The formation of rims on calcium-aluminum-rich inclusions: Step I - Flash heating

Wark-Lovering rims of six calcium-aluminum-rich inclusions (CAIs) represent ing the main CAI types and groups in Allende, Efremovka and Vigarano were m icrosurgically separated and analysed by neutron activation analysis (NAA). All the rims have similar similar to 4x enrichments, relative to the inter iors, of highly refractory lithophile and siderophile elements. The NAA res ults are confirmed by ion microprobe and scanning electron microscope (SEM) analyses of rim perovskites and rim metal grains. Less refractory Eu, Yb, V, Sr, Ca and Ni are less enriched in the rims. The refractory element patt erns in the rims parallel the patterns in the outer parts of the CAIs. In p articular, the rims on type B1 CAIs have the igneously fractionated rare ea rth element (REE) pattern of the melilite mantle below the rim and not the REE pattern of the bulk CAI, proving that the refractory elements in the ri ms were derived from the outer mantle and were not condensates onto the CAI s. The refractory elements were enriched in an Al2O3-rich residue < 50 mum thick after the most volatile similar to 80% of the outermost 200 mum of ea ch CAI had been volatilized, including much Mg, Si and Ca. Some volatilizat ion occurred below the rim, and created refractory partial melts that cryst allized hibonite and gehlenitic melilite. The required "flash heating" prob ably exceeded 2000 degreesC, but for only a few seconds, in order to melt o nly the outer CAI and to unselectively volatilize slow-diffusing O isotopes which show no mass fractionation in the rim. The volatilization did, howev er, produce "heavy" mass-fractionated Mg in rims. In some CAIs this was lat er obscured when "normal" Mg diffused in from accreted olivine grains at re latively high temperature (not the lower temperature meteorite metamorphism ) and created the similar to 50 mum set of monomineralic rim layers of pyro xene, melilite and spinel.