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Radiogenic isotopic evolution of the mantle and crust Radiogenic isotopic evolution of the mantle and crust

Radiogenic isotopic evolution of the mantle and crust - PowerPoint Presentation

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Radiogenic isotopic evolution of the mantle and crust - PPT Presentation

Matt Jackson and Bill McDonot SrNd isotope plot Global OIB ocean island basalts hotspots and MORB midocean ridge basalt We will model SrNd isotopic evolution by crustmantle differentiation ID: 289459

time melting isotopic mantle melting time mantle isotopic 144nd parent daughter radiogenic 143nd yrs 86sr initial melt 87sr role

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Slide1

Radiogenic isotopic evolution of the mantle and crust

Matt Jackson

and

Bill

McDonotSlide2

Sr-Nd

isotope plot

Global OIB (ocean island basalts, hotspots) and MORB (mid-ocean ridge basalt)

We will model Sr-Nd isotopic evolution by crust-mantle differentiation.

Hofmann (1997)Slide3

How to evolve radiogenic isotopic differences?

Step #1. Fractionate the radioactive parent (

87

Rb) from the radiogenic daughter (87

Sr).

Step #2. Wait.Slide4

Step 1: How to fractionate parent from daughter?

Answer

: Melt the mantle and extract the melt.Slide5

Batch melting

C

l

/Co = (Concentration in liquid)/(Concentration original

unmelted solid)Where

F is the amount of melting. Values range from 0 (no melting) to 1 (100% melting).

Partition coefficient (

D

):

When D < 1, incompatible

When D > 1, compatibleSlide6

Batch Melting

A plot of C

L

/C

O

vs. F for various values of

D

Figure 9.2.

Variation in the relative concentration of a trace element in a liquid vs. source rock as a

fiunction

of D and the fraction melted, using equation (9-5) for equilibrium batch melting. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

C

C

1

D

(1

F)

F

L

O

=

-

+

Rb

Sr

Batch Melting

Slide7

Rb-Sr

fractionation

during mantle melting

Rb

SrSlide8

Sr

Nd

<

SmSlide9

Sm-Nd

fractionation

during mantle melting

D

Sm>D

NdSlide10
Slide11

Step #2: Now that we have fractionated

parent (

Rb

) from daughter (Sr),

how do we generate isotopic differences?

Answer:

Wait

, and give the

87

Rb time to decay to

87

Sr.Slide12

87

Rb

87

Sr (t

1/2

=48.8 billion years)

λ

=ln(2)/t

1/2

(define decay constant)

87

Sr

meas = 87Srinitial +

87Rb(eλt-1) How to evolve radiogenic isotopic differences?

We measure

this

Parent-daughter ratio

Time in years

Decay constant

Questions:

When

87

Rb/

86

Sr is high, what happens to

87

Sr/

86

Sr over time?

When

87Rb/86Sr is low, what happens to 87Sr/

86Sr over time?

initial

(

)

y = b

+ x * mSlide13

87

Sr/

86

Sr

t = 0 yrs

t = 5 x 10

8

yrs

t = 1 x 10

9

yrs

y

=

b

+ (

x

)(

m

)

b

=

y

-intercept = initial

87

Sr/

86Sr

ratiom = slope (proportional to age)  t

= ln(m+1)/λ

liquid

Original source

MantleResidue

87

Rb/

86

SrSlide14
Slide15

147

Sm

143

Nd +

4

He (t

1/2

=106 billion years)

143

Nd

meas

=

143

Ndinitial + 147Sm(eλ

t-1)We measure

this

Parent-daughter ratio

Time in years

Decay constant

Questions:

When

147

Sm/

144

Nd is high, what happens to

143

Nd/

144

Nd over time?

When 147Sm/144Nd is low, what happens to 143Nd/144

Nd over time?initial

(y

= b + x * m

)

How to evolve radiogenic isotopic differences? Slide16

143

Nd/

144

Nd

t = 0 yrs

t = 5 x 10

8

yrs

t = 1 x 10

9

yrs

y

=

b

+

(

x

)(

m

)

b

=

y

-intercept = initial

143Nd/144Nd ratio

m = slope (proportional to age)  t = ln(m+1)/

λ

147

Sm/

144Nd

liquid

Original mantle

MantleResidueSlide17
Slide18

Radiogenic isotopes:

The role of parent-daughter fractionation

AND

timeSlide19

The

87

Sr/

86Sr – 143Nd/144Nd mantle arraySlide20

Sr and

Nd

isotopic evolution of the crust-mantle

Assume an initial uniform silicate Earth underwent melting at some time in the past to form continental crust (melt) and mantle (melting residue

):

Calculate the present-day Sr and Nd

isotopic composition of 1%, 2%, and 5% partial melts and respective melting residues, assuming the bulk partition coefficients given in

the spreadsheet.

Now assume

melting occurred at

different times (e.g., 1

Ga

, 2

Ga, 3 Ga

, etc). What happens to 143Nd/144Nd and 87Sr/86Sr in the melt and the residue.Now vary the starting composition of the silicate Earth.Slide21

Things to think about

Think about the role of time (bigger spread in

Sr

and Nd isotopes if fractionated earlier).Consider the role of melt fraction (F).What role does variability in the starting composition play?Can you match the global OIB-MORB array with this simple model?