THE ORIGIN OF CHROMITIC CHONDRULES AND THE VOLATILITY OF CR UNDER A RANGE OF NEBULAR CONDITIONS

We characterize ten chromitic chondrules, two spinelian chondrules and one spinel-bearing chondrule and summarize data for 120 chromitic inc lusions discovered in an extensive survey of ordinary chondrites. Comp ositional and petrographic evidence suggests that chromitic chondrules and inclusions are closely related. The Cr/(Cr + Al) ratios in the sp inel of these objects range from 0.5 to 0.9 and bulk Al2O3 contents ar e uniformly high (> 10 wt%, except for one with 8 wt%). No other eleme nts having comparable solar abundances are so strongly enriched, and a lkali feldspar and merrillite are more common than in normal chondrule s. The Cr/Mg ratios in chromitic chondrules are 180-750 x the ratios i n the bulk chondrite. With the possible exception of magnetic clumping of chromite in the presolar cloud, mechanical processes cannot accoun t for this enrichment. Examination of nebular equilibrium processes sh ows that 50%-condensation temperatures of Cr at pH 2/pH 20 of 1500 are several tens of degrees below those of Mg as Mg2SiO4; the condensatio n of Cr is primarily as MgCr2O4 dissolved in MgAl2O4 at nebular pressu res of 10(-4) atm or below. At pH2 = 10(-3) atm condensation as Cr in Fe-Ni is favored. Making the nebula much more oxidizing reduces the di fference in condensation temperatures but Mg remains more refractory. We conclude that nebular equilibrium processes are not responsible for the enhanced Cr/ Mg ratios. We propose that both Cr and Al became enr iched in residues formed by incomplete evaporation of presolar lumps. We suggest that spinels remained as solid phases when the bulk of the silicates were incorporated into the evaporating melt; vaporization of Al and Cr were inhibited by the slow kinetics of diffusion. Subsequen t melting and crystallization of these residues fractionated Cr from A l. The resulting materials constituted major components in the precurs ors of chromitic chondrules. Our model implies that chromitic chondrul es and inclusions preserve the Cr isotopic record of presolar sources.