High-temperature chondrule accretion in primitive chondrites and its relevance for theories of chondrule formation and planetary accretion

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Some primitive (unequilibrated) ordinary chondrites have been described to contain mutually indented chondrules. Own preliminary studies show that these textures are much more widespread than previously thought. They occur in clasts of specific chondritic rocks, which are named “cluster chondrites” hereafter. The textures of these rocks may be interpreted such that hot and hence plastic chondrules accreted together with rigid chondrules, followed by instantaneous compaction and lithification. This is in contrast to current views that chondrules were already solidified before parent body accretion. The ubiquity of cluster chondrite clasts may indicate that the process of hightemperature chondrule accretion was fundamental to the formation history of chondritic planetesimals. The present research proposal is intended to understand the formation conditions of cluster chondrites and their parental planetesimals. Their petrography, bulk and mineral chemistry, oxygen isotopic composition, inventory of primordial noble gases, and their presolar grains will be investigated. If it can be confirmed that cluster chondrites accreted while chondrules where still plastic, i.e. within hours to a few days after chondrule formation, we would get insight into very early rock forming processes of short duration and limited spatial extension of the solar nebula. This would inevitably influence the current models for chondrule formation and planetesimal accretion.