TRACE-ELEMENT ABUNDANCES AND MAGNESIUM, CALCIUM, AND TITANIUM ISOTOPIC COMPOSITIONS OF GROSSITE-CONTAINING INCLUSIONS FROM THE CARBONACEOUS CHONDRITE ACFER-182

The carbonaceous chondrite Acfer 182 contains Ca, Al-rich inclusions(CAIs) that differ from most CAIs in other meteorites. Many of them contain grossite (CaAl4O7), whose modal abundance exceeds 30 vol % in most of these CAIs (''grossite-rich inclusions''). Similar inclusions have been found only in the CH chondrite ALH85085 and the CR chondrite Acfer 059-EI Djouf 001. Ion microprobe analyses of trace elements were made on nineteen CAIs from Acfer 182 and two CAIs from Acfer 059-El Djouf 001, of Al-Mg isotopes on eighteen Acfer 182 and the two Acfer 059-El Djouf 001 inclusions, and of calcium isotopes on ten Acfer 182 inclusions (on six of them also of titanium isotopes) and one Acfer 059-El Djouf 001 inclusion. Trace element and isotopic signatures of grossite-containing inclusions from Acfer 182 resemble those of some inclusions from ALH85085. Volatility-fractionated trace element abundance patterns, ranging from ultrarefractory-depleted (similar to Group II pattern) to volatile-enhanced, are predominant (17 CAIs, including both inclusions from Acfer 059-El Djouf 001). Two inclusions have ultrarefractory patterns, one has a Group III pattern, and one inclusion has a Group III-related pattern similar to patterns found in HAL-type inclusions. Only one CAI from Acfer 182, but both inclusions from Acfer 059-El Djouf 001, have nonlinear excesses of Mg-26 corresponding to the initial Al-26/Al-27 ratio of similar to 5 x 10(-5). None of the twenty inclusions has a significant intrinsic isotopic mass fractionation of Mg. Titanium is normal in all analyzed CAIs and only small Ca-48 excesses (4.3 and 3.8 parts per thousand) are present in two inclusions. Isotopes of both Ca and Ti show no significant intrinsic mass fracrionations. Grossite-containing inclusions from Acfer 182 occur in different petrographic contexts. In many cases petrographic characteristics are correlated with trace element abundances. Almost all inclusions studied in this work have a condensation history but no evidence for extensive evaporation processes is present. Many of the CAIs must have formed by direct gas-solid condensation, while some inclusions with clearly identifiable igneous textures formed from a refractory melt. Grossite, whose formation was the subject of controversy in the past, probably condensed from a reservoir with a higher-than-chondritic Ca/Al-ratio and depleted in the highly refractory trace elements, os crystallized from a Ca,Al-rich melt. The two ultrarefractory inclusions from Acfer 182 have extremely high refractory trace element abundances(sometimes exceeding 10(5) x CI). The lack of isotopic fractionation indicates a formation at very high temperatures by condensation rather than evaporation. The lack of Mg-26 excesses in almost all inclusions from Acfer 182 is best explained by heterogeneous distribution of Al-26 in the primitive solar nebula. Grossiterich inclusions having volatility-fractionated ultrarefractory-depleted trace element abundance patterns and lacking Mg-26 excesses could be the complement of platy hibonites (PLACs) from Murchison with Group III or ultrarefractory patterns and likewise no Mg-26 excesses.