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

In situ ion microprobe analyses of spinel in refractory calcium-aluminium-rich inclusions (CAIs) from type 3 EH chondrites yield O-16-rich compositions (delta O-18 and delta O-17 about -40 parts per thousand). Spinel and feldspar in a CAI from an EL3 chondrite have significantly heavier isotopic compositions (delta O-18 and delta O-17 about -5 parts per thousand). A regression through the data results in a line with slope 1.0 on a three-isotope plot, similar to isotopic results from unaltered minerals in CAIs from carbonaceous chondrites. The existence of CAIs with O-16-rich and O-16-poor compositions in carbonaceous as well as enstatite chondrites indicates that CAIs formed in at least two temporally or spatially distinct oxygen reservoirs. General similarities in oxygen isotopic compositions of CAIs from enstatite, carbonaceous, and ordinary chondrites indicate a common nebular mechanism or locale for the production of most CAIs.