MATRIX AND FINE-GRAINED RIMS IN THE UNEQUILIBRATED CO3 CHONDRITE, ALHA77307 - ORIGINS AND EVIDENCE FOR DIVERSE, PRIMITIVE NEBULAR DUST COMPONENTS

The mineralogical and chemical characteristics of dark matrix and fine -grained rims in the unequilibrated CO3 chondrite ALHA773O7 have been investigated in detail by scanning electron microscopy, electron micro probe analysis, and transmission electron microscopy. Matrix and fine- grained rims on chondrules and other objects are compositionally homog eneous on the scale of 10 mum, and there are no compositional differen ces between matrix and rims. Mineralogically, both the matrix and rims are extremely diverse and consist of a highly unequilibrated assembla ge of Si- and Fe-rich amorphous material, olivine, pyroxene, Fe,Ni met al, magnetite, pentlandite, pyrrhotite, anhydrite, and mixed layer phy llosilicate phases. Several distinct components can be recognized with in the matrix and rims based on their textural and compositional chara cteristics, which appear to represent basic fine-grained units of nebu lar dust. The microstructures of these different components show that they have experienced significantly different thermal histories. Unlik e the ordinary chondrites, an origin for any significant component of the matrix or rims in ALHA77307 from chondrules is improbable, based o n compositional and microstructural evidence. The matrix and rim compo nents in ALHA77307 formed by disequilibrium condensation processes as fine-grained amorphous dust that is represented by the abundant amorph ous component in the matrix. Such condensation could have occurred und er a variety of conditions, at different times and locations within th e solar nebula, or possibly earlier in a circumstellar environment. Su bsequent thermal processing of this primitive condensate material, in a variety of environments in the nebula, resulted in partial or comple te recrystallization of the fine-grained dust. The intimate associatio n of fine-grained components with disparate compositions and thermal h istories shows that mixing of fine-grained dust within the nebula must have been extremely thorough.