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Abstract-Petrographic and mineralogic studies of amoeboid olivine inclusions (AOls) in C03 carbonaceous chondrites reveal that they are sensitive indicators ofparent-body aqueous and thermal alteration. As the petrologic subtype increases from 3.0 to 3.8, forsteritic olivine (FaO-l) is systematically converted into ferroan olivine (Fa60-75)' We infer that the Fe, Si and 0 entered the assemblage along grain boundaries, forming ferroan olivine that filled fractures and voids. As temperatures increased, Fe+ 2 from the new olivine exchanged with Mg+2 from the original AOI to form diffusive haloes around 10w-FeO cores. Cations of Mn+ 2, Ca+ 2 and Cr+ 3 were also mobilized.The systematic changes in AOI textures and olivine compositional distributions can be used to refine the classification of C03 chondrites into subtypes. In subtype 3.0, olivine occurs as small forsterite grains (FaD-I), free offerroan olivine. In petrologic subtype 3.2, narrow veins ofFeO-rich olivine have formed at forsterite grain boundaries. With increasing alteration, these veins thicken to form zones of ferroan olivine at the outside AOI margin and within the AOI interior. By subtype 3.7, there is a fairly broad olivine compositional distribution in the range Fa63-70, and by subtype 3.8, no forsterite remains and the high-Fa peak has narrowed, Fa64-67' Even at this stage, there is incomplete equilibration in the chondrite as a whole (e.g., data for coarse olivine grains in Isna (C03.8) chondrules and lithic clasts show a peak at Fa39)' We infer that the mineral changes in AOI identified in the low petrologic types required aqueous or hydrothermal fluids whereas those in subtypes~3.3 largely reflect diffusive exchange within and between mineral grains without the aid of fluids.

available online at Abstract-The presence of apparently unaltered, micron-sized Fe,Ni metal grains, juxtaposed against hydrated fine-grained rim materials in the CM2 chondrite Yamato (Y-) 791198 has been cited as unequivocal evidence of preaccretionary alteration. We have examined the occurrence, composition, and textural characteristics of 60 Fe,Ni metal grains located in fine-grained rims in Y-791198 using scanning electron microscopy (SEM) and electron microprobe analysis. In addition, three metal grains, prepared by focused ion beam (FIB) sample preparation techniques were studied by transmission electron microscopy (TEM). The metal grains are heterogeneously distributed within the rims. Electron microprobe analyses show that all the metal grains are kamacite with minor element contents (P, Cr, and Co) that lie either within or close to the range for other CM2 metal grains. X-ray maps obtained by electron microprobe show S, P, and/or Ca enrichments on the outermost parts of many of the metal grains. Z-contrast STEM imaging of FIB-prepared Fe,Ni metal grains show the presence of a small amount of a lower Z secondary phase on the surface of the grains and within indentations on the grain surfaces. Energy-filtered TEM (EFTEM) compositional mapping shows that these pits are enriched in oxygen and depleted in Fe relative to the metal. These observations are consistent with pitting corrosion of the metal on the edges of the grains and we suggest may be the result of the formation of Fe(OH) 2 , a common oxidation product of Fe metal. The presence of such a layer could have inhibited further alteration of the metal grains. These findings are consistent with alteration by an alkaline fluid as suggested by Zolensky et al. (1989), but the location of this alteration remains unconstrained, because Y-791198 was recovered from Antarctica and therefore may have experienced incipient terrestrial alteration. However, we infer that the extremely low degree of oxidation of the metal is inconsistent with weathering in Antarctica and that alteration in an extraterrestrial environment is more probable. Although the presence of unaltered or incipiently altered metal grains in these fine-grained rims could be interpreted as evidence for preaccretionary alteration, we suggest an alternative model in which metal alteration was inhibited by alkaline fluids on the asteroidal parent body.