Bland, P.A., Lee, M.R., Sexton, A.S., Franchi, I.A., Fallick, A.E., Miller, M.F., Cadogan, J.M., Berry, F.J., and Pillinger, C.T. (2000) Aqueous alteration without a pronounced oxygen-isotopic shift: implications for the asteroidal processing of chondritic materials. Meteoritics and Planetary Science, 35(6), pp. 1387-1395.
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Primitive meteorites exhibit certain features that are consistent with aqueous 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 take 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 possible explanation is found when we consider the alteration of terrestrial silicates. Numerous studies report pervasive development of channels a few to a few tens of nanometer wide in the incipient alteration of silicates. We observe a similar texture. Alteration involves a restructuring of clay minerals 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 inheritance of the original O structure by the clay. Our data suggests the latter: with extensive inheritance of structural polymers by the weathering product, the bulk O- isotopic composition is comparatively unaffected. This offers an explanation for the lack of an isotopic effect in the weathering of 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.
|Glasgow Author(s) Enlighten ID:||Fallick, Professor Anthony|
|Authors:||Bland, P.A., Lee, M.R., Sexton, A.S., Franchi, I.A., Fallick, A.E., Miller, M.F., Cadogan, J.M., Berry, F.J., and Pillinger, C.T.|
|Subjects:||Q Science > QB Astronomy|
|College/School:||College of Science and Engineering > Scottish Universities Environmental Research Centre|
|Journal Name:||Meteoritics and Planetary Science|