Absolute Impact Agesand

Cratering as a Function of Time

With contributions fromTimothy D. SwindleDonald D. Bogard

David A. Kring

• 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 -λt

• Age is: 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 of40K to 40Ca).

40K

40Ar,40Ca

K-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.

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.

0

1

2

3

4

5

0 1 2 3 439Ar / 36Ar

40A

r / 36

Ar

Ar-Ar Geochronology Method

• 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.

eucrite EET-90020,26

4.30

4.35

4.40

4.45

4.50

4.55

4.60

0.0 0.2 0.4 0.6 0.8 1.039Ar Cumulative Fraction

39A

r-4

0A

r A

GE

Ga

0.000

0.003

0.006

0.009

0.012

K /

Ca

Plateau Age =4,491 +-11 Myr

Simple Example of an Ar-Ar Age Spectrum

Yamaguchi et al. (2001)

Low temperatures High temperatures

Ar-Ar Geochronology Method (magmatic example)

Plateau ages of ~1375 Ma

Low temperatures High temperatures

Step Heating

Swindle & Olson (2004)

Ar-Ar Geochronology Method (magmatic example)

Low temperatures High temperatures

Step Heating

Low-T phases lost Ar or were “degassed” and, thus, do not reflectage of crystallization.

Swindle & Olson (2004)

Ar-Ar Geochronology Method (magmatic example)

Low temperatures High temperatures

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.

Swindle & Olson (2004)

Ar-Ar Geochronology Method (impact melt example)

Plateau age of 3800-3900 Ma

Degassing event <2000 Ma

Swindle et al. (2009)

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.

An Example of the Method’s Application

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 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.