| Chondrules and other components in seven CM carbonaceous chondrites contain a diverse but detailed record of aqueous alteration which has resulted from their interaction with an aqueous fluid during the early stages of our Solar System. Results obtained by optical microscopy, scanning electron microscopy, electron probe microanalyses and transmission electron microscopy strongly suggest that the aqueous alteration of these CM chondrites occurred largely or exclusively in their asteroidal parent body. In particular, textural and compositional evidence for equivalent alteration histories experienced by chondrules in an unbrecciated CM chondrite (Allan Hills 81002) appears to be entirely inconsistent with preaccretionary alteration processes. However, several aspects of previous parent body alteration models, such as a correlation between the extent of alteration and brecciation and the origin of PCP-rich objects in CM chondrites may need revisions. In situ alteration is also indicated by the presence of so-called Fe-rich aureoles in several of the samples studied. These aureoles have clearly resulted from the in situ exchange of elements, such as Fe and S, between altering metal grains and surrounding fine-grained matrix material on a millimeter-scale. The morphological appearance of Fe-rich aureoles also suggests that fluid flow did not play a major role during the alteration process. Observations on partially altered chondrules on a sub-micron scale provide new insights into the diverse alteration mechanisms of different primary chondrule phases and are consistent with an alteration process under retrograde conditions and low temperatures {dollar}rm ({lcub}<{rcub}100spcirc C).{dollar} An investigation of the compositional evolution of altered chondrule mesostasis in the CM chondrites studied indicates distinct increases in the Mg/(Mg+Fe) ratios as the alteration advanced. These observations confirm one of the central tenets of parent body alteration models which postulates an early Fe-rich fluid followed by more Mg- and Si-rich fluid compositions as the alteration proceeds. The mesostasis serpentine compositions also provide a relatively precise measure of the alteration degree experienced by individual chondrules, which can be applied to determine the degree of alteration and brecciation of CM chondrite clasts and entire thin sections. |
