
Isua
0.004
1.14170
3.8 Ga
0.002
1.14160
0.000
1.14150
1200
0
200
400
600
800
Time after accretion (Myr)
Coupled to the long-lived chronometer:
147Sm
143Nd (T
1/2 = 106 Ga)
147Sm abundance decreased by only 3% in 4.56 Ga
1000
142
Sm/144Sm
exists only in
the first 500 Ma of
Solar System history
146
146Sm
4.4 Ga
0.006
Nd/144Nd
Zircon
(T1/2= 103 Ma)
142Nd
Variation in Earth Materials
Limited and Restricted Only to Rocks Older than 3.5 Ga
142Nd
excesses measured in 3.8
Ga samples from SW Greenland
and Anshan, China (up to 0.15e).
142Nd deficiencies in
Nuvvuagittuq, Quebec, Canada
• Evidence for early
differentiation, but not all old rocks
show this
• No heterogeneities preserved
after 3.5 Ga in the convecting
Earth’s mantle
External Precision
Is “Terrestrial” 142Nd/144Nd Chondritic? – No!
Terrestrial
• 142Nd/144Nd ratios measured
in carbonaceous, ordinary and
some enstatite chondrites, and
eucrites, are lower than
laboratory standard and
terrestrial samples
St. Severin
Pasamonte
Nuevo Laredo
Bereba
Basaltic Eucrites
Abee
Indarch
E-Chondrites
Torino
Chico
NWA869
NWA800
Franconia
Sharps
O-Chondrites
Richardton
Dhajala
Bruderheim
Ucera
Homestead
Gladstone
• Excess 142Nd in Earth rocks
indicative of higher than
chondritic Sm/Nd ratio while
146Sm was still extant.
Allende
C-Chondrites
Mokoia
Grosnaja
Murchison
Murray
Mighei
Cold Bokkveld
Orgueil
-80
-60
-40
142 Nd/144 Nd
-20
0
20
40
Relative to La Jolla Nd (in ppm)
Open symbols show data from Nyquist et al., 1995; Andreasen and
Sharma, 2006; Rankenburg et al., 2006. Closed symbols are data
from Boyet and Carlson, 2005; Carlson et al., 2007.
Constraints on the Timing of Earth Differentiation
5 Ma, 147Sm/144Nd=0.209
30 Ma, 147Sm/144Nd=0.212
60 Ma, 147Sm/144Nd=0.216
100 Ma, 147Sm/144Nd=0.222
Mid-ocean
ridge basalts
chondritic evolution
Archean
samples
Differentiation event occurred during the first <30 Ma of Earth
“primordial” chondrite reservoir
(Ra)
Predicted Parental Mantle Reservoir from 142Nd
Overlaps with high 3He/4He Reservoir
Reservoir
parental to
terrestrial
mantle
The Broader Trace Element Characteristics
of this Ancient Depleted Source
Jackson et al., Nature 2010
Similar Normalized Incompatible Element Patterns Found for
Other Major Flood Basalts, in this case, Ontong-Java Basalts
10
1
The flat primitive-mantle-normalized
patterns defined by alteration-resistant
incompatible elements in the
Kwaimbaita- and Kroenke-type basalts
(see Fitton & Godard, 2004) point to a
mantle source not too different from
estimated primitive mantle in most of
its inter-element ratios. However, the
observed isotopic values (e.g., eNd(t) ~
+6) are clearly far-removed from those
estimated for primitive mantle (eNd =
0). (Tejada and Mahoney,
MantlePlumes.org)
10
1
All these samples have e143Nd between
+4 and +7
Size and Composition of the Reservoirs. So What?
Reservoir
Mass(1025g) Th(ppb)U(ppb) K(ppm) TW
Cont. Crust
2.26
5600 1300 15000
7.3
Enriched=D”
17
920
230
2650
9.3
Enriched>1600km
111
150
40
440
10.4
Primitive (60%)
242
79
20
240
11.7
Early Depleted
MORB Mantle
290-390
161
43-53 11-13 ~150 9.5-10.3
7.9
3.2
50
1.1
Two Ways to Create an EDR – EER Pair
Magma Ocean
Overturn
Shallow
Differentiation
Basal Magma Ocean (Labrosse et al., Nature 2007)
How Did the Non-Chondritic
Mantle Form?
Melting is the easiest way to
fractionate the lithophile
elements, but what were the
conditions of melting?
N-MORB
-MORB
OIB
Cont.
Crust
Gd Y Yb
Dy Er
Distribution Coefficient
100
Garnet
Clinopyroxene
10
1
0.1
0.01
0.001
Rb Th Nb Ce Sr Hf Sm Gd Y Yb
Ba U La Pb Nd Zr Eu Dy Er
Corgne et al., 2005 – 25 GPa
Signatures of Early Earth Differentiation in the Deep Mantle?
1) Earth accreted first, and mostly, from volatile-depleted material
2) Core formation occurred while the accreting material shifted from
volatile-poor (reduced?) to volatile-rich (oxidized)
• First ~85% of Earth’s mass mostly volatile-poor
3) What has been called “primitive” mantle is in fact incompatible
element depleted
• Earth is non-chondritic in refractory lithophile element
abundances?
• Signature of an early differentiation event?
• Deep fractionation of perovskite or subduction of a
shallow “KREEP” crust?
• Only the depleted reservoir is sampled at Earth’s surface –
the complementary enriched reservoir must be buried in
the deep mantle – LLSVPs?
When Did Earth’s Core Form?
If core formation were simple
33 ± 2 Ma after Solar System
formation or 4.534 Ga
Parts in 10,000
If Earth grew slowly and
involved many “accumulation
events”, then the answer
depends on the details of Earth
accumulation
Parts in 10,000
 182W (t1/2 = 9 Ma)
Chondrite Hf/W = 1
Metal Hf/W = 0
Mantle Hf/W = 10
182Hf
Pd-Ag Core Formation Timescale Too Fast for Hf-W!
Accrete volatile-rich material first, but this violates Mn-Cr
 107Ag (t1/2 = 6.5 Myr)
Pd/Ag CI = 3
Pd/Ag Earth = 13
Pd/Ag Core > 400
Pd/Ag Mantle = 0.5
107Pd
Dashed curves are for accumulation of material as volatile-depleted as Earth today
(Pd/Ag = 12.9). Solid curves are for accumulation of CV3 chondrites (Pd/Ag = 8.5).
Numbers along the curves give the mantle Pd/Ag ratio after core formation. If Earth
accumulated from volatile-rich material, then Pd-Ag offers no constraints on the timing
of core formation. (From Schonbachler et al., Science 2010)
The Importance of that Last 1%
Mantle Abundances
Silicate-Metal Distribution Coefficients
Earth = 6 x 1024 kg
Chondrites
1
Sample/CI Chondrites
0.1
Mantle After Core Formation
Plus 0.8% CI Chondrite
0.01
Ocean = 1.4 x 1021 kg
CI Chondrite = 18 wt%
H2O
0.001
1% Earth Mass of CI
Chondrite contains 1021
kg water
0.0001
10-5
10-6
Os
Re
Ir
Ru
Pt
Rh
Au
Pd
Early Earth 142Nd/144Nd and 143Nd/144Nd Evolution
The +15 ppm 142Nd/144Nd of the SW Greenland Archean rocks require
a 147Sm/144Nd > 0.225. The reduction in 142Nd/144Nd between 3.9 and
3.5 Ga requires mixing between high- and low-Sm/Nd reservoirs
formed within tens of Ma of Earth formation.
142Nd/144Nd
142Nd/144Nd
50
Initial e143Nd in Mantle-Derived Rocks
in Archean Mantle-Derived Rocks
Data from: Bennet et al., 38th LPSC, 2007;
Caro et al., GCA, 2006;
Boyet and Carlson, EPSL 2006.
Relative to
Chondritic
(in ppm)
e143Nd
Figure after Shirey et al., 2007
with data from numerous literature sources
10
8
40
20
6
Mixing with EER
at a rate of
10% of the mass of
the EER per 80 Ma.
Mixing begins at 3.9 Ga,
stops at 3.5 Ga.
30
End Reservoir has
147Sm/144Nd = 0.2105
10
4
2
Early Depletion with
147Sm/144 Nd = 0.2250
3.5
3.6
3.7
3.8
3.9
4.0 4.1
Age (Ga)
4.2
4.3
4.4
0
4.5
1.0
2.0
3.0
Age (Ga)
4.0
r-, s-process Variability Explains at Least some of the 142Nd/144Nd
Range Between C- and O-, E-Chondrites, but not the EarthChondrite Offset
Crust Formation (with its characteristic LREE enrichment) Started Early
Example – the 4.3 Ga Nuvvuagittuq Terrane, Quebec, Canada
3.8 Ga
4.0 Ga
O’Neil et al., Science, 2008