Modal mineral abundances and the differentiation trends in primitive achondrites

Abstract

The mineral distribution maps of polished thin sections (PTSs) of acapulcoites (Acapulco, Allan Hills (ALH) 77081,ALH78230,Elephant Moraine (EET) 84302 and Yamato (Y)-8307) and lodranites (Gibson, MacAlpine Hills (MAC) 88177,Y-74357,Y-75274,Y-791491 and Y-8002), winonaites (Tierra Blanca, Y-74025,Y-75305 and Y-8005), silicate inclusions in IAB iron meteorites (Y-791058 and Caddo County), and an H7 chondrite (Y-75008) were made from elemental distribution maps of the electron probe microanalyzer to obtain quantitative modal abundances of minerals, and to study variations in the distributions of minerals. The trends of variations of chemical compositions of minerals were also studied. Other related meteorites, ALH81187 and ALH81261 (acapulcoites) were also studied for comparison. Textures of these meteorites are various from fine-grained recrystallized chondrite-like materials including minerals known in chondrites, to coarse-grained materials with opaque mineral grains which have complex shapes, often with large aggregates of Fe-Ni metal. The variety of modal abundances and textures of primitive achondrites suggests that heterogeneity produced by local segregation of partially melted materials on their parent bodies can explain the origin of variation of primitive achondrites. Systematic variations in the relative abundances of olivine, augite+plagioclase, and orthopyroxene shows that the plagioclase-augite-rich region of Caddo County B3A (augite+plagioclase=96.8%) can be taken as an endmember of the variations observed in other primitive achondrites. The orthopyroxene-chromite-rich region of EET84302,19 (orthopyroxene=95.1%) and Y-74357 (olivine=83.2%) are other types of segregated materials. The mechanism proposed in this paper reveals the early differentiation processes of planetesimals from primitive source materials