Pre-Terrestrial Deformation Effects in Iron Meteorites

Abstract

Some of the macroscopic and microscopic features which are encountered in iron meteorites can be ascribed to mechanical deformation. Relatively mild deformation within the meteorite parent body before the kamacite has begun to precipitate may produce macroscopic twins which are subsequently decorated by the precipitation of kamacite. In addition, the stresses which arise by virtue of the volume changes during the precipitation of kamacite may produce transformation twins on a microscopic scale within the kamacite. These transformation twins appear similar to partially annealed Neumann lines and are decorated by the subsequent precipitation of rhabdite. Decorated transformation twins are usually accompanied by a later generation of fresh or undecorated Neumann lines. More violent deformation effects may arise when the original meteorite parent bodies are fragmented, and also when subsequent collisions occur between fragments in space. Similar eifects may sometimes be produced by particularly violent impact with the earth, but a satisfactory distinction can be made between pre-terrestrial damage and damage during earth residence, if the heat alteration zone which is produced during atmospheric entry can be shown to be of later origin than the mechanical deformation.The most intensively studied aspects of pre-terrestrial mechanical damage are Neumann lines, and the acicular kamacite which results from shock-induced polymorphism. By contrast there has been relatively little study of the more intense local displacements of material within iron meteorites. Local displacements may be encountered in normal or in acicular kamacite, and may take the form of relatively broad deformation bands or of more sharply defined surfaces of displacement. Broad deformation bands are usually encountered entirely within the grains of metal, but in polycrystalline octahedrites such deformation may be encountered also at the boundaries between differently oriented gamma grains. Sharply defined surfaces of displacement in iron meteorites are analogous to veins in stone meteorites, and some of the effects in iron meteorites are encountered on a smaller scale within the metal fraction of stone meteorites.Care must be exercised in the study of microscopic displacements in iron meteorites, because structures which appear similar to mechanically displaced fields of plessite may arise through special circumstances of the nucleation and growth of kamacite in octahedrites, where the Widmanstatten pattern is modified by the presence of sulphide globules.KeywordsShear DisplacementShear SurfaceBritish MuseumParent BodyIron MeteoriteThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.