Petrographic constraints on the formation of silica‐rich igneous rims around chondrules in <scp>CR</scp> chondrites

In the CR (Renazzo-like) chondrite group, many chondrules have successive igneous rim (IR) layers, with an outer layer that contains a silica mineral and/or silica-rich glass (silica-rich rims, SIRs). Models for SIR formation include (1) accretion of Si-rich dust onto solid chondrule surfaces, followed by heating and cooling (2) condensation SiO(gas) surface partially molten chondrules. We evaluate these models, based on petrographic study five Antarctic chondrites undergone minimal secondary alteration. obtained electron microprobe analyses minerals quantitative wavelength-dispersive spectroscopy mapping, identified polymorphs Raman spectroscopy. Common SIRs contain silica, low-Ca pyroxene, Ca-rich Fe,Ni metal, ± plagioclase rare olivine. also describe near-monomineralic where narrow zone cristobalite occurs at edge chondrule. All crystalline is cristobalite, except one consists tridymite. Some rims (>80 wt% SiO2) but no mineral. Features such as sharp interfaces compositional boundaries between indicate were formed from precursors. Consideration stability fields computed liquidus temperatures indicates heated to >1500°C limited time periods, rapid cooling, similar conditions formation. infer in region, same mechanism was repeated multiple times while chemical composition nebular gas evolved highly fractionated compositions.