Matrix mineralogy of the Lancé CO3 carbonaceous chondrite: A transmission electron microscope study

The Lancé CO3 carbonaceous chondrite (CC) is less altered than the CI and CM chondrites and so provides a view of the mineralogy and textures resulting from the earliest stages of aqueous alteration of CCs. Matrix olivine in Lancé has been partly altered to fine-grained, Fe-bearing serpentine and poorly crystalline Fe³⁺ oxide, a process that required both hydration and oxidation. Serpentine occurs as discrete packets separated from the olivine surfaces by the Fe³⁺ oxide. The Fe released during the dissolution of olivine was partly incorporated into the serpentine; the remainder was oxidized to form Fe³⁺ oxide. Matrix metal was also altered to produce Fe oxides, leaving the residual metal enriched in Ni. Olivine grains in Lancé matrix contain channels along their [100]and [001]directions. The formation and convergence of such channels resulted in a grain-size reduction of the olivine. The alteration was pervasive but incomplete, suggesting a limited availability of fluid. A brief study of two other CO chondrites, Kainsaz and Warrenton, shows that these meteorites do not contain phyllosilicates in their matrices, although both contain Fe³⁺ oxide between olivine grains. Prior to its alteration, Lancé probably resembled Kainsaz, an unaltered CO3 chondrite. The alteration assemblage in Lancé is only slightly different from that in Mokoia and essentially the same as that in C3 xenoliths from Murchison. Alteration products in Lancé show greater similarities to CI than to CM chondrites.