Trace Element Constraints on Processes and Precursors of Chondrule Formation in Unequilibrated Ordinary Chondrites

Chondrules are silicate spherules in chondritic meteorites thought to have formed as free-floating melt droplets created by a transient heat source in the early solar system. Due to their ubiquity in asteroids, determining the origin of chondrules is key to understanding the conditions and processes occurring within the early protoplanetary disk. Recent models for chondrule formation involve the formation of chondrules by nebular shock waves or by protoplanetary impacts. Here, elemental abundances in 153 chondrules from four unequilibrated ordinary chondrites (UOCs) were analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to test these models. Chondrules in UOCs are mixtures of mineral phases within a glassy mesostasis. Obtaining a representative bulk chondrule elemental composition by a microanalytical technique is challenging since chondrules are mineralogically heterogeneous at the scale of an analyzed spot. This study, therefore, placed reliance on inter-element ratios of incompatible elements (elements concentrated into the melt phase) that would be representative of the bulk chondrule composition. Most chondrules exhibit both positive and negative Ce and Eu anomalies, and some chondrules even display resolvable negative Yb anomalies, indicating a chondrule forming process that occurred within a chemical microenvironment of varying oxygen fugacity (fO2). The modeled effects of planetesimal collisions within the early solar nebula create rapidly varying fO2 microenvironments in impact vapor plumes that begin oxidized but end reduced. Such models are necessary to explain the observed Ce-Eu-Yb anomalies. Aluminum-rich chondrules contain Nb depletions that indicate inheritance from distinct precursors to ferromagnesian chondrules, possibly containing refractory inclusions. Only a few chondrules exhibit significant fractionations in incompatible refractory element ratios distinctive of differentiated precursors (Zr/Th, Zr/Hf, Nb/Ta), indicating that most UOC chondrule precursors were derived from undifferentiated sources. The anomalies in Ce and Eu and the presence of a significant fraction of chondrules implying involvement of a differentiated precursor provide compelling evidence that chondrules from UOCs formed as a byproduct of planetesimal collisions between differentiated and undifferentiated bodies before nebular hydrogen dissipated. Models of chondrule formation that involved nebular shockwaves or planetesimal collisions after dissipation of nebular gas could not account for the Ce-Eu-Yb anomalies observed in UOC chondrules.