Ca-,Al-rich inclusions in the unique chondrite ALH85085: Petrology, chemistry and isotopic compositions

Abstract

The unique chondrite ALH85085 contains Ca-,Al-rich inclusions (CAIs) that differ from CAIs found in other types of meteorites. The ALH85085 inclusions are smaller (5–80 μm) and more refractory than their counterparts in carbonaceous chondrites. We have performed a comprehensive study of these inclusions: forty-two for petrography and mineralogy, fifteen 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 compositions were determined in minerals from a single inclusion (CAI 125). The mineralogy of the CAIs is dominated by oxide phases. About half of the inclusions contain either hibonite (hib) or spinel (sp), commonly associated with melilite (mel) or perovskite (pv); the others contain calcium dialuminate (CaA2), rarely found in CAIs from other meteorites. Most CAIs are spherical in shape and many exhibit igneous textures indicating crystallization from a melt. Textures in others are reminiscent of recrystallization due to thermal processing. Many of the CAIs 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 normal isotopic compositions except for two that have nonlinear 48Ca excesses of 7.4 and 11.7‰, respectively. Only three CAIs show 26Mg excesses, two of them have ( 26Al 27Al ) o ~ 5 × 10 −5 , the third has 2 × 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 16O excesses of ~5%, indicating that ALH85085 inclusions never experienced the thermal event that caused the oxygen isotopic equilibration 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 fractionated (refractory-depleted) Group II-related trace element patterns, one a Group III pattern with a slightly ultrarefractory trace element signature, and one a rather fiat pattern with depleted Eu. There is no correlation between mineralogy, major element chemistry and trace element abundances. It is further shown that the high-temperature geochemical behavior of ultrarefractory trace elements is decoupled from that of the major elements Ca and Ti (Ti is correlated with the relatively volatile elements Mb and Yb) implying that perovskite is of only minor importance as carrier of ultrarefractories. ALH85085 inclusions with refractory-depleted trace element abundances and no 26Mg excesses could be the complement to platy hibonites (PLACs) with Group III trace element patterns from the Murchison meteorite.