ESS 109C Isotope Geochemistry Notes

April 23, 2007

High-precision Geochronology

1. Class notes & homeworks are available online –
   http://www2.ess.ucla.edu/~schauble/Isotope_geochemistry/
2. 2^nd homework due Today (4-23-07).
   
   
3. High-precision Geochronology
1. Most precise, “first-choice” geochronometers are not simple isochron systems.

i. Most long-lived radioactive isotopes have ~0.5-1% uncertainty in decay constants.

1. ~2Ma error at 200Ma, ~30Ma at 4.5 Ga.

ii. Need to separately analyze daughter and parent abundances can also introduce error.

iii. Often difficult to discern minor, or even major addition/loss of parent and daughter isotopes after crystallization.

2. ⁴⁰Ar*/³⁹Ar, U,Th/Pb typically yield highest precisions, in applicable geological contexts.

i. Both systems have “tricks” to overcome some aspects of three drawbacks to isochron methods.

ii. Serendipitous “flukes” of nuclear chemistry & crystal chemistry

4. ⁴⁰Ar*/³⁹Ar: neutrons are the secret!
1. Despite name, based on decay of ⁴⁰K to ⁴⁰Ar (e- capture)
2. Difference from K-Ar à partial conversion of ³⁹K to ³⁹Ar by exposure to neutron source (nuclear reactor

i. ³⁹K + ¹n à ³⁹Ar + ¹p⁺ [³⁹K (n, p) ³⁹Ar]

ii. ³⁹K is stable, assumed to occur in constant proportion to ⁴⁰K

iii. ³⁹Ar unstable (t_1/2 = 269 years), assumed absent before exposure

iv. Standard K-bearing crystals used to monitor progress of conversion (neutrons are hard to control with precision: can’t use a mass-spectrometer or ion optics!).

³⁹Ar(irradiation) = ³⁹K*t_irradiation*ƒ(neutron flux, cross-section, neutron energy)

³⁹K/⁴⁰K(today) = constant

⁴⁰K(today) = ⁴⁰K/³⁹K(today) * ³⁹Ar(measured)/(t _irradiation*ƒ(neutron flux, cross-section, neutron energy))

v. Before using standard decay equation, need to account for branched decay of ⁴⁰K (most goes to ⁴⁰Ca):

⁴⁰Ar*/³⁹Ar = (e^lt – 1) * l_e/l_tot *⁴⁰K/³⁹K(today)/(t _irradiation*ƒ(neutron flux, cross-section, neutron energy))

Everything after the exponential term is rolled into a function “J”.

vi. Thus, if we have samples of known age, and an Ar-mass spectrometer, we can measure the knowns and solve for “J”:

⁴⁰Ar*/³⁹Ar(std, measured) = (e^lt(known) – 1)/J

The same J is then assumed to apply to the unknown.

3. Advantage: both daughter and parent are analyzed on the same mass spectrometer
4. Initial ⁴⁰Ar can be assessed by analyzing ³⁶Ar and ³⁸Ar, which are stable istopes of Ar.

i. Works perfectly if ⁴⁰Ar/³⁸Ar/³⁶Ar always occur in constant proportion

ii. Ar-isotope standard?

iii. Deviations from standard?

5. Further advantage: both are noble gasses, and can be liberated from a sample by simply heating it, and gas sent (almost) directly through a mass spectrometer.

i. Gas may be released in increments (step heating), or on specific portions of a sample (laser heating)

6. Limitations:

i. Must have enough K and age to generate ³⁹Ar and ⁴⁰Ar*

ii. Results dependent on well-characterized standard materials of independently known age (typically via U,Th-Pb dating of zircons).

iii. Irradiation may produce ³⁹Ar from other nuclei besides ³⁹K, esp. isotopes of Ca

1. May not work well for high Ca/K materials.

iv. Ar diffuses rapidly at high temperatures

1. “Cooling ages” rather than crystallization age

2. a bug and a feature (wait ‘til Thursday)!

7. Sample types (high K/Ar, K/Ca, Ar-retentive at ambient temperature):

i. Biotite, muscovite, K-spar (volcanics best)

ii. Hornblende (K-Ca substitution, but can have high Ca/K)

iii. Volcanic glass (Ar diffusion in & out may be significant)

iv. Glauconite or clay (often not reliable at high precision)

v. Fluid inclusions (quartz)!

8. Excess ⁴⁰Ar