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Abstract: We have performed an experimental study of the aqueous alteration of the Allende CV3 carbonaceous chondrite under highly oxidizing conditions, in order to examine the alteration behavior of Allende’s anhydrous mineralogy. The experiments were carried out at temperatures of 100, 150, and 200°C, for time periods between 7 and 180 days, with water/rock ratios ranging from 1:1 to 9:1. Uncrushed cubes of Allende were used so that the spatial relationships between reactant and product phases could be examined in detail. Scanning electron microscope studies show that in all the experiments, even those of short duration (7 days), soluble salts of Ca and Mg (CaSO4, CaCO3, and MgSO4) precipitated on the sample surface, indicating that these elements are rapidly mobilized during alteration. In addition, iron oxides and hydroxides formed on the sample surfaces. The sulfates, carbonates, and the majority of the iron-bearing secondary minerals are randomly distributed over the surface of samples. In some instances the iron oxides and hydroxides are constrained to the boundaries of altering mineral grains. Transmission electron microscope studies show that the FeO-rich olivine in the interior of the samples has altered to form interlayered serpentine/saponite and Fe-oxyhydroxides. The degree of alteration increases significantly with increasing water/rock ratio, and to a lesser extent with increasing duration of heating. The serpentine/saponite forms both by direct replacement of the olivine in crystallographically oriented intergrowths, and by recrystallization of an amorphous Si-rich phase that precipitates in pore space between the olivine grains. The alteration assemblage bears many similarities to those found in altered carbonaceous chondrites, although in detail there are important differences, which we attribute to (a) the relatively high temperatures of our experiments and (b) comparatively short reaction times compared with the natural examples. In terms of mineral assemblage, our experiments most closely resemble alteration in the CI chondrites, although the degree of alteration of our experiments is much lower. CI chondrites contain serpentine/saponite intergrowths and veins of Ca-sulfate and Ca-carbonate as well as the Fe-oxyhydroxide, ferrihydrite. However, the phyllosilicate phases formed in our experiments are somewhat coarser-grained than the finest phyllosilicate fraction present in CI chondrites, suggesting that alteration of the CI chondrites occurred at lower temperatures. In terms of mineral assemblage, our experiments also appear to come close to matching CR chondrites, although we infer that CR alteration probably occurred at temperatures <100°C, based on the very fine-grained size of phyllosilicates in CR matrices. [Copyright &y& Elsevier]

A petrographic and scanning electron microscopic study of the four CO3 chondrites Kainsaz, Ornans, Lance´, and Warrenton reveals for the first time that dark inclusions (DIs) occur in all the meteorites. DIs are mostly smaller in size than those reported from CV3 chondrites. They show evidence suggesting that they were formed by aqueous alteration and subsequent dehydration of a chondritic precursor and so probably have a formation history similar to that of DIs in CV3 chondrites. DIs in the CO3 chondrites consist mostly of fine-grained, Fe-rich olivine and can be divided into two types on the basis of texture. Type I DIs contain rounded, porous aggregates of fine grains in a fine-grained matrix and have textures suggesting that they are fragments of chondrule pseudomorphs. Veins filled with Fe-rich olivine are common in type I DIs, providing evidence that they experienced aqueous alteration on the parent body. Type II DIs lack rounded porous aggregates and have a matrix-like, featureless texture. Bulk chemical compositions of DIs and mineralogical characteristics of olivine grains in DIs suggest that these two types of DIs have a close genetic relationship.The DIs are probably clasts that have undergone aqueous alteration and subsequent dehydration at a location different from the present location in the meteorites. The major element compositions, the mineralogy of metallic phases, and the widely dispersed nature of the DIs suggest that their precursor was CO chondrite material. The CO parent body has been commonly regarded to have been dry, homogeneous, and unprocessed. However, the DIs suggest that the CO parent body was a heterogeneous conglomerate consisting of water-bearing regions and water-free regions and that during asteroidal heating, the water-bearing regions were aqueously altered and subsequently dehydrated. Brecciation may also have been active in the parent body.The DIs and the matrices are similarly affected by thermal metamorphism in their own host CO3 chondrites (petrologic subtypes 3.1 to 3.6), but the degree of the secondary processing (aqueous alteration and subsequent dehydration) of the DIs has no apparent correlation with the petrologic grades of the host chondrites. These observations suggest that the DIs had been incorporated into the host chondrites before the thermal metamorphism took place and that the secondary processes that affected the DIs largely occurred before the thermal metamorphism. [Copyright &y& Elsevier]