Chronological evidence for the Late Heavy Bombardment in ordinary chondrite meteorites

1. Chronological evidence for the Late Heavy Bombardment in ordinary chondrite meteorites and David A. Kring Lunar and Planetary Institute Houston TX Timothy D. Swindle Lunar and Planetary Laboratory University of Arizona Tucson AZ

2. Moon has been center of discussion • “Lunar cataclysm” proposal • Turner et al. (1973), Tera et al. (1974) • Graham Ryder’s resurrection of cataclysm (1990) • Why the lunar focus? • Abundant, well-studied samples with known geological context

3. But the Moon isn’t the only object recording the impact flux … Asteroids should record any effect not local to Earth-Moon system And most meteorites come from asteroids … You are here Location of known comets (solid blue squares), NEOs (red), Main Belt asteroids (green) and Trojan asteroids (open blue), along with planetary orbits.

4. How to date a crater • Date last episode of isotopic equilibration (or thermal loss) • Usually K-Ar or U/Th-Pb • In impact, more material moved than heated • Large impacts heat more material than small impacts • Material that cools slowly more likely to be degassed/reequilibrated • Recrystallized melt better sample than glass • Clasts next to melt may be better than melt (diffusion path length?) Painting by W. K. Hartmann

5. HED meteorites (4 Vesta) • Bogard noted large number of ages 3.4-4.1 Ga • Not a local event • Not consistent with “classic” cataclysm (spike at ~3.9 Ga) • When first noted (1995), little evidence of similar effect in ordinary chondrites • Bogard et al. (2000) found impact age of ~3700 Ma in 3 (of 8) IIE irons Age-probability curve for Ar-Ar ages of eucrites (from asteroid 4 Vesta), from Bogard (2005, LPSC XXXVI, #1131). Dark line is (renormalized) sum of individual meteorites’ analyses.

6. Growing the OC database • With vast number of Antarctic, hot desert meteorites, now finding several impact melts or impact melt breccias per year • Several groups working on these (Bogard, Trieloff, us) • UA/LPI approach: multiple samples, look for common signature Three samples of an impact melt all give plateaus with apparent ages between 3900 Ma and 4000 Ma (from Swindle et al., submitted).

7. Growing the OC database • With vast number of Antarctic, hot desert meteorites, now finding several impact melts or impact melt breccias per year • Several groups working on these (Bogard, Trieloff, us) • UA/LPI approach: multiple samples, look for common signature Gao-Guenie samples are more complicated. Three samples of unmelted material (A1) all have small plateaus or minima at ~300 Ma. Two impact melt samples (A2) disagree with each other and with the unmelted material, making any further chronological interpretation problematic (from Swindle et al., submitted).

8. H chondrites • Three consistent with accretion-era ages (>4400 Ma) • Six at 3600-4000 Ma, one more at ~4100 Ma • Nothing 1500-3600 Ma • Very similar to eucrites except for young ages • More similar to lunar glasses, lunar meteorite impact melts

9. L chondrites • Dominated by 470 Ma breakup • Most shocked OC’s are L chondrites from this event • Dominated early studies of shocked chondrites • Six at 4400-4500 Ma • One at 3800 Ma, two at 3000-3200 Ma • Gaps 4400-4000 Ma, 3200-3800 Ma (?), 1200-3000 Ma • Accretion-era impact melts can definitely survive

10. LL chondrites • Dominated by ages of 4200-4300 Ma! • Most, but not all, from Dixon et al. (2003) • Two at ~3900 Ma • Nothing 2000-3900 Ma • Only sample set where pre-1 Ga ages aren’t dominated by either 3500-4000 Ma or >4400 Ma ages

11. Summary & Conclusions • Ages 3500-4000 Ma found in all three types of OC, plus HEDs, IIE irons • Dominate early events in HED, H, IIE • Accretion-era (>4400 Ma) impact melts found in H, L • It is possible for them to survive • Ages 4000-4400 Ma rare (except for LL, where they dominate) • Ages 1500-3000 Ma almost nonexistent (1 meteorite), 3000-3500 Ma rare but not nonexistent