New petrographic and trace element data on thermally metamorphosed carbonaceous chondrites

Mineral grains and matrix of heated chondrites Yamato (Y)-82162,Belgica (B)-7904,Y-86720,and Asuka (A)-881655 were examined for major elements and, where appropriate, phyllosilicate and matrix samples were characterized by TEM. CM chondrites A-881655 and B-7904 were only partially aqueously altered before thermal metamorphism initiated dehydration and recrystallization. Tochilinite is absent in both A-881655 and B-7904 probably due to mild thermal metamorphism. Phyllosilicates in B-7904 are dehydrated but not completely recrystallized to olivine and pyroxene. Y-86720 experienced a history very different from other CM chondrites : its chondrules and other coarse-grained components were completely altered by aqueous fluids. Fine-grained olivine was subsequently replaced during an episode of thermal metamorphism producing compositions near Fo_<70> and complete destruction of serpentine occurred. Y-82162 comes from a CI parent and is characterized by complete destruction of preexisting anhydrous silicates during aqueous alteration. Subsequent heating at 600-700℃ resulted in dehydration of phyllosilicates. The matrix contains abundant fine-grained olivine which would not likely survive the original aqueous alteration and therefore must have been formed by phyllosilicate metamorphism. Alternating episodes of oxidation and sulfidization following aqueous alteration are evident in all but Y-86720. Either Y-86720 escaped an oxidizing event or effects of such an event were obliterated by later sulfidization. Based on RNAA data for thermally mobile trace elements in them and in Murchison CM2 samples heated for one week at 500°, 600°, and 700℃ under conditions reasonable for interiors of primitive parent bodies, the four thermally metamorphosed carbonaceous chondrites can be ordered by the severity of open-system heating as 500°≤A-881655<B-7904<Y-82162<Y-86720≤700℃. Petrographic studies of these meteorites indicate that each bears the signature of a unique sequence of oxidation/reduction/hydration episodes followed by a late stage thermal metamorphic event which occurred in an asteroidal setting, on at least two separate asteroids (the CM and CI parent bodies). Thermal alterations in these meteorites can be used to identify other carbonaceous chondrites that experienced thermal metamorphism in their parent bodies. Interestingly, all thermally metamorphosed carbonaceous chondrites identified to date were found in Antarctica, mainly in Queen Maud Land, and none are observed falls. Apparently, carbonaceous chondrite parent sources sampled in near-Earth space in the past, differ from those being sampled by the Earth today.