PETROLOGY AND CHEMISTRY OF MESOSIDERITES .I. TEXTURES AND COMPOSITION OF NICKEL-IRON

Metal-silicate textures in ten mesosiderites indicate that metal structures formed in situ, after the presently observed physical relationships were established. The Widmanstätten structure is observed only occasionally in the mesosiderites. More often kamacite nuoleation appears to have been influenced by grain boundaries, resulting in irregular structures. Characteristic features of the metal are the invariably high Ni contents (>35 wt. %) of the centers of taenite masses and a general absence of plessite. These related features are most likely the result of Type II impingement during the later stages of kamacite growth. The bulk Ni contents of the metal of ten mesosiderites were determined by X-ray fluorescence and show a narrow range of 7.4-8.8 wt.%. Electron microprobe analyses reveal that taenite and kamacite masses in mesosiderites show distinct composition-dimension correlations related to cooling rate in the temperature range 500-350°C. Cooling rate determinations for 9 mesosiderites give the same result of 0.1°C 106yr. Combined metal and silicate features are possibly consistent either with a thermal history involving continuous but markedly decelerated cooling, or with rapid initial cooling to low temperatures followed by reheating to 500°C with subsequent very slow cooling to at least 350°C. Mesosiderite cooling rates are the slowest yet determined for any meteorites. This fact is discussed with respect to its genetic implications. Udei Station, long considered to be a mesosiderite, possesses metal features which indicate a closer affinity to the octahedrites than to the mesosiderites. It has a metal bulk Ni content of 9.4 wt. % and a cooling rate of 6-10°C 106 yr (from 600-450°C). Its metal features, together with its silicate mineralogy, indicate it should be classified as an iron with silicate inclusions rather than as a mesosiderite. © 1969.