LPI-JSC Center for Lunar Science and Exploration

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Absolute Impact Ages and Cratering as a Function
of Time With contributions from Timothy D.
Swindle Donald D. Bogard David A. Kring
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K-Ar Geochronology Method

• 40K (half-life 1.3 Ga) decays to 40Ca (89) and
• 40Ar (11) like sand through an
  hourglass.
• Rate proportional only to amount of 40K T1/2
• Measure amount 40K remaining 40Ar formed.
• Decay Eq ln (N / No) e -?t
• Age is t (1/?) ln ((40Ar/40K) (?/?e)
  1)
• where ? ln 2 / T1/2 is the total decay constant
  and the sum of ?e (decay of 40K to 40Ar) and ?ß
  (decay of
• 40K to 40Ca).

40K
40Ar, 40Ca
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Ar-Ar Geochronology Method

• Irradiate a K-bearing sample with neutrons to
  produce 39Ar from 39K
• (The nuclear reaction is 39K (n, p) 39Ar )
• 39Ar becomes a proxy for K is located in same
  lattice site as 40Ar from 40K
• Precisely measure with a mass spectrometer the
  Ar isotopic ratio, 40Ar/39Ar, eliminating the
  need to measure absolute concentrations of both K
  and Ar.
• Age given by t (1/?) ln
  ((40Ar/39Ar) J 1)
• J is a factor calculated from standards of known
  age irradiated with unknown samples. Age, t, is
  thus calculated relative to a standard age.
• The Ar-Ar method is more reliable than the K-Ar
  technique for most samples is now almost
  exclusively used. It is also ideal for small
  samples (e.g., impact melts from the Moon and in
  meteorites).
• Commonly degas measure Ar from sample in
  increasing temperature steps to examine age in
  different lattice sites.

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Ar-Ar Geochronology Method

• Some Issues
• Age of unknown sample only as accurate as age of
  standard sample.
• Sample may have contained 40Ar at the timeof
  formation. Resolve with isochron plot of
  40Ar/36Ar vs. 39Ar/36Ar (shown here) or
• 36Ar/40Ar vs. 39Ar/40Ar.
• Age is calculated from the slope
• Inherited 40Ar is given by the intercept
• Sample may have lost some 40Ar by diffusion out
  of grain surfaces.
• Prior loss typically revealed in Ar released at
  lower extraction temperatures.

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Simple Example of an Ar-Ar Age Spectrum

• Age boxes in red, K/Ca ratio in blue, for each
  temperature step.

• Slight prior diffusion loss of 40Ar at
  low-temperature.
• Varying K/Ca ratios indicate different K-bearing
  phases with same K-Ar age.

Low temperatures
High temperatures
Yamaguchi et al. (2001)
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Ar-Ar Geochronology Method
(magmatic example)
Step Heating
Plateau ages of 1375 Ma
Low temperatures
High temperatures
Swindle Olson (2004)
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Ar-Ar Geochronology Method
(magmatic example)
Step Heating
Low-T phases lost Ar or were degassed and,
thus, do not reflect age of crystallization.
Low temperatures
High temperatures
Swindle Olson (2004)
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Ar-Ar Geochronology Method
(magmatic example)
Step Heating
The nuclear reaction may create a recoil
effect that moves 39Ar from a K-rich phase into
a high-Ca, low-K phase, in this case pyroxene,
producing a fictitiously low age in the highest
T steps.
Low temperatures
High temperatures
Swindle Olson (2004)
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Ar-Ar Geochronology Method
(impact melt example)
Plateau age of 3800-3900 Ma
Degassing event lt2000 Ma
Swindle et al. (2009)
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An Example of the Methods Application
Apollo The radiometric ages of rocks from
the lunar highlands indicated the lunar
crust had been thermally metamorphosed 3.9
4.0 Ga. A large number of impact
melts were also generated at the same
time. This effect was seen in the Ar-Ar
system (Turner et al., 1973) and the U-Pb
system (Tera et al., 1974). It was also
preserved in the more easily reset Rb-Sr
system. (Data summary, left, from Bogard,
1995.) A severe period of bombardment was
inferred.
Bogard (1995)
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References
D.D. Bogard (1995) Impact ages of meteorites A
synthesis. Meteoritics 30, 244-268. T.D.
Swindle, C.E. Isachsen, J.R. Weirich, and D.A.
Kring (2009) 40Ar-39Ar ages of H-chondrite impact
melt breccias. Meteoritics Planet. Sci. 44,
747-762. T.D. Swindle and E.K. Olson (2004)
40Ar-39Ar studies of whole-rock nakhlites
Evidence for the timing of aqueous alteration on
Mars. Meteoritics Planet. Sci. 39, 755-766. F.
Tera, D.A. Papanastassiou, and G.J. Wasserburg
(1974) Isotopic evidence for a terminal lunar
cataclysm. Earth Planet. Sci. Lett. 22,
1-21. G. Turner, P.H. Cadogan, and C.J. Yonge
(1973) Argon selenochronology. Proc. Lunar
Planet. Sci. Conf. 4th, 1889-1914. A. Yamaguchi,
G.J. Taylor, K. Keil, C. Floss, G. Crozaz, L.E.
Nyquist, D.D. Bogard, D.H. Garrison, Y.D. Reese,
H. Wiesmann, and C.Y. Shih (2001)
Post-crystallization reheating and partial
melting of eucrite EET90020 by impact into the
hot crust of asteroid 4Vesta 4.50 Ga ago.
Geochim. Cosmochim. Acta 65, 3577-3599.