Thermal and Impact Histories of Meteorites and Their Parent Asteroids Ed Scott

1. Thermal and Impact Histories of Meteorites and Their Parent Asteroids Ed Scott HIGP, University of Hawaii, Honolulu, USA Yang J.*, Goldstein J. I.*, and Scott E. R. D. (2007) Nature. Yang, Goldstein and Scott (2008) Geochim. Cosmochim Acta. Goldstein, Scott, and Chabot (2008) Chemie de Erde, in preparation. Goldstein et al. (2008) Meteoritics & Planet. Sci., submitted. * University of Massachusetts Amherst Ascona Planets 2008 Conference

2. Igneous meteorites Chondrites Achondrites Irons Stony irons Carbon-aceous Ordinary Enstatite HEDs (basalts etc), Aubrites, Ureilites, Angrites H L 12 Groups (IAB, IC, IIAB etc.) LL EH EL CI CR CV CH Pallasites, Mesosiderites CM CO CK From asteroids that didn’t melt… Meteorite types From asteroids that did melt…

3. Igneous meteorites come from asteroids that melted to form metallic cores and silicate mantles Achondrites Irons Stony irons: pallasites Vesta: 500 km. Basaltic surface Kleopatra 220x95 km; iron

4. Ages of irons, basalts, CAIs and chondrules • Accretion of asteroids lasted 5 Myr • Asteroids that accreted in <1.5 Myr after CAIs melted • Asteroids that accreted at 2-5 Myr supply chondrites and did not melt. Sanders and Scott; Goldstein, Chabot, Scott

5. Radiometric ages consistent with 26Al heating Al isotopes 26Al half-life 730,000 yr 27Al stable • Early accreted bodies heated to higher temperatures • Impact melting less important Based on Sanders & Taylor (2005) Chondrites Igneous Meteorites

6. Why study thermal histories of meteorites? • Constrain parent body size • Elucidate planetesimal accretion • Identify and date impact heated rocks to constrain impact histories. E.g., • Did Moon-sized protoplanets form in a massive asteroid belt causing early high impact rates? • Did inefficient accretion over 5 Myr preclude protoplanet formation in asteroid belt leading to low total mass and low impact rates?

7. Did asteroids collide when hot or molten? No! H chondrites derived from 100 km radius body that cooled without disturbance (Kleine et al., 2008; Trieloff et al. 2005)

8. Did asteroids collide when hot or molten? Yes! H Chondrites s • Cooling rates from metallic Fe,Ni grains show weak correlation with extent of heating. • No evidence of impact heating in these meteorites. • H chondrite asteroid was partly scrambled by impact(s) as it cooled.

9. Structure of Iron Meteorites  how slowly they cooled • Oriented pattern caused by growth of Fe,Ni crystals in solid at ~800-900 K • Width and composition of crystals depend on bulk composition and cooling rate • Irons cooled at 1-5000 C/Myr • If iron cores cooled in silicate mantles, bodies were 5-300 km across Gibeon iron meteorite Compositions of Iron Meteorites  how they crystallized from melt

10. IVA iron meteorites • 60 meteorites with 7.5-10.5%Ni from one asteroid • Cooling rates at 800-900 K were 100-5000 K/Myr; decreased with increasing Ni • Variation within core insulated by silicate mantle should be < factor 2 • Either • Systematic variation with Ni is an artefact, or • IVA irons did not cool in core of asteroid Gibeon: mhmeteorites.com

11. Determining Cooling rates of Fe-Ni metal • Cooling rate calculation requires • Ni concentration profiles across oriented crystals • Equilibrium compositions • Diffusion rates • Nucleation mechanism

12. IVA Irons (Yang et al., 2008) Inverse correlation between cooling rates at ~500°C and bulk Ni incompatible with cooling in asteroidal core

13. Cooling rates from cloudy taenite • Submicrometer intergrowth of high- and low-Ni phases • Dimensions depend on cooling rate at 600 K Scanning electron microscope image of etched surface Transmission electron microscope image

14. Particle sizes in cloudy taenite correlate with cooling rate • Provides an independent measure of cooling rate at 600 K Particle sizes from Yang et al. 1997; Yang et al. 2007 Cooling rates from Yang et al. (2007), Yang and Goldstein (2006), Hopfe and Goldstein (2001), Yang et al. (1997), Buseck and Goldstein (1969)

15. Cloudy taenite in IVA irons • Use transmission electron microscopy • cloudy taenite size increases with increasing Ni • cooling rate decreases by factor ~15 across group ±1 Goldstein et al. (2008) Two independent techniques show IVA irons have diverse cooling rates  did not cool in core of asteroid

16. IVA irons cooled in metallic body without silicate mantle • Slowly cooled irons (100 K/Myr) fix radius at 150±50 km • Fast cooled irons (5000 K/Myr derived from ~0.97R • Metallic body initially at 1750 K -- surface temperature 200 K. • <1 km of silicate on surface • TWP and TCZ indicate the temperatures at which oriented crystals (Widmanstatten pattern) and cloudy taenite form R=150 km Yang et al. (2007)

17. How did the 300 km diameter metallic body form? • Impact capable of removing mantle would demolish and scramble core • Cooling rate-Ni correlation suggests molten metal crystallized inwards after impact

18. Use fractional crystallization model to determine radial position of IVA irons Ir (ppm) Au (ppm) r radial distance from center R radius • Ir and Au trends match with 3 wt.% S • Ga and Ge trends match with 9 wt.% S • (Yang et al., 2007b; Chabot 2004)

19. Combining fractional crystallization and thermal models gives a relationship between Ni and cooling rate IVA irons crystallized and cooled in a metallic body of radius 150 ± 50 km Yang et al. (2008)

20. Origin of IVA irons • Metal derived from core of 600+ km body (larger than Vesta) • Impact created molten body 300 km in diameter with <1 km of silicate • Irons crystallized and cooled in metallic body

21. Origin of iron and stony-iron meteorites in “hit-and-run” collisions -- Asphaug, Agnor, and Williams (2006) Chain of metal-enriched bodies • Oblique collision between Moon-sized projectile and Mars-sized target • Projectile drawn into a chain of objects with diverse compositions

22. Pallasites: samples from core-mantle boundaries? • Samples from at least six bodies • Main group compositionally linked to largest group of irons—IIIAB. • Main group pallasites thought to have formed around IIIAB core 1 cm  Olivine fragments in Fe-Ni metal (white)

23. Cooling rates of main-group pallasites from cloudy taenite • Cooled slower than IIIAB irons (25-70 K/Myr) • Did not cool around IIIAB or any other core • Cooling rates of 2-10 K/Myr indicate diverse burial depths Yang et al. (2007)

24. Origin of pallasites • Pallasites are mixtures of core and mantle materials but did not form at core-mantle boundary • Mantle fragments and small amount of molten Fe-Ni mixed in asteroidal or protoplanetary impact

25. Mesosiderites: stony iron meteorites made of metallic Fe-Ni and basalt, gabbro, dunite etc • Molten metal and cooler rocks were mixed in major collision that created a 300 km ball of broken rock and metal-rock mixtures

26. Ureilite achondrites • Igneous rocks formed deep inside partly molten 200 km wide asteroid • Catastrophically destroyed at ~4.55 Gyr so that rocks cooled from 1100°C in days • Impact debris reaccreted into new body

27. Iron meteorites with silicate inclusions IAB Landes: Jim Strope IAB: Four Corners: G. Notkin, Oscar Monnig Collection • Two groups: IAB and IIE • Both formed by impacts at 4.55 Gyr • Melting due to 26Al or impact

28. Conclusions Many asteroids disrupted or severely damaged by impacts when hot or molten at 4.55 Gyr Consistent with early massive belt Impact heating was minor relative to 26Al Some meteorites derived from collisions between protoplanets.