Refractory inclusions as Type IA chondrule precursors: Constraints from melting experiments

The formation of chondrules involved major processes in the protoplanetary disk and therefore needs to be understood. Identifying possible precursors and the conditions of their transformation into chondrules is an essential step. Here we investigate whether refractory inclusions (RI) can be converted into Type IA chondrule analogs by isothermal heating and dynamic crystallization experiments, and report a new constraint on chondrule peak temperatures. We prepared synthetic calcium-aluminum-rich inclusions (CAI) by sintering <20 µm An + Di + Sp powder at 1200 °C and synthetic AOA analogs from crushed <5 µm Fo gel or San Carlos olivine mixed with nuggets of synthetic CAI. We used the AOA analogs as starting materials in experiments and were able to reproduce the textures and mesostasis compositions of Type IA chondrules. However, in the charges, the olivine lacks asymmetric zonation and our mesostasis compositions show olivine fractionation trends, two differences from Type I chondrules indicating the requirement of condensation of Mg and SiO in the latter. Relict spinel is present in isothermal runs up to 1550 °C, but is totally resorbed by 1600 °C. We conclude that CAI and AOA were sintered essentially at their condensation temperatures and are appropriate precursors for chondrules. Chondrules with relict spinel must have formed at <1600 °C, much lower than their liquidus temperatures (∼1750 °C). Such peak temperatures are consistent with models of condensation during chondrule formation. In typical chondrules with no inclusions of AOA or CAI, spinel is an indicator of their near complete assimilation. Grains of spinel (sensu stricto) in chondrules are relicts of RI and constitute a largely untapped cosmochemical resource for the investigation of chondrule provenance.