HR: 09:30h
AN: P31C-07 INVITED     [Abstracts]
TI: A Framework for Chondrite Formation in the Nebula
AU: * Wood, J A
EM: jwood@cfa.harvard.edu
AF: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138-2501 United States
AB: More than a dozen subclasses of chondritic meteorites, the most primitive accessible samples of planetary material, are recognized. It has long been assumed that these subclasses formed at different times and/or places in what is now the asteroid belt. Recently the ages of individual chondrule in particularly primitive chondrites have been determined by radiometric techniques, and these data, though still fragmentary, permit placement of the subclasses in a crude time - radial distance matrix. Distances are related to spectral correspondences between the chondrite subclasses and present families of asteroids, and times to the age ranges found for chondrules in the various subclasses, which are unexpectedly long and which seem to be offset from one another. The variable abundance of refractory inclusions, which formed in a brief period at the beginning of nebular activity, in chondrite subclasses provide another age criterion. The mm-sized chondrules that comprise the bulk of most chondritic material were created by brief energetic events in the solar nebula, probably the action of energetic shock waves on precursor solids (this, at least, is the conventional wisdom). The process was modulated by the presence of carbon (probably presolar organic compounds; Nakano et al., 2003) in the precursor material, which during chondrule melting reduced Fe oxides to varying degrees, providing a basis for metal/silicate fractionation and the variability in bulk Fe/Si that is one of the properties differentiating the chondrite subclasses. Greatly enhanced system C/O was needed at the time and place when the end-member enstatite chondrite subclass were formed. The source of the putative chondrule-forming shocks constitutes an interesting problem. Chondrules were formed over too long a time period (3 Myr or more) for the shocks to have been caused by (early) gravitational instability in the nebula. Tidal disturbances by early stellar companions of the sun (Larson, 2002) are a more plausible mechanism, but they may be inconsistent with the present orderly geometry of the planetary system.
DE: 6008 Composition
DE: 6040 Origin and evolution
DE: 6045 Physics and chemistry of materials
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting