The oxygen-isotopic record in enstatite meteorites

Oxygen-isotopic compositions were determined for a suite of enstatite chond rites and aubrites. In agreement with previous work (Clayton et al., 1984), most samples have O-isotopic compositions close to the terrestrial fractio nation line (TFL), and there appear to be no significant differences in O-i sotopic compositions between individual EH and EL chondrites and aubrites. Five enstatite meteorites have O-isotopic compositions that are significant ly different from the other samples and >0.2 parts per thousand away from t he TFL. Two of these have petrographic evidence of brecciation and interact ion between other meteorite types; for the other three, similar scenarios a re suggested. There appears to be a systematic increase in delta(18)O from enstatite chondrites (both EH and EL) of petrologic type 3 to those of type 6. There is also good evidence that the EH meteorites do not fall along a mass fractionation line but along a line slope 0.66. At the present time, d etailed understanding of the origin of these O-isotopic systematics remain elusive but clearly point to a complex accretion history, parent-body evolu tion, or both.