Isotopes. Update: Midterm graded Today: What are isotopes Radioactive decay Age dating Isotopes as...

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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

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.

56 58 60 62 64 66 68 70 72

Element

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

56 58 60 62 64 66 68 70 72

67% Ol 17% Opx 17% Cpx

56 58 60 62 64 66 68 70 72

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

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

ExamplesExamples

Batch Melting Batch Melting

0 0.2 0.4 0.6 0.8 1

D = 0.001

D = 0.1

D = 0.5D = 1

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)

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

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#

time time

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

Isochron TechniqueIsochron Technique

a b ca1

b1c1a2

b2c2 t1

t286Sr87Sr

86Sr87Rb

Isochron resultsIsochron results

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

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

Isotopes. Update: Midterm graded Today: What are isotopes Radioactive decay Age dating Isotopes as...

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