Chondrites and the protoplanetary disk

Major advances in deciphering the record of nebula processes in chondrites can be attributed to analytical improvements that allow coordinated isotopic and mineralogical studies of components in chondrites and to a wealth of new meteorites from hot and cold deserts. These studies have identified a few rare pristine chondrites that largely escaped heating and alteration in asteroids, which have matrices composed of submicrometer-sized grains of enstatite and forsterite and amorphous silicates, as found in comets. Isotopic analyses of components in pristine chondrites using short-lived nuclide chronometers, Pb-Pb dating, and oxygen isotopes aided by laboratory and theoretical studies of chondrites and differentiated meteorites have provided key constraints on the processes that shaped the early solar system. These processes were once thought to have followed one another sequentially over a period of several million years: chondrule formation; planetesimal accretion; alteration, metamorphism, and melting in planetesimals; and finally, high-velocity collisions between asteroids. Radiometric dating shows, however, that these processes overlapped so that chondrules were still forming in the nebula several million years after early-formed planetesimals had melted and collided. Chondrites are extraordinary mixtures of presolar and solar nebula materials and asteroidal debris.