Experimental constraints on formation of silica-rich igneous rims around chondrules in CR chondrites

Abstract

Silica-rich igneous rims (SIRs) occur commonly as an outer rim layer on porphyritic chondrules in CR (Renazzo-like) chondrites, as well as more rarely in other chondrite groups. Formation conditions for SIRs can provide insight into chemical and physical conditions in the chondrule-forming region of the protoplanetary disk, as well as helping to understand temporal changes to successive thermal events within the CR and other chondrule formation regions. Both accretion and condensation models have been proposed as formation mechanism of SIRs, but a lack of robust thermal constraints prevents a detailed interpretation. We have carried out an experimental study aimed to define the conditions of SIR formation by reproducing the texture, mineralogy, mineral chemistry, and silica polymorphs present in natural SIRs. Experiments conducted on an SIR bulk composition, at peak temperatures of 1310–1507 °C with linear cooling rates between 30 and 90 °C/hr successfully reproduce natural SIRs that contain cristobalite, low-Ca pyroxene, Ca-rich pyroxene, and glass. Microcrystalline mesostasis was formed in an experiment with lower cooling rates at low temperatures (6 °C/hr from <1200 °C). All silica in the experiments was cristobalite, including in experiments with maximum temperatures as low as 1310 °C. Since the cristobalite stability field in the silica phase diagram is >1470 °C, it is clear that silica polymorphs are not robust temperature indicators and that the presence of cristobalite in natural SIRs does not necessarily indicate high peak temperatures. We suggest that accretion of SIR precursors onto solidified chondrules, followed by melting, is the most likely scenario for their formation. Our constraints on SIR formation are similar to those that are usually discussed for chondrules. Therefore, in the CR chondrule-forming region, a repeatable heating mechanism is required that does not change significantly during the time in which condensing material evolved to highly silica-rich compositions. The need for recurring heating events is a general constraint for modeling chondrule formation.