Evidence for primitive nebular components in chondrules from the Chainpur chondrite

The least equilibrated ordinary chrondrites contain chondrules which have experienced little change since the time of their formation in the early solar system. These chondrules are excellent indicators of the physical and chemical nature of the solar nebula. We separated 36 chondrules from the Chainpur (LL3.4) chondrite and analyzed each for 20 elements and petrographic properties. Sampling biases were minimized as far as possible.Chondrules seem to have formed through the melting of random mixtures of grains comprising a limited number of nebular components. The identity of these components can be deduced from chondrule compositions. The dominant components appear to be: 1) a mixture of metal and sulfide with composition similar to whole-rock metal and sulfide; 2) refractory (Ir-rich) metal; 3) refractory, olivine-rich silicates; 4) low-temperature, pyroxene-rich silicates, and, possibly, 5) a component containing the more volatile lithophiles.Most of the textural types of chondrules formed from the same set of precursor components. In some cases chondrules having different textures are almost identical in composition. A few, unusual chondrule types seem to mainly consist of uncommon nebular components, possibly indicating different modes of formation.Etching experiments confirm that chondrule rims are enriched in metal, troilite and moderately volatile elements relative to the bulk chondrules. However, a large fraction of the volatiles remains in the unetched interior.