Absolute Impact Ages
and
Cratering as a Function of Time
With contributions from
Timothy D. Swindle
Donald D. Bogard
David A. Kring
LPI-JSC Center for Lunar Science and Exploration
2011
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
40K
• Measure amount 40K remaining & 40Ar formed.
• Decay Eq:
• Age is:
ln (N / No) = e -λt
t = (1/λ) ln ((40Ar*/40K) (λ/λe) + 1)
where λ = ln 2 / T1/2 is the total decay constant and the
sum of λe (decay of 40K to 40Ar) and λβ (decay of
40K
to 40Ca).
LPI-JSC Center for Lunar Science and Exploration
2011
40Ar,
40Ca
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/λ) 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.
LPI-JSC Center for Lunar Science and Exploration
2011
Ar-Ar Geochronology Method
• Some Issues:
• Age of unknown sample only as accurate as age of standard sample.
• Age is calculated from the slope
5
36
Ar / Ar
4
3
2
40
• Sample may have contained 40Ar at the time
of formation. Resolve with isochron plot of
40Ar/36Ar vs. 39Ar/36Ar (shown here) or
36Ar/40Ar vs. 39Ar/40Ar.
1
0
• Inherited 40Ar is given by the intercept
0
1
2
39
3
4
36
Ar / Ar
• Sample may have lost some 40Ar by diffusion out of grain surfaces.
Prior loss typically revealed in Ar released at lower extraction temperatures.
LPI-JSC Center for Lunar Science and Exploration
2011
Simple Example of an Ar-Ar Age Spectrum
• Age ‘boxes’ in red, K/Ca ratio in blue, for each temperature step.
• Slight prior diffusion
4.60
loss of 40Ar at lowtemperature.
eucrite EET-90020,26
4.55
0.009
4.50
4.45
0.006
K / Ca
Ar-40Ar AGE Ga
Plateau Age =4,491 +-11 Myr
39
• Varying K/Ca ratios
indicate different Kbearing “phases” with
same K-Ar age.
0.012
4.40
0.003
4.35
4.30
0.000
0.0
0.2
0.4
39
Low temperatures
LPI-JSC Center for Lunar Science and Exploration
2011
0.6
0.8
1.0
Ar Cumulative Fraction
High temperatures
Yamaguchi et al. (2001)
Ar-Ar Geochronology Method
(magmatic example)
Step Heating
Plateau ages of ~1375 Ma
Low temperatures
LPI-JSC Center for Lunar Science and Exploration
2011
High temperatures
Swindle & Olson (2004)
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
LPI-JSC Center for Lunar Science and Exploration
2011
High temperatures
Swindle & Olson (2004)
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
LPI-JSC Center for Lunar Science and Exploration
2011
High temperatures
Swindle & Olson (2004)
Ar-Ar Geochronology Method
(impact melt example)
Plateau age of 3800-3900 Ma
Degassing event <2000 Ma
LPI-JSC Center for Lunar Science and Exploration
2011
Swindle et al. (2009)
An Example of the Method’s 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.
LPI-JSC Center for Lunar Science and Exploration
2011
Bogard (1995)
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 Hchondrite 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.
LPI-JSC Center for Lunar Science and Exploration
2011