Electron microscopy of some stony meteorites

Abstract

High-voltage (1 MV) electron microscopy has been used to study substructures within grains of the minerals olivine and pyroxene, in stony meteorites of a type that probably originated by aggregation of solid particles in the early Solar System (ordinary chondrites). In specimens that have not suffered later heating or shockdeformation, the substructures were largely formed during cooling after early hightemperature events. The meteorite Saint-Severin was deformed while slowly cooling, after the rock had acquired a coarse metamorphic texture. The recovered substructures in its olivine are distinguished from cold shock-deformation and from shock-heating effects. In three meteorites (Hedjaz, Parnallee and Chainpur) with relatively low-temperature histories during and after aggregation, inclusions with prior high-temperature histories (chondrules) contain pyroxenes showing effects of relatively rapid cooling. The predominant calcium-poor pyroxenes contain planar defects and submicroscopic twinning. Exsolution lamellae in calcium-bearing pyroxenes, and antiphase domains in one grain of pigeonite, are interpreted in terms of cooling rate by comparison with similar substructures in volcanic pyroxenes. Results of electron microscope microanalysis of pyroxenes in the fine-grained matrix of the low-temperature aggregates indicate that a range of calcium-bearing compositions has been inherited from environments pre-dating aggregation.