ESS 109C Isotope Geochemistry Notes

April 19, 2007

 

Isochrons & planetary evolution

 

  1. Class notes & homeworks are now available online –
                                  http://www2.ess.ucla.edu/~schauble/Isotope_geochemistry/
  2. 2nd homework will be due next Monday (4-23-07).
    I will be traveling Thursday afternoon – Sunday, email for help.

  3. Isochron dating (87Rb/87Sr)
    1. The initial abundance of daughter nuclei in a well-mixed system can be determined if it subsequently fractionated, using the isochron method.

                                                          i.      D/S =  N/S(exp( lt) – 1) + D0/S

                                                        ii.      System must be isotopically uniform, then differentiate, then close.

1.     analyses of minerals will form linear array

2.     slope dependent on age (m = exp( lt) – 1)

3.     intercept dependent on initial concentration of daughter (b = D0/S)

    1. Pitfalls – persuasive nonsense

                                                          i.      Inherited heterogeneity/incomplete mixing

                                                        ii.      Daughter loss/daughter addition

    1. Isochron systems –

                                                          i.      Typically dispersed elements with long half-lives

1.     Dispersed: need a range of parent/daughter element ratios

2.     Long half-life: age resolution decreases, mixing errors worse for lt >> 1

                                                        ii.      87Rb ˆ 87Sr stable denominator 86Sr

1.     4.88x1010 year half life

2.     Both elements disperse, typically substitute for major elements K and Ca, respectively.

3.     Large Rb/Sr, 87Sr/86Sr variation in country rocks leads to mixing confusion

4.     Rb often very rare in basaltic rocks

                                                      iii.      147Sm ˆ 143Nd stable denominator 144Nd

1.     1.06x1011 year half life

2.     Subtle dispersal: both are rare-earth elements, modest substitution for Ca, but also concentrate in trace minerals

3.     Sm generally more ÒcompatibleÓ – concentrated in crystals

4.     Parent/daughter ratio variability can be small, limiting resolution (exceptions for garnet, REE-minerals)

5.     Both elements rare in many common minerals

                                                      iv.      176Lu ˆ 176Hf

1.     Very similar to Sm/Nd, with greater range of parent/daughter ratios

                                                        v.      187Re ˆ 187Os stable denominator 186Os

1.     4.3x1010 year half life

2.     Mysterious dispersal: both highly concentrated in sulfides/metal alloy grains, but apparently not in the same types.

3.     Os tends to be ÒcompatibleÓ – does not go into melt easily, Re incompatible

4.     Both elements very rare in most rocks

5.     Re commonly added after crystallization (open system)

6.     Os difficult to ionize to cation

                                                      vi.      40K ˆ 40Ca stable denominator 42Ca

1.     1.25x109 year half life

2.     Parent & daughter both major elements, strong dispersal

3.     Low 40K/40Ca typical, may limit resolution

4.     Much less well studied than 40K ˆ 40Ar, despite being most common decay mechanism.

    1. Long-lived radioactivity, isochrons, and the evolution of planetary reservoirs

                                                          i.      Rb/Sr, 87Sr/86Sr in seawater

                                                        ii.      Calcite as a recorder of ancient 87Sr/86Sr

                                                      iii.      Sources of radiogenic, depleted Sr to the oceans.