Calcium-aluminum-rich inclusions in enstatite chondrites (II): Oxygen isotopes

In situ ion microprobe analyses of spinel in refractory calcium-aluminium-r ich inclusions (CAIs) from type 3 EH chondrites yield O-16-rich composition s (delta O-18 and delta O-17 about -40 parts per thousand). Spinel and feld spar in a CAI from an EL3 chondrite have significantly heavier isotopic com positions (delta O-18 and delta O-17 about -5 parts per thousand). A regres sion through the data results in a line with slope 1.0 on a three-isotope p lot, similar to isotopic results from unaltered minerals in CAIs from carbo naceous chondrites. The existence of CAIs with O-16-rich and O-16-poor comp ositions in carbonaceous as well as enstatite chondrites indicates that CAI s formed in at least two temporally or spatially distinct oxygen reservoirs . General similarities in oxygen isotopic compositions of CAIs from enstati te, carbonaceous, and ordinary chondrites indicate a common nebular mechani sm or locale for the production of most CAIs.