Compositional Model for the Mantle beneath the
Pacific Plate
Rhea Workman
Outline:
1. Concepts of trace element and isotope geochemistry for the
Earth’s mantle
2. Derivation of upper mantle’s composition
3. Some updates
4. Composition of uppermost 100km
Mid-Ocean Ridge Spreading Center :
Mantle Melting and Production of Crust Removes U and Th from the Mantle
~100 km deep
Underwater Basaltic Eruption, Hawaii
QuickTime™ and a
Sorenson Video decompressor
are needed to see this picture.
“Pele Meets the sea” by Pyle et al. (1990), Lava video productions
U, Th and K also removed by continental crust formation
Depleted Mantle
upwelling beneath ridges
Partial Melting Leads to Trace Element Partitioning
Olivine
(Mg,Fe)2SiO4
Melt
U, Th and K
all prefer the melt phase
With Melt/Residue ~ 1000
Wark et al. (2003)
Orthopyroxene
(Mg,Fe)SiO3
Partial Melting Leads to Trace Element Partitioning
100
Element Concentrations
PUM Normalized Concentrations
Normalized to Bulk Silicate Earth
1.E+02
10
1.E+01
Bulk Silicate Earth (Mantle before any crust was formed)
1
1.E+00
0.1
1.E-01
Increasing Compatibility in Solid Residue
0.01
1.E-02
Rb Ba Th
U Nb Ta La Ce Pb Pr Nd Sr Zr Hf Sm Eu Ti Gd Tb Dy Ho
Y
Er Yb Lu
Partial Melting Leads to Trace Element Partitioning
100
Element Concentrations
PUM Normalized Concentrations
Normalized to Bulk Silicate Earth
1.E+02
Mantle melt
(Ocean Crust)
10
1.E+01
1
1.E+00
0.1
1.E-01
Increasing Compatibility in Solid Residue
0.01
1.E-02
Rb Ba Th
U Nb Ta La Ce Pb Pr Nd Sr Zr Hf Sm Eu Ti Gd Tb Dy Ho
Y
Er Yb Lu
Partial Melting Leads to Trace Element Partitioning
100
Element Concentrations
PUM Normalized Concentrations
Normalized to Bulk Silicate Earth
1.E+02
Mantle melt
(Ocean Crust)
10
1.E+01
1
1.E+00
Mantle residue
after melt removal
0.1
1.E-01
Increasing Compatibility in Solid Residue
0.01
1.E-02
Rb Ba Th
U Nb Ta La Ce Pb Pr Nd Sr Zr Hf Sm Eu Ti Gd Tb Dy Ho
Y
Er Yb Lu
If trace element fractionation happened a long time ago…
87Rb
86Sr
87Sr
is not radiogenic
Isotopic Compositions of Mid-Ocean-Ridge Basalts
Elemental Abundances in Modern Ocean Crust
100
Normalized to Bulk Silicate Earth
PUMElement
Normalized Concentrations
Concentrations
1.E+02
10
1.E+01
1
1.E+00
0.1
1.E-01
Mid-Ocean Ridge Basalts (MORBs)
Model melt from BSE
0.01
1.E-02
Rb Ba Th
U
Nb Ta La Ce Pr Nd Sr
Zr
Hf Sm Eu
Ti
Gd Tb Dy Ho
Y
Er Yb
Lu
Elemental Abundances in Modern Ocean Crust
100
Normalized to Bulk Silicate Earth
PUMElement
Normalized Concentrations
Concentrations
1.E+02
10
1.E+01
1
1.E+00
0.1
1.E-01
Mid-Ocean Ridge Basalts (MORBs)
Model melt from BSE
Upper Mantle Source for MORBs
0.01
1.E-02
Rb Ba Th
U
Nb Ta La Ce Pr Nd Sr
Zr
Hf Sm Eu
Ti
Gd Tb Dy Ho
Y
Er Yb
Lu
**Upper mantle has ~3% mafic melt removal - a big effect for
incompatible trace elements (like Th, U and K).
**Seismic properties, based on major element chemistry, don’t
change much from small degrees of melt extraction.
Calculated by L. Stixrude
Constraints on the Trace Element Composition of DMM
1. Abyssal Peridotites =
Define trends of melt depletion for the upper mantle
(same assumptions as McDonough and Sun (1995)
2. Isotopic composition of Mid-Ocean Ridge Basalts =
Parent/daughter ratios in DMM
(Rb/Sr, Sm/Nd, U/Pb, Th/Pb, Lu/Hf)
Requires 1 more assumption than BSE calculation
3. Canonical Ratios =
Some trace element ratios are nearly constant in MORBs and
assumed to be the same in the MORB source (Ce/Pb, Nb/Ta, Nb/U,
Ba/Rb)
Workman and Hart (2005)
1. Abyssal Peridotites - samples of mantle with melt removed
From Henry Dick
Element Concentrations
Normalized to Bulk Silicate Earth
1. Abyssal Peridotites - samples of mantle with melt removed
Data from: Dick (1984), Dick (1989), Johnson et al. (1990), Johnson & Dick (1992), Dick & Natland (1996),
Salters & Dick (2002), Hellebrand et al. (2002), Tartorotti et al. (2002)
1. Abyssal Peridotites - samples of mantle with melt removed
-1
** Slope is a function of
relative partitioning of the
two elements.
Bulk Silicate Earth (BSE)
McDonough & Sun (1995)
-2
-3
** Where is modern upper
mantle on this trend?
ln(Eu)
-4
-5
** Use Sm-Nd isotope
system to plot position of
upper mantle…BUT need
to know information about
the AGE of mantle
depletion!
-6
-7
-8
-9
-9
-8
-7
-6
-5
-4
ln(Sm)
-3
-2
-1
0
The only solid material we know has definitely been extracted from
the mantle and STAYED extracted from the mantle is the
continental crust.
Depleted Mantle
upwelling beneath ridges
Continental Growth Models
---> identify age (i.e. history) of mantle depletion
**A consensus is merging toward the middle
2. Isotopic composition of Oceanic Crust
Melt is continually removed from the upper mantle
through time, starting at 3 Ga
Sm/Nd = 0.411
(Calculated)
Present day Nd
Isotopic value
(Observed)
Defining a unique position on the mantle depletion trends
1
BSE
0
-1
ln(Nd)
-2
-3
-4
-5
-6
-7
-8
-7
-6
-5
-4
-3
ln(Sm)
-2
-1
0
3. “Canonical” ratios
Some trace elements don’t fractionate from each other!
So ratio in melt equals ratio in residue
Spreading Center Lavas
PETDB Database
Composing Trace Element Composition of Upper Mantle
Abyssal Peridotite Constraints
Element Concentrations
PUM Normalized
Bulk Silicate Earth
Normalized toConcentrations
1.00
0.10
0.01
Rb Ba Th
U Nb Ta La Ce Pb Pr Nd Sr Zr Hf Sm Eu Ti Gd Tb Dy Ho
Y
Er Yb Lu
Composing Trace Element Composition of Upper Mantle
Parent/Daughter Constraints
Element Concentrations
PUM Normalized
Bulk Silicate Earth
Normalized toConcentrations
1.00
0.10
0.01
Rb Ba Th
U Nb Ta La Ce Pb Pr Nd Sr Zr Hf Sm Eu Ti Gd Tb Dy Ho
Y
Er Yb Lu
Composing Trace Element Composition of Upper Mantle
Cannonical Ratios Constraints
Element Concentrations
PUM Normalized
Bulk Silicate Earth
Normalized toConcentrations
1.00
0.10
0.01
Rb Ba Th
U Nb Ta La Ce Pb Pr Nd Sr Zr Hf Sm Eu Ti Gd Tb Dy Ho
Y
Er Yb Lu
Composing Trace Element Composition of Upper Mantle
Connecting the Dots…
Element Concentrations
PUM Normalized
Bulk Silicate Earth
Normalized toConcentrations
1.00
0.10
-- Internally consistent model
(error for many elements is 1-5%)
-- Is it accurate?
0.01
Rb Ba Th
U Nb Ta La Ce Pb Pr Nd Sr Zr Hf Sm Eu Ti Gd Tb Dy Ho
Y
Er Yb Lu
So how much U, Th and K is that?
U = 3.2 ± 0.5 ppb
(16% of the BSE value)
Th = 7.9 ± 1.0 ppb
(10% of the BSE value)
K = 50 ppm
(20% of the BSE value)
Workman and Hart likely gives minimum values.... New
information is coming out to suggest this.
New evidence from a short lived isotope
informs us about the Early Earth (>4 billion
years ago)…
Shows that a crust was formed early in earth
history, creating a very old depleted mantle.
146Sm
--> 142Nd
t1/2 = 103 My
Boyet and Carlson (2006)
Using a similar approach as I showed, they get:
U = 5.4 ppb
Th = 16 ppb
K = 68.4 ppm
(About 1.4 - 2x higher than our previous estimate)
These numbers are only valid for the modern
UNMELTED upper mantle
What about the upper ~100km that has melt removed (and
hence much to all of the U and Th removed)??
Estimating the Compositional Structure of Oceanic Lithosphere
•
Use the pHMELTS model: most recent iteration of a thermodynamic model
for phase equilibria (Ghiorso and Sack, 1995; Asimow and Ghiorso, 1998;
Ghiorso et al., 2002, Asimow and Langmuir, 2003; Asimow et al., 2004)
•
Assume DMM composition (average of W&H, 2005 and B&C, 2006)
•
Water content is set 120 ppm
1. Range of water = 70-200 ppm (Michael, 1988; Michael et al., 1995;
Danyushevsky et al., 2000; Saal et al., 2002; Workman and Hart, 2005)
2. Water content that generates a MORB with 0.2 wt% H2O at 8 wt% MgO
•
Find the potential temperature needed to make oceanic crust
What is the Potential Temperature of the Mantle?
pHMELTS
model runs
+
Error is ±50°
Roughly 1km
for every 25 degrees
Effect of water on the mantle’s melting temperature
A
B
Hirth and Kohlstedt (1996)
Recent iteration by Asimow and Langmuir (2003)
Melt Extraction from Upper Mantle
U and Th (ppb), K (ppm)
0
crust
0
20
40
60
crust
10
20
30
F = 0.5%
40
50
Dry solidus
U
60
70
120 ppm H2O
80
90
100
Th
K
0
0
20
40
60
U (ppb)
crust
0
U and Th (ppb)
K (ppm)
1
1500
2000
Sediments
Altered Ocean Crust
30
2
40
50
U
60
Th
3
Unaltered Ocean Crust,
56 ppb
4
K
70
5
80
90
100
Depth (km)
Depth (km)
1000
0
10
20
500
6
?
How deep? At least ~500 km.
Maybe higher U, Th, K at depth…PM values?
2500
Estimates for U, T and K concentrations in the upper mantle
Workman and Hart (2005) estimate
Average depleted upper mantle -- likely a minimum bound:
"Not so depleted" depleted upper mantle:
CONCLUSIONS
Salters and Stracke (2004)
Simple isotope evolution model,
Relies heavily on observations of oceanic crust
Th (ppb)
U (ppb)
K (ppm)
7.9 ± 1.0
15.7
3.2 ± 0.5
5.2
50
13.7 ± 30%
4.7 ± 30%
60 ± 28%
16
5.4
68.4
Th (ppb)
12.0
6.5
1.6
0.4
0.08
<0.001
U (ppb)
4.3
2.3
0.6
0.14
0.03
<0.001
K (ppm)
59.2
32
7.9
1.9
0.4
<0.001
U (ppb)
1000-3000
K (ppm)
4000-25000
56
698
Boyet and Carslon (2006)
More constrained isotope evolution model,
based on 142Nd evidence for early earth differentiation
Lithospheric upper mantle (mantle depleted of melt)
Calculations by R. Workman using the pHMELTS
thermodynamics model for phase equlibria
(see Asimow et al., 2004 for pHMELTS description)
Starting values for U, Th, K are an average of
Workman and Hart (2005) and Boyet and Carlson (2006 )
Estimates for U, T and K concentrations in other reservoirs
Marine sediments (Plank and Langmuir, 1998)
Seafloor sediments extremely variable, depending on lithology
Depth (km)
>90
85
80
75
70
<60
Thickness (m)
Th (ppb)
200 ± 150
2000-18000
(for mid-Pacific)
Mid-Ocean Ridge Crust (Hofmann et al., 1988)
"Parental" mantle melt -- represents the whole crust,
which in reality has compositional layering
Hofmann values are corrected for 20% partial crystallization
6500 ± 500
Altered Oceanic Crust (Kelley et al., 2003)
Enriched in U and K relative to unaltered ocean crust
Upper 500m
Workman and Hart (2005), Earth and Planetary Science Letters v. 231 p. 53-72
Salters and Stracke (2004), Geochem. Geophys. Geosyst., v. 5, Q05004, doi:10.1029/2003GC000597
Boyet and Carlson (2006), Earth and Planetary Science Letters v. 250 p. 254-268
Asimow et al. (2004), Geochem. Geophys. Geosyst., v. 5, Q01E16, doi:10.1029/2003GC000568
Plank and Langmuir (1998), Chemical Geology, v. 145, p. 325-394
Hofmann (1988), Earth and Planetary Science Letters v.90 p. 297-314
For global sediment thickness, see http://www.ngdc.noaa.gov/mgg/sedthick/sedthick.html
Kelley et al. (2003), Geochem. Geophys. Geosyst., v. 4 no. 6, 8910, doi:10.1029/2002GC000435.
148
U and K can be 2-8 times enriched
Peridotites = Residues of DMM Melting
Fractional Melting:
(Sobolev & Shimizu, 1993;
Johnson et al., 1990; Johnson and Dick, 1992)
1
1)
Cs
 (1 F) D
Co
(
Reconstituted peridotites:

(No plag peridotites)
D 
CW holeRock  Ccpx  bulk 
 Dcpx 
Dbulk  xol Dol  xopxDopx  xcpx Dcpx  xspDsp

Peridotites = Residues of DMM Melting
Linearized relationship
between two elements, A & B, in
a residue of fractional melting:
-1
Primitive Upper Mantle (PUM)
McDonough & Sun (1995)
-2
-3
-4
ln(Eu)
 A 
Co 
A
B

ln Cs  Rln Cs  ln
B R 

Where slope, R
Co  
-5
-6
-7
-8
-9
DB (1 DA )
R
DA (1 DB )
-9
-8
-7
-6
-5
-4
ln(Sm)
-3
-2
-1
0
87 Rb
87Sr
t1/2 = 49 Byr
Mineralogy and Buoyancy of the Lithosphere
50 Ma
0 Ma
Solidus