Aqueous alteration without a pronounced oxygen-isotopic shift: Implications for the asteroidal processing of chondritic materials

Primitive meteorites exhibit certain features that are consistent with aque ous and thermal alteration on asteroids, but O-isotopic analyses show only a modest heavy-isotope shift, interpreted as indicating modification in the nebula. To understand the isotopic effects of asteroidal alteration, we ta ke the L-group ordinary chondrites weathered in Antarctica as an analogue. The data show that alteration is a two-stage process, with an initial phase producing only a negligible isotopic effect. Although surprising, a possib le explanation is found when we consider the alteration of terrestrial sili cates. Numerous studies report pervasive development of channels a few to a few tens of nanometer wide in the incipient alteration of silicates. We ob serve a similar texture. Alteration involves a restructuring of clay minera ls along these narrow channels, in which access of water is restricted. The clay shows a topotactic relationship to the primary grain, which suggests either epitaxial growth of the clay using the silicate as a substrate or in heritance of the original O structure by the clay. Our data suggests the la tter: with extensive inheritance of structural polymers by the weathering p roduct, the bulk O-isotopic composition is comparatively unaffected. This o ffers an explanation for the lack of an isotopic effect in the weathering o f the L chondrites. If substantial modification of chondritic materials may occur without a pronounced isotopic effect, it also reconciles existing O analyses of CV chondrites with an asteroidal model of aqueous alteration.