An analytical electron microscope investigation of some pallasites

Abstract

Four pallasites (Brenham, Brahin, Esquel, and Omolon) were investigated by analytical transmission electron microscopy. All show very similar compositions and defect microstructures. The olivine grains contain a low density of c dislocations organized in tilt subgrain boundaries, indicating that the meteorites were annealed for a long time at high temperature. They also contain straight fractures parallel to ‘{1k0’} planes (with k = 0, 2, and 3) filled with a mixture and sulfide, and alignments of tiny inclusions of metal + troilite. Both types of defects are shock indicators, the former ones are common in shocked olivine and the later ones must result from earlier shock events and healed during a post-shock annealing episode. The Widmanstätten patterns in the metal are similar to those observed in iron meteorites. By using the calibration charts published by the Goldstein group the cooling rates of the pallasites were deduced from the Ni concentration profiles across taenite bands. These cooling rates are quite low (≤ 5 K/MY). Kamacite represents approximately 90% of the metal and is constituted of large grains with low densities of free dislocations (≤ 10 13 m −2) exhibiting a marked screw character. One also detects some twin lamellae and a number of phosphide precipitates with compositions M 3P and M 2P as well (M = Fe and Ni). The twins probably result from a shock with moderate intensity. The M 2P phosphides have never been characterized before. They probably precipitated at low temperature when their Gibbs energy became lower than the one of schreibersite (Fe, Ni) 3P. The shock indices in both olivine and kamacite correspond to a moderate shock intensity. As their parent body (or bodies) suffered at least one strong shock which fragmented it, the source region of pallasites must lie at great depth within their parent bodies.