Compositional Model for the Mantle beneath the Pacific Plate Rhea Workman

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

Mantle Melting and Production of Crust Removes U and Th from the Mantle

Mid-Ocean Ridge Spreading Center :

~100 km deep

QuickTime™ and aSorenson Video decompressorare needed to see this picture.

Underwater Basaltic Eruption, Hawaii

“Pele Meets the sea” by Pyle et al. (1990), Lava video productions

Depleted Mantleupwelling beneath ridges

U, Th and K also removed by continental crust formation

Olivine(Mg,Fe)2SiO4

Orthopyroxene(Mg,Fe)SiO3

Partial Melting Leads to Trace Element Partitioning

Wark et al. (2003)

U, Th and K all prefer the melt phase

With Melt/Residue ~ 1000

1.E-02

1.E-01

1.E+00

1.E+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

PUM Normalized Concentrations

Bulk Silicate Earth (Mantle before any crust was formed)

Increasing Compatibility in Solid Residue

Partial Melting Leads to Trace Element PartitioningE

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

Mantle melt(Ocean Crust)

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

Mantle melt(Ocean Crust)

Mantle residue after melt removal

If trace element fractionation happened a long time ago…

87Rb 87Sr86Sr is not radiogenic

Isotopic Compositions of Mid-Ocean-Ridge Basalts

Ancient depletion of upper mantle

1.E-02

1.E-01

1.E+00

1.E+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

Mid-Ocean Ridge Basalts (MORBs)

Model melt from BSE

Elemental Abundances in Modern Ocean Crust

1.E-02

1.E-01

1.E+00

1.E+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

Mid-Ocean Ridge Basalts (MORBs)

Model melt from BSE

Upper Mantle Source for MORBs

Elemental Abundances in Modern Ocean Crust

**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 Constraints on the Trace Element Composition of DMMDMM

1. Abyssal Peridotites =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 =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 = 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)

From Henry Dick

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 removedE

-9 -8 -7 -6 -5 -4 -3 -2 -1 0

ln(Sm)

ln(Eu)

Bulk Silicate Earth (BSE)McDonough & Sun (1995)

ount of M

1. Abyssal Peridotites - samples of mantle with melt removed

** Slope is a function of relative partitioning of the two elements.

** Where is modern upper mantle on this trend?

** Use Sm-Nd isotope system to plot position of upper mantle…BUT need to know information about the AGE of mantle depletion!

Depleted Mantleupwelling beneath ridges

The only solid material we know has definitely been extracted from the mantle and STAYED extracted from the mantle is the continental crust.

Continental Growth ModelsContinental Growth Models

---> identify age (i.e. history) of mantle depletion---> identify age (i.e. history) of mantle depletion

**A consensus is merging toward the middle

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)

2. Isotopic composition of Oceanic Crust

-7 -6 -5 -4 -3 -2 -1 0

ln(Sm)

ln(Nd)

Sm/Nd = 0.411

Defining a unique position on the mantle depletion trends

Some trace elements don’t fractionate from each other!So ratio in melt equals ratio in residue

3. “Canonical” ratios

Spreading Center LavasPETDB Database

Composing Trace Element Composition of Upper Mantle

Abyssal Peridotite Constraints

Parent/Daughter Constraints

Composing Trace Element Composition of Upper MantleE

Cannonical Ratios Constraints

Connecting the Dots…

-- Internally consistent model (error for many elements is 1-5%)

-- Is it accurate?

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.

146Sm --> 142Ndt1/2 = 103 My

New evidence from a short lived isotope New evidence from a short lived isotope informs us about the Early Earth (>4 informs us about the Early Earth (>4 billion years ago)…billion years ago)…

Shows that a crust was formed early in Shows that a crust was formed early in earth history, creating a very old earth history, creating a very old depleted mantle.depleted mantle.

Boyet and Carlson (2006)

Using a similar approach as I showed, they get:U = 5.4 ppbTh = 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 LithosphereEstimating the Compositional Structure of Oceanic Lithosphere

• Use the pHMELTS model: most recent iteration of a thermodynamic model Use the pHMELTS model: most recent iteration of a thermodynamic model

for phase equilibria (Ghiorso and Sack, 1995; Asimow and Ghiorso, 1998; for phase equilibria (Ghiorso and Sack, 1995; Asimow and Ghiorso, 1998;

Ghiorso et al., 2002, Asimow and Langmuir, 2003; Asimow et al., 2004)Ghiorso et al., 2002, Asimow and Langmuir, 2003; Asimow et al., 2004)

• Assume DMM composition (average of W&H, 2005 and B&C, 2006)Assume DMM composition (average of W&H, 2005 and B&C, 2006)

• Water content is set 120 ppmWater content is set 120 ppm

1. Range of water = 70-200 ppm (Michael, 1988; Michael et al., 1995; 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) Danyushevsky et al., 2000; Saal et al., 2002; Workman and Hart, 2005)

2. Water content that generates a MORB with 0.2 wt% H2. Water content that generates a MORB with 0.2 wt% H22O at 8 wt% MgOO at 8 wt% MgO

• Find the potential temperature needed to make oceanic crustFind the potential temperature needed to make oceanic crust

What is the Potential Temperature of the Mantle?What is the Potential Temperature of the Mantle?

pHMELTS model runs

Roughly 1km for every 25 degrees

+Error is ±50°Error is ±50°

Effect of water on the mantle’s melting temperature

Hirth and Kohlstedt (1996)Recent iteration by Asimow and Langmuir (2003)

Dry solidus

120 ppm H2O

Melt Extraction from Upper Mantle

0 20 40 60

U and Th (ppb), K (ppm)

F = 0.5%

0 20 40 60

U and Th (ppb)K (ppm)

?How deep? At least ~500 km. Maybe higher U, Th, K at depth…PM values?

0 500 1000 1500 2000 2500

U (ppb)

Depth (km)

Sediments

Altered Ocean Crust

Unaltered Ocean Crust, 56 ppb

Estimates for U, T and K concentrations in the upper mantle Th (ppb) U (ppb) K (ppm)Workman and Hart (2005) estimate

Average depleted upper mantle -- likely a minimum bound: 7.9 ± 1.0 3.2 ± 0.5 50"Not so depleted" depleted upper mantle: 15.7 5.2

Salters and Stracke (2004)Simple isotope evolution model, 13.7 ± 30% 4.7 ± 30% 60 ± 28%Relies heavily on observations of oceanic crust

Boyet and Carslon (2006)More constrained isotope evolution model, 16 5.4 68.4based on 142Nd evidence for early earth differentiation

Lithospheric upper mantle (mantle depleted of melt) Depth (km) Th (ppb) U (ppb) K (ppm)Calculations by R. Workman using the pHMELTS >90 12.0 4.3 59.2thermodynamics model for phase equlibria 85 6.5 2.3 32(see Asimow et al., 2004 for pHMELTS description) 80 1.6 0.6 7.9Starting values for U, Th, K are an average of 75 0.4 0.14 1.9Workman and Hart (2005) and Boyet and Carlson (2006 ) 70 0.08 0.03 0.4

<60 <0.001 <0.001 <0.001

Estimates for U, T and K concentrations in other reservoirsMarine sediments (Plank and Langmuir, 1998) Thickness (m) Th (ppb) U (ppb) K (ppm)

Seafloor sediments extremely variable, depending on lithology 200 ± 150 2000-18000 1000-3000 4000-25000(for mid-Pacific)

Mid-Ocean Ridge Crust (Hofmann et al., 1988) 6500 ± 500 148 56 698"Parental" mantle melt -- represents the whole crust, which in reality has compositional layeringHofmann values are corrected for 20% partial crystallization

Altered Oceanic Crust (Kelley et al., 2003) Upper 500m U and K can be 2-8 times enrichedEnriched in U and K relative to unaltered ocean crust

Workman and Hart (2005), Earth and Planetary Science Letters v. 231 p. 53-72Salters and Stracke (2004), Geochem. Geophys. Geosyst., v. 5, Q05004, doi:10.1029/2003GC000597Boyet and Carlson (2006), Earth and Planetary Science Letters v. 250 p. 254-268Asimow et al. (2004), Geochem. Geophys. Geosyst., v. 5, Q01E16, doi:10.1029/2003GC000568Plank and Langmuir (1998), Chemical Geology, v. 145, p. 325-394Hofmann (1988), Earth and Planetary Science Letters v.90 p. 297-314For global sediment thickness, see http://www.ngdc.noaa.gov/mgg/sedthick/sedthick.htmlKelley et al. (2003), Geochem. Geophys. Geosyst., v. 4 no. 6, 8910, doi:10.1029/2002GC000435.

Peridotites = Residues of DMM Melting

= (1− F)(

D−1)

Dbulk = xolDol + xopxDopx + xcpxDcpx + xspDsp

CWholeRock = CcpxDbulkDcpx

⎝ ⎜

⎠ ⎟

Fractional Melting:

Reconstituted peridotites:

(Sobolev & Shimizu, 1993; Johnson et al., 1990; Johnson and Dick, 1992)

(No plag peridotites)

Peridotites = Residues of DMM Melting

Linearized relationshipbetween two elements, A & B, in a residue of fractional melting:

Where slope, R

ln CsA

( ) = R ln CsB

( ) + lnCoA

⎜ ⎜

⎟ ⎟

R =DB (1−DA )

DA (1−DB )

-9 -8 -7 -6 -5 -4 -3 -2 -1 0

ln(Sm)

ln(Eu)

Primitive Upper Mantle (PUM)McDonough & Sun (1995)

tion B

87 Rb 87Srt1/2 = 49 Byr

Mineralogy and Buoyancy of the LithosphereMineralogy and Buoyancy of the Lithosphere

Solidus

Compositional Model for the Mantle beneath the Pacific Plate Rhea Workman

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