CA-,AL-RICH INCLUSIONS IN THE UNIQUE CHONDRITE ALH85085 - PETROLOGY, CHEMISTRY, AND ISOTOPIC COMPOSITIONS

The unique chondrite ALH85085 contains Ca-,Al-rich inclusions (CAIs) t hat differ from CAIs found in other types of meteorites. The ALH85085 inclusions are smaller (5-80 mum) and more refractory than their count erparts in carbonaceous chondrites. We have performed a comprehensive study of these inclusions: forty-two for petrography and mineralogy, f ifteen for bulk major and minor element chemical composition, six for Mg-Al isotopic systematics, ten for Ca isotopes, nine for Ti isotopes, and six for trace element abundances. In addition, oxygen-isotopic co mpositions were determined in minerals from a single inclusion (CAI 12 5). The mineralogy of the CAIs is dominated by oxide phases. About hal f of the inclusions contain either hibonite (hib) or spinel (sp), comm only associated with melilite (mel) or perovskite (pv); the others con tain calcium dialuminate (CaA2), rarely found in CAIs from other meteo rites. Most CAIs are spherical in shape and many exhibit igneous textu res indicating crystallization from a melt. Textures in others are rem iniscent of recrystallization due to thermal processing. Many of the C AIs are coated with a double layer of gehlenitic melilite and diopside or a single layer of diopside. The CAIs analyzed for Ca and Ti have n ormal isotopic compositions except for two that have nonlinear 48Ca ex cesses of 7.4 and 11.7 parts per thousand, respectively. Only three CA Is show Mg-26 excesses, two of them have (Al-26/Al-27)0 approximately 5 X 10(-5), the third has 2 X 10(-6). There is no obvious correlation between Al-Mg systematics and any other properties of the inclusions. All minerals (sp, pv, hib, CaA2), including melilite, in CAI 125 have O-16 excesses of approximately 5%, indicating that ALH85085 inclusions never experienced the thermal event that caused the oxygen isotopic e quilibration of melilite in Allende CAIs. The ALH85085 inclusions are fractionated in their refractory element abundances: Most of them have very high Al/Ca and Ti/Ca ratios, four CAIs have volatility fractiona ted (refractory-depleted) Group II-related trace element patterns, one a Group III pattern with a slightly ultrarefractory trace element sig nature, and one a rather flat pattern with depleted Eu. There is no co rrelation between mineralogy, major element chemistry, and trace eleme nt abundances. It is further shown that the high-temperature geochemic al behavior of ultrarefractory trace elements is decoupled from that o f the major elements Ca and Ti (Ti is correlated with the relatively v olatile elements Nb and Yb) implying that perovskite is of only minor importance as carrier of ultrarefractories. ALH85085 inclusions with r efractory-depleted trace element abundances and no Mg-26 excesses coul d be the complement to platy hibonites (PLACs) with Group III trace el ement patterns from the Murchison meteorite.