Allende Xenolith AF: Undisturbed Record of Condensation and Aggregation of Matter in the Solar Nebula

Abstract

In this paper an unusual, cm-sized xenolith from the Allende meteorite is described. Xenolith Allende-AF (All-AF) is chemically similar to bulk Allende, its texture is, however, completely different. It consists mainly of highly porous silicate-rich aggregates embedded in a largely opaque matrix. Occasionally olivine-sulfide-metal aggregates and large sulfide-andradite objects are found. Chondrules are absent and grain size in matrix and aggregates is the same, quite different from normal Allende. The main mineral in All-AF is olivine (22–41 wt% FeO), low Ca-pyroxene is absent, but a variety of clinopyroxenes were encountered. Rare phases found throughout All-AF are refractory metal nuggets (Ir, Os, Ru etc.), HgS, and Ca-phosphate. Associated with sulfide-andradite objects are native-Cu. Ti-magnetite, perovskite, barite, and calcite. Morphology, crystalline state and minor element contents of olivine strongly suggest an origin by condensation from a gas. Al2O3-contents, for example, range from 0.5 to 3% and Cr203-contents reach 2.5%. Texture of All-AF indicates that condensation of olivine continued during formation of aggregates. Relictic low-Fe aluminous diopsides suggest that olivine was initially FeO-poor. Before final accretion into a solid rock exchange reactions between condensed phases and an (increasingly oxidized) vapor established the high FeO-content of olivine. A similar process led to sulfurization of most of the original metal. The exotic sulfide-andradite objects are probably alteration products of unusual, metal-rich calcium-aluminium-rich inclusions. It appears that All-AF has preserved a unique record of condensation, hierarchical aggregation, and metasomatic exchange reactions in the solar nebula. © 1989, Verlag der Zeitschrift. All rights reserved.