1. LPI-JSC Center for Lunar Science and Exploration 2011 Absolute Impact Ages and Cratering as a Function of Time With contributions from Timothy D. Swindle Donald D. Bogard David A. Kring

2. LPI-JSC Center for Lunar Science and Exploration 2011 40 K (half-life 1.3 Ga) decays to 40 Ca (89%) and 40 Ar (11%) – like sand through an hourglass. Rate proportional only to amount of 40 K & T 1/2 Measure amount 40 K remaining & 40 Ar formed. Decay Eq: ln (N / N o ) = e -λt Age is: t = (1/λ) ln (( 40 Ar*/ 40 K) (λ/λ e ) + 1) where λ = ln 2 / T 1/2 is the total decay constant and the sum of λ e (decay of 40 K to 40 Ar) and λ β (decay of 40 K to 40 Ca). 40 K 40 Ar, 40 Ca K-Ar Geochronology Method

3. LPI-JSC Center for Lunar Science and Exploration 2011 Irradiate a K-bearing sample with neutrons to produce 39 Ar from 39 K (The nuclear reaction is 39 K (n, p) 39 Ar ) 39 Ar becomes a proxy for K & is located in same lattice site as 40 Ar from 40 K Precisely measure with a mass spectrometer the Ar isotopic ratio, 40 Ar/ 39 Ar, eliminating the need to measure absolute concentrations of both K and Ar. Age given by: t = (1/λ) ln (( 40 Ar*/ 39 Ar) 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

4. LPI-JSC Center for Lunar Science and Exploration 2011 Some Issues: Age of unknown sample only as accurate as age of standard sample. Sample may have contained 40 Ar at the time of formation. Resolve with isochron plot of 40 Ar/ 36 Ar vs. 39 Ar/ 36 Ar (shown here) or 36 Ar/ 40 Ar vs. 39 Ar/ 40 Ar. Age is calculated from the slope Inherited 40 Ar is given by the intercept Sample may have lost some 40 Ar by diffusion out of grain surfaces. Prior loss typically revealed in Ar released at lower extraction temperatures. Ar-Ar Geochronology Method

5. LPI-JSC Center for Lunar Science and Exploration 2011 Age ‘boxes’ in red, K/Ca ratio in blue, for each temperature step. Slight prior diffusion loss of 40 Ar at low- temperature. Varying K/Ca ratios indicate different K- bearing “phases” with same K-Ar age. Simple Example of an Ar-Ar Age Spectrum Yamaguchi et al. (2001) Low temperatures High temperatures

6. LPI-JSC Center for Lunar Science and Exploration 2011 Ar-Ar Geochronology Method (magmatic example) Plateau ages of ~1375 Ma Low temperatures High temperatures Step Heating Swindle & Olson (2004)

7. LPI-JSC Center for Lunar Science and Exploration 2011 Ar-Ar Geochronology Method (magmatic example) Low temperatures High temperatures Step Heating Low-T phases lost Ar or were “degassed” and, thus, do not reflect age of crystallization. Swindle & Olson (2004)

8. LPI-JSC Center for Lunar Science and Exploration 2011 Ar-Ar Geochronology Method (magmatic example) Low temperatures High temperatures Step Heating The nuclear reaction may create a “recoil” effect that moves 39 Ar 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)

9. LPI-JSC Center for Lunar Science and Exploration 2011 Ar-Ar Geochronology Method (impact melt example) Plateau age of 3800-3900 Ma Degassing event <2000 Ma Swindle et al. (2009)

10. LPI-JSC Center for Lunar Science and Exploration 2011 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)

11. LPI-JSC Center for Lunar Science and Exploration 2011 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) 40 Ar- 39 Ar ages of H- chondrite impact melt breccias. Meteoritics Planet. Sci. 44, 747-762. T.D. Swindle and E.K. Olson (2004) 40 Ar- 39 Ar 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. 4 th, 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.