Isotopes

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

Spider Diagrams Fig 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

Element sample/chondrite 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. REE/Spider Diagrams II 1 2

sample/chondrite La Ce Nd Sm Eu Tb Er Yb Lu 67% Ol 17% Opx 17% Cpx sample/chondrite La Ce Nd Sm Eu Tb Er Yb Lu 57% Ol 14% Opx 14% Cpx 14% Grt sample/chondrite 60% Ol 15% Opx 15% Cpx 10%Plag La Ce Nd Sm Eu Tb Er Yb LuExamples

Batch Melting F D = 1 = even split, F D enriched in melt F D > 1 = compatible in minerals => depleted in melt Figure 9-2. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. C L, C O = liquid, solid concentration F = fraction melt produced = melt/(melt + rock) CC 1 What’s D i ? DiDiDiDi(1F)F L O  

Fractional melting, and others Separation of each melt drop as it formed C L /C O = (1/D) * (1-F) (1/D -1) l Crystallization like melting l Wall-rock assimilation l Zone refining l Combinations of processes Cox, Bell, Pankhurst

The Nature of Matter - Elements The Nature of Matter - Elements

Isotopes

# Protons is fixed, # Neutrons can vary (isotopes with different mass #) i Hydrogen (1,2,3) (Average in nature is 1.008) i Iron (54,56,57,58) (Average in nature is 55.85) i Uranium (234,235,238) (Average in nature is ) s 88 Naturally occurring elements - some have >1 isotopes s 8 elements make up > 98% of Earth’s crust (O, Si, Al, Fe, Ca, Mg, K, Na) The Nature of Matter - Elements The Nature of Matter - Elements

Isotopes in Nature

Radioactive Isotopes

Isotopic variations between rocks, etc. due to: 1. Mass fractionation (as for stable isotopes) 2. Daughters produced in varying proportions resulting from previous event of chemical fractionation separating parent from daughter 3. Time…(next slide) Example: 40 K  40 Ar by radioactive decay Basalt  rhyolite by FX (a chemical fractionation process) Rhyolite has more K than basalt 40 K  more 40 Ar over time in rhyolite than in basalt 40 Ar/ 39 Ar ratio will be different in each Isotopic fractionation

Radioactive Isotopes & Decay # parent atoms time  1½¼

Ages through isotopes

Rb-Sr System

y = b + x m y = b + x m = equation for a line in 87 Sr/ 86 Sr vs. 87 Rb/ 86 Sr plot Slope = (e t -1) Divide by stable 86 Sr: 87 Sr/ 86 Sr = ( 87 Sr/ 86 Sr) o + ( 87 Rb/ 86 Sr)(e t -1) = 1.4 x a -1 = 1.4 x a -1 Isochron method

a bc toto 86 Sr 87 Sr o () 86 Sr 87 Sr 86 Sr 87 Rb

a bc a1a1 b1b1 c1c1 t1t1 toto 86 Sr 87 Sr 86 Sr 87 Rb 86 Sr 87 Sr o ()

a bc a1a1 b1b1 c1c1 a2a2 b2b2 c2c2 t1t1 toto t2t2 86 Sr 87 Sr 86 Sr 87 Sr o () 86 Sr 87 Rb

Isochron Technique a bc a1a1 b1b1 c1c1 a2a2 b2b2 c2c2 t1t1 toto t2t2 86 Sr 87 Sr 86 Sr 87 Rb

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

Figure After Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer. Making Sr isotope reservoirs

The Sm-Nd System

147 Sm  143 Nd by alpha decay = 6.54 x a -1 (half life 106 Ga) = 6.54 x a -1 (half life 106 Ga) l Decay equation ( 144 Nd non-radiogenic) F 143 Nd/ 144 Nd = ( 143 Nd/ 144 Nd) o + ( 147 Sm/ 144 Nd)(e t -1) + ( 147 Sm/ 144 Nd)(e t -1) Decay equation Sm-Nd

Evolution opposite to Rb - Sr Figure After Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer.

Nd isotopes vs Sr isotopes W. White