Compositional Model for the Mantle beneath the Pacific Plate
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Compositional Model for the Mantle beneath the Pacific Plate Rhea Workman. Outline: Concepts of trace element and isotope geochemistry for the Earth’s mantle Derivation of upper mantle’s composition Some updates Composition of uppermost 100km. Mid-Ocean Ridge Spreading Center :.

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Compositional Model for the Mantle beneath the Pacific Plate

Rhea Workman

Outline:

Concepts of trace element and isotope geochemistry for the Earth’s mantle

Derivation of upper mantle’s composition

Some updates

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

“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

10

1

0.1

0.01

Element Concentrations

Normalized to Bulk Silicate Earth

Bulk Silicate Earth (Mantle before any crust was formed)

Increasing Compatibility in Solid Residue


Partial Melting Leads to Trace Element Partitioning

100

10

1

0.1

0.01

Mantle melt

(Ocean Crust)

Element Concentrations

Normalized to Bulk Silicate Earth

Increasing Compatibility in Solid Residue


Partial Melting Leads to Trace Element Partitioning

100

10

1

0.1

0.01

Mantle melt

(Ocean Crust)

Element Concentrations

Normalized to Bulk Silicate Earth

Mantle residue

after melt removal

Increasing Compatibility in Solid Residue



Isotopic Compositions of Mid-Ocean-Ridge Basalts

Ancient depletion of upper mantle


Elemental Abundances in Modern Ocean Crust

100

10

1

0.1

0.01

Element Concentrations

Normalized to Bulk Silicate Earth


Elemental Abundances in Modern Ocean Crust

100

10

1

0.1

0.01

Element Concentrations

Normalized to Bulk Silicate Earth


**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
Constraints on the Trace Element Composition of DMM incompatible trace elements (like Th, U and K).

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 incompatible trace elements (like Th, U and K).

From Henry Dick


1. Abyssal Peridotites - samples of mantle with melt removed incompatible trace elements (like Th, U and K).

Element Concentrations

Normalized to Bulk Silicate Earth

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 incompatible trace elements (like Th, U and K).

** 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!

Bulk Silicate Earth (BSE)

McDonough & Sun (1995)

Increasing Amount of Melt Removal


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 extracted from the mantle and STAYED extracted from the mantle is the continental crust.

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

**A consensus is merging toward the middle


2. Isotopic composition of Oceanic Crust extracted from the mantle and STAYED extracted from the mantle is the continental 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 extracted from the mantle and STAYED extracted from the mantle is the continental crust.

BSE

Sm/Nd = 0.411


3. “Canonical” ratios extracted from the mantle and STAYED extracted from the mantle is the continental crust.

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 extracted from the mantle and STAYED extracted from the mantle is the continental crust.

Abyssal Peridotite Constraints

Element Concentrations

Normalized to Bulk Silicate Earth


Composing Trace Element Composition of Upper Mantle extracted from the mantle and STAYED extracted from the mantle is the continental crust.

Parent/Daughter Constraints

Element Concentrations

Normalized to Bulk Silicate Earth


Composing Trace Element Composition of Upper Mantle extracted from the mantle and STAYED extracted from the mantle is the continental crust.

Cannonical Ratios Constraints

Element Concentrations

Normalized to Bulk Silicate Earth


Composing Trace Element Composition of Upper Mantle extracted from the mantle and STAYED extracted from the mantle is the continental crust.

Connecting the Dots…

Element Concentrations

Normalized to Bulk Silicate Earth

-- Internally consistent model

(error for many elements is 1-5%)

-- Is it accurate?


So how much U, Th and K is that? extracted from the mantle and STAYED extracted from the mantle is the continental crust.

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: Early Earth (>4 billion years ago)…

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? Lithosphere

pHMELTS

model runs

+

Error is ±50°

Roughly 1km

for every 25 degrees


Effect of water on the mantle’s melting temperature Lithosphere

A

B

Hirth and Kohlstedt (1996)

Recent iteration by Asimow and Langmuir (2003)


Melt Extraction from Upper Mantle Lithosphere

U and Th (ppb), K (ppm)

crust

crust

F = 0.5%

Dry solidus

120 ppm H2O


crust Lithosphere

U and Th (ppb)

K (ppm)

Sediments

Altered Ocean Crust

Depth (km)

Unaltered Ocean Crust,

56 ppb

?

How deep? At least ~500 km.

Maybe higher U, Th, K at depth…PM values?


CONCLUSIONS Lithosphere


Peridotites = Residues of DMM Melting Lithosphere

Fractional Melting:

(Sobolev & Shimizu, 1993;

Johnson et al., 1990; Johnson and Dick, 1992)

Reconstituted peridotites:

(No plag peridotites)


Peridotites residues of dmm melting
Peridotites = Residues of DMM Melting Lithosphere

Primitive Upper Mantle (PUM)

McDonough & Sun (1995)

Linearized relationship

between two elements, A & B, in

a residue of fractional melting:

Where slope, R

Depletion By Fractional Melting


87 Rb 87Sr Lithosphere

t1/2 = 49 Byr


Mineralogy and Buoyancy of the Lithosphere Lithosphere

50 Ma

0 Ma

Solidus


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