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Abstract-The oxidized CV3 chondrites can be divided into two major subgroups or lithologies, Bali-like (cV3,,~) and d), in which chondrules, calcium-aluminum-rich inclusions (CAIs) and matrices show characteristic alteration features Krot et al., 1997d;Kimura and Ikeda, 1997). The cv30,B lithology is present in Bali, Kaba, parts of the Mokoia breccia and, possibly, in Grosnaja and Allan Hills (ALH) 85006. It is characterized by the presence of the secondary low-Ca phyllosilicates (saponite and sodium phlogopite), magnetite, Ni-rich sulfides, fayalite (Fa>go), Ca-Fe-rich pyroxenes (Fs 10-5oW045-50) and andradite. Phyllosilicates replace primary Ca-rich minerals in chondrules and CAIs, which suggests mobilization of Ca during aqueous alteration. Magnetite nodules are replaced to various degrees by fayalite, Ca-Fe-rich pyroxenes and minor andradite. Fayalite veins crosscut fine-grained rims around chondrules and extend into the matrix. Thermodynamic analysis of the observed reactions indicates that they could have occurred at relatively low temperatures (<300 "C) in the presence of aqueous solutions. Oxygen isotopic compositions of the coexisting magnetite and fayalite plot close to the terrestrial fractionation line with large A1*Ofayal,te-magnetlte fractionation (-20%0). We infer that phyllosilicates, magnetite, fayalite, Ca-Fe-rich pyroxenes and andradite formed at relatively low temperatures (<300 "C) by fluid-rock interaction in an asteroidal environment.Secondary fayalite and phyllosilicates are virtually absent in chondrules and CAIs in the CV3,d lithology, which is present in Allende and its dark inclusions, Axtell, ALHA81258, ALH 84028, Lewis Cliff (LEW) 86006, and parts of the Mokoia and Vigarano breccias. Instead secondary nepheline, sodalite, and fayalitic olivine are common. Fayalitic olivine in chondrules replaces low-Ca pyroxenes and rims and veins forsterite grains; it also forms coarse lath-shaped grains in matrix. Secondary Ca-Fe-rich pyroxenes are abundant. We infer that the CV3,d lithology experienced alteration at higher temperatures than the cv3,.& lithology. The presence of the reduced and CV3,d lithologies in the Vigarano breccia and W 3 , d and cV3,,~ lithologies in the Mokoia breccia indicates that all CV3 chondrites came from one heterogeneously altered asteroid. The metamorphosed clasts in Mokoia (Krot and Hutcheon, 1997) may be rare samples of the hotter interior of the CV asteroid. We conclude that the alteration features observed in the oxidized CV3 chondrites resulted from the fluid-rock interaction in an asteroid during progressive metamorphism of a heterogeneous mixture of ices and anhydrous materials mineralogically similar to the reduced CV3 chondrites.

Fifteen of the sixteen known enstatite chondrites were studied microscopically in reflected and transmitted light, and their modal compositions were determined by point‐counting techniques. Compositions of clinoenstatite, orthoenstatite, plagioclase, kamacite, taenite, troilite, oldhamite, daubreelite, niningerite, ferroan alabandite, and schreibersite were determined with the electron microprobe X‐ray analyzer. Chemical composition, mineral occurrence, and mineral composition were found to depend on degree of recrystallization of the chondrites as judged by, for example, distinctness of chondrules and coarseness of silicates. On the basis of these parameters, three groups of enstatite chondrites can be distinguished and are referred to as type I, intermediate type, and type II. Differences between types I and II are pronounced, whereas intermediate type is transitional. The suggestion of Van Schmus and Wood that type II enstatite chondrites originated from type I by reheating is reviewed in the light of the new data. It is concluded that, although many of the chemical‐mineralogical parameters of type II chondrites could be explained as being the result of reheating of type I chondrites, there are some that would require rather stringent environmental conditions during reheating. For example, lower iron and sulfur contents in type II chondrites would presumably require reheating of type I chondrites to ≥975°C, the lowest temperature at which a melt would appear in the Fe‐Ni‐S system of type I composition and at which physical separation of the liquid from the silicates could occur. Differences in Si/Mg ratios would require reheating to even higher temperatures and fractionation in an open system. Furthermore, observed differences in nitrogen and sinoite contents between type I and type II are difficult to explain unless reheating took place in a closed system, or under oxygen fugacities low enough to allow nitrogen to react with SiO2 and Si to form Si2N2O. An alternative model to the one by Van Schmus and Wood is discussed; it assumes that major differences in chemical and mineralogical composition between type I and type II were essentially established before or during chondrule formation and agglomeration by, for example, igneous differentiation or fractionation during condensation from a solar nebula, and that differences in texture are due either to different cooling rates of type I and type II chondrites during and after agglomeration of chondrules or to mild reheating to temperatures ≤975°C. This model does not, however, readily explain why only enstatite chondrites of type II bulk chemical composition (i.e. low Fe, S) cooled slowly or were reheated, and why chondrites of type I composition (high Fe, S) were always quenched to temperatures low enough to prevent recrystallization and were not reheated.

Abstract-We studied 26 IAB iron meteorites containing silicate-bearing inclusions to better constrain the many diverse hypotheses for the formation of this complex group. These meteorites contain inclusions that fall broadly into five types: (1) sulfide-rich, composed primarily of troilite and containing abundant embedded silicates; (2) nonchondritic, silicate-rich, comprised of basaltic, troctolitic, and peridotitic mineralogies; (3) angular, chondritic silicate-rich, the most common type, with approximately chondritic mineralogy and most closely resembling the winonaites in composition and texture; (4) rounded, often graphite-rich assemblages that sometimes contain silicates; and ( 5 ) phosphate-bearing inclusions with phosphates generally found in contact with the metallic host. Similarities in mineralogy and mineral and 0-isotopic compositions suggest that IAB iron and winonaite meteorites are from the same parent body.We propose a hypothesis for the origin of IAB iron meteorites that combines some aspects of previous formation models for these meteorites. We suggest that the precursor parent body was chondritic, although unlike any known chondrite group. Metamorphism, partial melting, and incomplete differentiation (i.e., incomplete separation of melt from residue) produced metallic, sulfide-rich and silicate partial melts (portions of which may have crystallized prior to the mixing event), as well as metamorphosed chondritic materials and residues. Catastrophic impact breakup and reassembly of the debris while near the peak temperature mixed materials fiom various depths into the re-accreted parent body. Thus, molten metal from depth was mixed with near-surface silicate rock, resulting in the formation of silicate-rich IAB iron and winonaite meteorites. Results of smoothed particle hydrodynamic model calculations support the feasibility of such a mixing mechanism. Not all of the metal melt bodies were mixed with silicate materials during this impact and reaccretion event, and these are now represented by silicate-free IAB iron meteorites. Ages of silicate inclusions and winonaites of 4. 40-4.54 Ga indicate this entire process occurred early in solar system history.