AQUEOUS ALTERATION AND BRECCIATION IN BELLS, AN UNUSUAL, SAPONITE-BEARING, CM CHONDRITE

The petrological and mineralogical characteristics of the unusual CM2 chondrite, Bells, have been investigated in detail by scanning electro n microscopy (SEM), electron microprobe analysis (EPMA), and transmiss ion electron microscopy (TEM). Bells is a highly brecciated chondrite which contains few intact chondrules, a very low abundance of refracto ry inclusions, and is notable in having an unusually high abundance of magnetite, which is disseminated throughout the fine-grained matrix. Fragmental olivines and pyroxenes are common and, based on composition al data, appear to have been derived from chondrules as a result of ex tensive brecciation. The fine-grained mineralogy of matrix in Bells di ffers considerably from other CM chondrites and has closer affinities to matrix in CI chondrites. The dominant phases are fine-grained sapon ite interlayered with serpentine, and phases such as tochilinite and c ronstedtite, which are typical of CM chondrite matrices, are entirely absent. Pentlandite, pyrrhotite, magnetite, anhydrite, calcite, and ra re Ti-oxides also occur as accessory phases. Based on its oxygen and n oble gas isotopic compositions (Zadnik, 1985; Rowe et al., 1994), Bell s can be considered to be a CM2 chondrite, although its bulk compositi on shows some departures from the typical range exhibited by this grou p. However, these variations in bulk chemistry are entirely consistent with the observed mineralogy of Bells. The unusual fine-grained miner alogy of Bells matrix can be reasonably attributed to the combined eff ects of aqueous alteration and advanced brecciation in a parent body e nvironment. Extensive brecciation has assisted aqueous alteration by r educing chondrules and mineral grains into progressively smaller grain s with high surface areas, which are more susceptible to dissolution r eactions involving aqueous fluids. This has resulted in the preferenti al dissolution of Fe-rich chondrule olivines, which are now completely absent in Bells although present in other CM chondrites. The formatio n of saponite in Bells probably resulted from the dissolution of relat ively silica-rich phases, such as pyroxene and olivine, that were deri ved from chondrules. The result of such dissolution reactions would be to increase the activity of silica in the fluid phase, at least on a localized scale, stabilizing saponite in preference to serpentine. An increase in aSiO(2) would also have destabilized preexisting cronstedt ite which may have reacted to form magnetite and Mg-Fe serpentine unde r conditions of constant fO(2).