Petrogenesis of silicate inclusions in the Weekeroo Station IIE iron meteorite: Differentiation, remelting, and dynamic mixing

The Weekeroo Station IIE iron meteorite contains a variety of felsic and ma fic inclusions enclosed in an FeNi-metal host. Petrographic, EMP, and SIMS data suggest that the petrogenesis of the silicates was complex, and includ ed differentiation, remelting, FeO-reduction, and dynamic mixing of phases. Differentiation produced a variety of olivine-free inclusion assemblages, r anging from pyroxene + plagioclase + tridymite with peritectic compositions , to coarse orthopyroxene, to plagioclase + tridymite and its glassy equiva lent. Individual phases have similar trace-element abundances and patterns, despite large variations in inclusion textures, modes, and bulk compositio ns, probably as a result of mechanical separation of pre-existing phases in an impact event that dynamically mixed silicates with the metallic host. T race-element data imply that augite and plagioclase grains in different inc lusions crystallized from the same precursor melt, characterized by relativ ely unfractionated REE abundances of similar to 20-30 x CI-chondrites excep t for a negative Eu anomaly. Such a precursor melt could have been produced by similar to 2-5% equilibrium partial melting of an H-chondrite silicate protolith, or by higher degrees of partial melting involving subsequent fra ctional crystallization. Glass appears to have formed by the remelting of p re-existing plagio-clase and orthopyroxene, in a process that involved eith er disequilibrium or substantial melting of these phases. During remelting, silicate melt reacted with the FeNi-metal host, and FeO was reduced to Fe- metal. Following remelting and metal-silicate mixing, inclusions apparently cooled at different rates in a near-surface setting on the parent body; gl ass- or pigeonite-bearing inclusions cooled more rapidly (greater than or e qual to 2.5 degrees C/hr between 1000-850 degrees C) than pigeonite-free, l argely crystalline inclusions. The results of this study point to two likely models for forming IIE iron m eteorites, both involving collision between an FeNi-metal impactor and eith er a differentiated or undifferentiated silicate-rich target of H-chondrite affinity. Each model has difficulties and it is possible that both are req uired to explain the diverse IIE group. Copyright (C) 1999 Elsevier Science Ltd.