PETROLOGY OF ALLENDE DARK INCLUSIONS

Bulk compositions acquired by neutron activation analysis are reported for eight Allende dark inclusions (DIs) and compared with composition s of five DIs previously reported in the literature. Enrichments of th e elements Na, K, Br, Ba, Au, and As are generally correlated with inc reased proportions of porous aggregates predominantly composed of fine -grained, platy to fibrous olivine. These aggregates have been interpr eted by some workers as chondrules and inclusions that first were expo sed to aqueous alteration, converting anhydrous silicates to phyllosil icates, and later were dehydrated by thermal metamorphism, converting phyllosilicates to olivine. Our data support this interpretation and s uggest that the aqueous alteration was open system in character. Felds pathoid grains intergrown with platy to fibrous olivines in porous, ol ivine-rich aggregates and inclusions of sodalite, nepheline, pentlandi te, ilmenite, and spinel in these olivines suggest that phyllosilicate s produced during aqueous alteration could accommodate Na, K, Ni, S, e tc and are most consistent with a mixture of both saponite and serpent ine. Dehydration of saponite would require that the temperature of the rmal metamorphism was higher than previously suggested and may have be en as high as 800 degrees-900 degrees C. Nonporous rims containing Ca- rich minerals (andradite, hedenbergite) surrounding dark inclusions co ntaining highest proportions of porous, olivine-rich aggregates and sl ight to moderate depletions of these clasts in Ca relative to bulk All ende suggest that these rims were also the result of redistribution of some elements during dehydration of these clasts caused by thermal me tamorphism. Copyright (C) 1997 Elsevier Science Ltd.