Our main goal is to characterize the different O-rich reservoirs inferred to have been present in the solar nebula region where individual chondrite groups formed, or to have been present in the chondrite parent bodies and have been responsible for alteration effects increasingly recognized in the unequilibrated chondrites. There is increasing evidence that the oxygen-isotopic composition of the nebular gas evolved during the period recorded in the various classes on chondritic meteorites. We will use the UCLA Cameca 1270 ion-microprobe to study O-isotopes in key phases that were formed at low temperatures in the solar nebula or during aqueous alteration that occurred within the parent asteroid. The high efficiency and high resolution of the Cameca 1270 permits us to determine the three oxygen isotopes in phases exposed in petrographic thin sections, thus textural and chemical information about the formation process is preserved. During the first two years of this grant we plan to focus our studies on the alteration product magnetite in R and ordinary chondrites. When magnetite forms from metal or FeS, all the O comes from the oxidant, inferred to have been H2O in most cases. We will also study another alteration product, the fayalite present in CV chondrites.
