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Compositional Model for the Mantle beneath the Pacific Plate Rhea Workman

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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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slide1

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

slide2

Mid-Ocean Ridge Spreading Center :

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

~100 km deep

slide3

Underwater Basaltic Eruption, Hawaii

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

slide4

U, Th and K also removed by continental crust formation

Depleted Mantle

upwelling beneath ridges

slide5

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

slide6

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

slide7

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

slide8

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

slide10

Isotopic Compositions of Mid-Ocean-Ridge Basalts

Ancient depletion of upper mantle

slide11

Elemental Abundances in Modern Ocean Crust

100

10

1

0.1

0.01

Element Concentrations

Normalized to Bulk Silicate Earth

slide12

Elemental Abundances in Modern Ocean Crust

100

10

1

0.1

0.01

Element Concentrations

Normalized to Bulk Silicate Earth

slide13

**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

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)

slide16

1. Abyssal Peridotites - samples of mantle with melt removed

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)

slide17

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!

Bulk Silicate Earth (BSE)

McDonough & Sun (1995)

Increasing Amount of Melt Removal

slide18

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

slide19

Continental Growth Models

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

**A consensus is merging toward the middle

slide20

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)

slide23

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

slide24

Composing Trace Element Composition of Upper Mantle

Abyssal Peridotite Constraints

Element Concentrations

Normalized to Bulk Silicate Earth

slide25

Composing Trace Element Composition of Upper Mantle

Parent/Daughter Constraints

Element Concentrations

Normalized to Bulk Silicate Earth

slide26

Composing Trace Element Composition of Upper Mantle

Cannonical Ratios Constraints

Element Concentrations

Normalized to Bulk Silicate Earth

slide27

Composing Trace Element Composition of Upper Mantle

Connecting the Dots…

Element Concentrations

Normalized to Bulk Silicate Earth

-- Internally consistent model

(error for many elements is 1-5%)

-- Is it accurate?

slide28

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.

slide29

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)

slide30

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)??

slide31

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
slide32

What is the Potential Temperature of the Mantle?

pHMELTS

model runs

+

Error is ±50°

Roughly 1km

for every 25 degrees

slide33

Effect of water on the mantle’s melting temperature

A

B

Hirth and Kohlstedt (1996)

Recent iteration by Asimow and Langmuir (2003)

slide34

Melt Extraction from Upper Mantle

U and Th (ppb), K (ppm)

crust

crust

F = 0.5%

Dry solidus

120 ppm H2O

slide35

crust

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?

slide37

Peridotites = Residues of DMM Melting

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

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

slide39

87 Rb 87Sr

t1/2 = 49 Byr

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