Isotopes. Update: Midterm graded Today: What are isotopes Radioactive decay Age dating Isotopes as fingerprint Today’s lecture.
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IsotopesIsotopes
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
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.
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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
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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
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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
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sam
ple
/ch
on
dri
te
60% Ol 15% Opx 15% Cpx 10%Plag
La Ce Nd Sm Eu Tb Er Yb Lu
ExamplesExamples
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
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
The Nature of Matter - The Nature of Matter - ElementsElements
IsotopesIsotopes
# 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
Isotopes in NatureIsotopes in Nature
Radioactive IsotopesRadioactive Isotopes
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
Radioactive Isotopes & DecayRadioactive Isotopes & Decay#
pare
nt a
tom
s#
pare
nt a
tom
s
time time
11
½½
¼¼
Ages through isotopesAges through isotopes
Rb-Sr System Rb-Sr System
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
a b c to86Sr
87Sr
o( )
86Sr
87Sr
86Sr
87Rb
a b c
a1b1
c1t1
to
86Sr
87Sr
86Sr
87Rb
86Sr
87Sr
o( )
a b c
a1b1
c1a2
b2
c2t1
to
t2
86Sr
87Sr
86Sr
87Sr
o( )
86Sr
87Rb
Isochron TechniqueIsochron Technique
a b ca1
b1c1a2
b2c2 t1
to
t286Sr87Sr
86Sr87Rb
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.
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
The Sm-Nd System The Sm-Nd System
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
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.
Nd isotopes vs Sr isotopesNd isotopes vs Sr isotopes
W. WhiteW. White
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