Isotopes

Update:Midterm graded

Today:What are isotopesRadioactive decayAge datingIsotopes as fingerprint

Todays lecture

Spider DiagramsFig. 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.

Figure 9-5. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.REE/Spider Diagrams II12

Examples

Batch Melting D = 1 = even split, D < 1 = incompatible in minerals => enriched in meltD > 1 = compatible in minerals => depleted in melt

Figure 9-2. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.CL, CO = liquid, solid concentrationF = fraction melt produced = melt/(melt + rock)

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

CL/CO = (1/D) * (1-F) (1/D -1)

Crystallization like meltingWall-rock assimilationZone refiningCombinations of processes

Cox, Bell, Pankhurst

The Nature of Matter - Elements

Isotopes

# Protons is fixed, # Neutrons can vary (isotopes with different mass #)Hydrogen (1,2,3) (Average in nature is 1.008) Iron (54,56,57,58) (Average in nature is 55.85) Uranium (234,235,238) (Average in nature is 238.03)88 Naturally occurring elements - some have >1 isotopes8 elements make up > 98% of Earths crust (O, Si, Al, Fe, Ca, Mg, K, Na) 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 daughter3. Time(next slide)Example: 40K 40Ar by radioactive decayBasalt 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 eachIsotopic fractionation

Radioactive Isotopes & Decay# parent atomstime 1

Ages through isotopes

Rb-Sr System

y = b + x m = equation for a line in 87Sr/86Sr vs. 87Rb/86Sr plotSlope = (elt -1)Divide by stable 86Sr:

87Sr/86Sr = (87Sr/86Sr)o + (87Rb/86Sr)(elt -1)l = 1.4 x 10-11 a-1Isochron method

abcto

abca1b1c1t1to

abca1b1c1a2b2c2t1tot2

Isochron Technique

abca1b1c1a2b2c2t1tot2

Isochron results 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.Making Sr isotope reservoirs

The Sm-Nd System

147Sm 143Nd by alpha decayl = 6.54 x 10-13 a-1 (half life 106 Ga)Decay equation (144Nd non-radiogenic)143Nd/144Nd = (143Nd/144Nd)o + (147Sm/144Nd)(elt-1)Decay equation Sm-Nd

Evolution opposite to Rb - Sr

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

Nd isotopes vs Sr isotopes

W. White

***Order of elements based on estimates of increasing incompatibility from right to left in a "typical" mantle undergoing partial melting Elements are all incompatible (D (-) slope Additionally, FX subsequent to magma segregation from the source should tip the pattern even further **Eu* is the value Eu should have if Eu+2 did not -> plagioclase Another example of how RATIOS can helpEu alone is inconclusive (low REE of low Eu)Sm/Eu is slope or Eu anomaly trough (Use Eu*/Eu anyway)

****D = 1.0 No fractionation so CL/CO = 1 for all values of F**Can also apply the Rayleigh equation to Rayleigh fractional melting

**************Bt - Ms - Kfs******