TRACE-ELEMENT CONTENTS OF CHONDRULE RIMS AND INTERCHONDRULE MATRIX IN ORDINARY CHONDRITES

The abundances of forty elements have been measured in the rims and matrices of the unequilibrated ordinary chondrites Semarkona, Bishunpur, Chainpur, and Krymka. The rims appear to be mixtures of matrix-like material and material that was volatilized during chondrule formation. Iron, silicon, manganese, and copper all appear to have behaved as volatiles during chondrule formation and their enrichments are generally confined to the rims, suggesting recondensation of these elements was rapid and localized. This is supported by petrologic observations that show many chondrules were still partially molten or plastic when the rims accreted. Some rims also exhibit a decrease in both their FeO content and the degree of mineral re-equilibration away from the chondrule. The alkali metals are enriched in both the rims and matrices. The alkalis may have recondensed onto the matrix as well as the rims, but their mobility is such that they have probably undergone extensive redistribution after accretion. Some Na may even have been able to diffuse back into chondrules either during chondrule cooling or during metamorphism/alteration. Most refractory lithophiles are depleted with respect to Al in the rims and matrices in all four UOCs. These depletions are probably due to postaccretional redistribution of these elements into phosphates and other secondary minerals that form in and around metal/sulfide grains. The matrix is probably composed of chondrule fragments and processed nebular dust, but the presence of carbonaceous presolar grains in the matrix indicates that there must be unprocessed presolar silicates present too. On average, the presolar silicates presumably have a CI-like composition but a CI-like signature is not seen in the matrix compositions. Simple estimates suggest that the abundances of carbonaceous presolar grains are consistent with there being at most 10 wt% presolar silicates present in UOC matrix, which would be hard to detect in this study.