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Isotopes Isotopes

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Page 1: Isotopes. Update: Midterm graded Today: What are isotopes Radioactive decay Age dating Isotopes as fingerprint Today’s lecture

IsotopesIsotopes

Page 2: Isotopes. Update: Midterm graded Today: What are isotopes Radioactive decay Age dating Isotopes as fingerprint Today’s lecture

Update:Update:

Midterm gradedMidterm graded

Today:Today:

What are isotopesWhat are isotopes

Radioactive decayRadioactive decay

Age datingAge dating

Isotopes as fingerprintIsotopes as fingerprint

Today’s lectureToday’s lecture

Page 3: Isotopes. Update: Midterm graded Today: What are isotopes Radioactive decay Age dating Isotopes as fingerprint Today’s lecture

Spider DiagramsSpider Diagrams

1

10

100

1000

Rb Ba Th Nb K La Ce Sr Nd Sm Zr Ti Gd Y

Rock/Chondrites

Fig. 9-5. Spider diagram for an alkaline basalt from Gough Island, southern Atlantic. After Sun and MacDonough (1989). In A. D. Saunders and M. J. Norry (eds.), Magmatism in the Ocean Basins. Geol. Soc. London Spec. Publ., 42. pp. 313-345.

Page 4: Isotopes. Update: Midterm graded Today: What are isotopes Radioactive decay Age dating Isotopes as fingerprint Today’s lecture

0.00

2.00

4.00

6.00

8.00

56 58 60 62 64 66 68 70 72

Element

sam

ple

/ch

on

dri

te

Eu*

La Ce Nd Sm Eu Tb Er Yb Lu

Figure 9-5. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.Figure 9-5. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

REE/Spider Diagrams IIREE/Spider Diagrams II

1122

Page 5: Isotopes. Update: Midterm graded Today: What are isotopes Radioactive decay Age dating Isotopes as fingerprint Today’s lecture

0.00

2.00

4.00

6.00

8.00

10.00

56 58 60 62 64 66 68 70 72

sa

mp

le/c

ho

nd

rite

La Ce Nd Sm Eu Tb Er Yb Lu

67% Ol 17% Opx 17% Cpx

0.00

2.00

4.00

6.00

8.00

10.00

56 58 60 62 64 66 68 70 72

sa

mp

le/c

ho

nd

rite

La Ce Nd Sm Eu Tb Er Yb Lu

57% Ol 14% Opx 14% Cpx 14% Grt

0.00

2.00

4.00

6.00

8.00

10.00

sam

ple

/ch

on

dri

te

60% Ol 15% Opx 15% Cpx 10%Plag

La Ce Nd Sm Eu Tb Er Yb Lu

ExamplesExamples

Page 6: Isotopes. Update: Midterm graded Today: What are isotopes Radioactive decay Age dating Isotopes as fingerprint Today’s lecture

Batch Melting Batch Melting

0.1

1

10

100

1000

0 0.2 0.4 0.6 0.8 1

F

D = 0.001

D = 0.1

D = 0.5D = 1

D = 2

D = 4

D = 10

CL/CO D = 1 = even split, D = 1 = even split, D < 1 = incompatible in D < 1 = incompatible in

minerals => enriched in meltminerals => enriched in melt D > 1 = compatible in D > 1 = compatible in

minerals => depleted in meltminerals => depleted in melt

Figure 9-2. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.Figure 9-2. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

CCLL, C, COO = liquid, solid concentration = liquid, solid concentration

F = fraction melt produced F = fraction melt produced

= melt/(melt + rock)= melt/(melt + rock)

CCCC

1 What’s D1 What’s Dii? ? DDii(1(1 F)F) FF

LL

OO

Page 7: Isotopes. Update: Midterm graded Today: What are isotopes Radioactive decay Age dating Isotopes as fingerprint Today’s lecture

Fractional melting, and othersFractional melting, and othersSeparation of each melt drop as it formed Separation of each melt drop as it formed

CCLL/C/COO = (1/D) * (1-F) = (1/D) * (1-F) (1/D -1)(1/D -1)

Crystallization like meltingCrystallization like melting

Wall-rock assimilationWall-rock assimilation Zone refiningZone refining Combinations of processes Combinations of processes

Cox, Bell, PankhurstCox, Bell, Pankhurst

Page 8: Isotopes. Update: Midterm graded Today: What are isotopes Radioactive decay Age dating Isotopes as fingerprint Today’s lecture

The Nature of Matter - The Nature of Matter - ElementsElements

Page 9: Isotopes. Update: Midterm graded Today: What are isotopes Radioactive decay Age dating Isotopes as fingerprint Today’s lecture

IsotopesIsotopes

Page 10: Isotopes. Update: Midterm graded Today: What are isotopes Radioactive decay Age dating Isotopes as fingerprint Today’s lecture

# Protons is fixed, # Neutrons can vary (# Protons is fixed, # Neutrons can vary (isotopes with different mass #isotopes with different mass #))

Hydrogen (1,2,3) (Average in nature is 1.008)Hydrogen (1,2,3) (Average in nature is 1.008)

Iron (54,56,57,58) (Average in nature is 55.85) Iron (54,56,57,58) (Average in nature is 55.85)

Uranium (234,235,238) (Average in nature is 238.03)Uranium (234,235,238) (Average in nature is 238.03)

88 Naturally occurring elements - some have >1 isotopes88 Naturally occurring elements - some have >1 isotopes

8 elements make up > 98% of Earth’s crust 8 elements make up > 98% of Earth’s crust

(O, Si, Al, Fe, Ca, Mg, K, Na)(O, Si, Al, Fe, Ca, Mg, K, Na)

The Nature of Matter - The Nature of Matter - ElementsElements

Page 11: Isotopes. Update: Midterm graded Today: What are isotopes Radioactive decay Age dating Isotopes as fingerprint Today’s lecture

Isotopes in NatureIsotopes in Nature

Page 12: Isotopes. Update: Midterm graded Today: What are isotopes Radioactive decay Age dating Isotopes as fingerprint Today’s lecture

Radioactive IsotopesRadioactive Isotopes

Page 13: Isotopes. Update: Midterm graded Today: What are isotopes Radioactive decay Age dating Isotopes as fingerprint Today’s lecture

Isotopic variations between rocks, etc. due to:Isotopic variations between rocks, etc. due to:1. Mass fractionation (as for stable isotopes)1. Mass fractionation (as for stable isotopes)

2. Daughters produced in varying proportions 2. Daughters produced in varying proportions resulting from previous event of chemical resulting from previous event of chemical fractionation separating parent from daughterfractionation separating parent from daughter

3. Time…(next slide)3. Time…(next slide)

Example: 40K 40Ar by radioactive decay

Basalt rhyolite by FX (a chemical fractionation process)

Rhyolite has more K than basalt40K more 40Ar over time in rhyolite than in basalt40Ar/39Ar ratio will be different in each

Isotopic fractionationIsotopic fractionation

Page 14: Isotopes. Update: Midterm graded Today: What are isotopes Radioactive decay Age dating Isotopes as fingerprint Today’s lecture

Radioactive Isotopes & DecayRadioactive Isotopes & Decay#

pare

nt a

tom

s#

pare

nt a

tom

s

time time

11

½½

¼¼

Page 15: Isotopes. Update: Midterm graded Today: What are isotopes Radioactive decay Age dating Isotopes as fingerprint Today’s lecture

Ages through isotopesAges through isotopes

Page 16: Isotopes. Update: Midterm graded Today: What are isotopes Radioactive decay Age dating Isotopes as fingerprint Today’s lecture

Rb-Sr System Rb-Sr System

Page 17: Isotopes. Update: Midterm graded Today: What are isotopes Radioactive decay Age dating Isotopes as fingerprint Today’s lecture

y = b + x m y = b + x m

= equation for a line in = equation for a line in 8787Sr/Sr/8686Sr vs. Sr vs. 8787Rb/Rb/8686Sr plotSr plot

Slope = (eSlope = (ett -1) -1)

Divide by stable Divide by stable 8686Sr:Sr:

8787Sr/Sr/8686Sr = (Sr = (8787Sr/Sr/8686Sr)Sr)oo + ( + (8787Rb/Rb/8686Sr)(eSr)(ett -1) -1)

= 1.4 x 10= 1.4 x 10-11-11 a a-1-1

Isochron methodIsochron method

Page 18: Isotopes. Update: Midterm graded Today: What are isotopes Radioactive decay Age dating Isotopes as fingerprint Today’s lecture

a b c to86Sr

87Sr

o( )

86Sr

87Sr

86Sr

87Rb

Page 19: Isotopes. Update: Midterm graded Today: What are isotopes Radioactive decay Age dating Isotopes as fingerprint Today’s lecture

a b c

a1b1

c1t1

to

86Sr

87Sr

86Sr

87Rb

86Sr

87Sr

o( )

Page 20: Isotopes. Update: Midterm graded Today: What are isotopes Radioactive decay Age dating Isotopes as fingerprint Today’s lecture

a b c

a1b1

c1a2

b2

c2t1

to

t2

86Sr

87Sr

86Sr

87Sr

o( )

86Sr

87Rb

Page 21: Isotopes. Update: Midterm graded Today: What are isotopes Radioactive decay Age dating Isotopes as fingerprint Today’s lecture

Isochron TechniqueIsochron Technique

a b ca1

b1c1a2

b2c2 t1

to

t286Sr87Sr

86Sr87Rb

Page 22: Isotopes. Update: Midterm graded Today: What are isotopes Radioactive decay Age dating Isotopes as fingerprint Today’s lecture

Isochron resultsIsochron results

0.710

0.715

0.720

0.725

0 2 4 6 8 10 12 14

Rb-Sr Isochron, Eagle Peak Pluton, Sierra Nevada Batholith

87Sr/86Sr = 0.00127 (87Rb/86Sr) + 0.70760

87Rb/86Sr

x

87 S

r/8

6 S r

Figure 9-9. After Hill et al. (1988). Amer. J. Sci., 288-A, 213-241.

Page 23: Isotopes. Update: Midterm graded Today: What are isotopes Radioactive decay Age dating Isotopes as fingerprint Today’s lecture

Figure 9-13. After Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer.Figure 9-13. After Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer.

Making Sr isotope reservoirsMaking Sr isotope reservoirs

Page 24: Isotopes. Update: Midterm graded Today: What are isotopes Radioactive decay Age dating Isotopes as fingerprint Today’s lecture

The Sm-Nd System The Sm-Nd System

Page 25: Isotopes. Update: Midterm graded Today: What are isotopes Radioactive decay Age dating Isotopes as fingerprint Today’s lecture

147147Sm Sm 143143Nd by alpha decayNd by alpha decay = 6.54 x 10= 6.54 x 10-13-13 a a-1 -1 (half life 106 Ga)(half life 106 Ga)

Decay equation (Decay equation (144144Nd non-radiogenic)Nd non-radiogenic)

143143Nd/Nd/144144Nd = (Nd = (143143Nd/Nd/144144Nd)Nd)oo

+ (+ (147147Sm/Sm/144144Nd)(eNd)(ett-1)-1)

Decay equation Sm-NdDecay equation Sm-Nd

Page 26: Isotopes. Update: Midterm graded Today: What are isotopes Radioactive decay Age dating Isotopes as fingerprint Today’s lecture

Evolution opposite to Rb - SrEvolution opposite to Rb - Sr

Figure 9-15. After Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer.Figure 9-15. After Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer.

Page 27: Isotopes. Update: Midterm graded Today: What are isotopes Radioactive decay Age dating Isotopes as fingerprint Today’s lecture

Nd isotopes vs Sr isotopesNd isotopes vs Sr isotopes

W. WhiteW. White