Metasomatic alterations of olivine inclusions in the Budulan mesosiderite

Abstract

Mesosiderites are thermal metamorphic breccias consisting of fragments of pyroxene-plagioclase rocks and FeNi metal. The silicate constituent of mesosiderites has a chemical and oxygen isotopic composition analogous to those of meteorites of the HED group: howardites, eucrites, and diogenites. The hypothesis currently most widely accepted for the genesis of mesosiderites is the impact mixing of the material of a differentiated asteroid and an iron meteorite. In contrast to many other classes of meteorites, mesosiderites exhibit no traces of metasomatic processes. The Budulan mesosiderite is the first meteorite of this type in which traces of metasomatism under the effect of an anhydrous fluid were detected. The metasomatic alterations are manifested as chemical zoning of olivine, aggregates of secondary minerals, and the mobilization and redeposition of iron and nickel in the form of metals and sulfides. These alterations were most probably caused by a reaction of olivine with S- and/or CO-bearing gases of endogenic or supergenic provenance. Traces of such metasomatic alterations were previously found in some meteorites and lunar rocks, and these processes could likely play a certain role in the differentiation of chondritic bodies.