Absolute Impact Ages and Cratering as a Function of Time With contributions from

1. Absolute Impact Ages and Cratering as a Function of Time With contributions from Timothy D. Swindle Donald D. Bogard David A. Kring

2. 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 -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 of • 40K to 40Ca). 40K 40Ar, 40Ca

3. 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.

4. 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.

5. 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)

6. Ar-Ar Geochronology Method (magmatic example) Step Heating Plateau ages of ~1375 Ma Low temperatures High temperatures Swindle & Olson (2004)

7. 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)

8. 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)

9. Ar-Ar Geochronology Method (impact melt example) Plateau age of 3800-3900 Ma Degassing event <2000 Ma Swindle et al. (2009)

10. 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. Bogard (1995)

11. 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.