ESS 109C Isotope Geochemistry Notes

April 30, 2007

 

Cosmogenic Nuclides

 

  1. Class notes & homeworks are available online –
                                  http://www2.ess.ucla.edu/~schauble/Isotope_geochemistry/
  2. 3rd homework due Wednesday (5-2-07).

  3. Cosmogenesis
    1. All the decay systems so far stem from parents with half-lives on the same order as the age of the solar system (or longer).

                                                          i.      Leftovers from nucleosynthesis, condensation of the Earth.

    1. Short-lived parents also occur in nature – must be produced within the Solar System/Earth
    2. Most important – interaction of stable nuclei with cosmic rays

                                                          i.      Cosmic ray : high-energy nucleus (usually 1H+ or 4He2+) accelerated towards Earth with ~109 eV

1.     Also Solar energetic particles, generally much lower in energy. (< MeV)

                                                        ii.      First (primary) reaction is usually fragmentation (spallation) of a target to protons & neutrons

1.     Recall binding energies typically ~8 MeV/nucleon, << cosmic ray

2.     Creates shower of neutrons, protons, and muons (like a massive electron)

3.     Lower energy, but still able to react with nuclei.

                                                      iii.      Particle reactions most common in upper atmosphere (~10-20km)

1.     Too high – not enough overlying primary spallations.

2.     Too low – spallation neutrons largely absorbed.

3.     Most common products formed from N, O, Ar

4.     Small amount of cosmogenic nuclide formation in uppermost ocean, crust, mainly from O, Si, etc.

  1. 14C (radiocarbon)
    1. Half-life ~5730 years.
    2. Produced mainly from 14N:
      147N + 10n ˆ 146C + 11p
      also written 14N (n,p) 14C
    3. Originally assumed to be produced at a roughly constant rate.

      14C/Ctotal (atm) Å constant.

    4. Any organism that fixes carbon will incorporate this ratio. Once fixed, organic 14C will decay (no longer taking in fresh carbon)

      A = A0e–lt

      rearranging, t = (lnA0 – lnA)/l

    5. Early dates based on counting of decays in a known mass of carbon from sample. In this case, A = –d14C /dt = –l14C
    6. More recent measurements take advantage of accelerator mass spectrometry.

                                                          i.      Most mass spectrometers not capable of separating rare 14C from 14N

                                                        ii.      Allow to measure 14C directly, rather than wait for rare decay events. Much better sensitivity.

  1. Complications
    1. Inheritance of aged carbon
    2. Secular variation of 14C production.