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Queen Alexandra Range 97990 (QUE97990) and Yamato 791198 (Y791198) are among the least aqueously altered CM carbonaceous chondrites known to date. Our mineralogical and petrographic study reveals that chondrule mesostases in QUE97990 contain dense arrays of parallel, thin lath-shaped crystallites of diopside. The diopside crystallites are primary quenched products formed during chondrule formation. Thus their presence suggests that QUE97990 preserves a very early state of aqueous alteration. On the other hand, chondrule mesostases in Y791198 consist largely of serpentine without such quenched crystallites. In Y791198, quenched crystallites have probably been replaced by serpentine due to a higher degree of alteration than in QUE97990. Our study also reveals that chondrules in both QUE97990 and Y791198 show topographic depressions on their surfaces. The texture and mineralogy of the depressions suggest that they formed by replacing opaque nodules in their host chondrules during aqueous alteration. These imply that, in spite of the evidence of incipient alteration in the chondrule interiors, there was considerable alteration reaction between the chondrule margins and their surrounding rims in QUE97990. We suggest that the aqueous alteration in both meteorites occurred after the chondrules acquired their own rims on the meteorite parent bodies.

Early aqueous alteration process in the QUE97990 and Y791198 CM carbonaceous chondrites