Silicate Inclusions in Iron Meteorites
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There are 12 major groups of iron meteorites and some of them contain silicate inclusions. The origin of these inclusions, their relationship (if any) to chondritic or achondritic meteorites, the significance of their dispersal in iron meteorites, and the record of their history prior to, during and after incorporation into these metallic melts, are some of the questions that continue to be addressed. Three major groups of iron meteorites contain silicate inclusions. These are: (1) the IAB-IIICD iron group (originally separate groups) and a related silicate-rich group called winonaites, (2) IIE irons, and (3) IVA irons. A fourth group, some metal-rich, is called the lodranite-acapulcoite group. All of these groups are primitive achondrites, intermediate between chondrites and achondrites in that they retain many chemical characteristics of chondrites (but have no chondrules) and have undergone some degree of partial melting. The IAB-IIICD-Winonaite group has undergone a small degree of partial melting, and its oxygen isotopes indicate no genetic relationship to any known meteoritic group. The IIE group is genetically related to H-group ordinary chondrites which were probably melted by impact processes. IIE irons with silicates display a complete range evolution from iron meteorites with chondrule-bearing inclusions, to inclusions with melted but undifferentiated chondrite, to inclusions with highly differentiated silicates derived from chondrites. The silicate inclusions in IVA irons are highly differentiated silica-pyroxene assemblages, with oxygen isotopes related to L-group ordinary chondrites, and they too probably formed by impact processes. The lodranite-acapulcoite group members are mainly silicate-rich meteorites which are not genetically related to any known meteoritic group. The mesosiderite and pallasite meteorite groups are achondrites that are generally half silicate and half metal, and are not considered to be iron meteorites. The silicates are genetically related to the howardite-eucrite-diogenite (HED) meteorites which may come from the Vesta asteroid. Nevertheless, the questions regarding the relationships of the silicates and olivine crystals with the metal are basically the same as those for the iron meteorites with silicate inclusions. All of these types of meteorites continue to be studied here.
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