In situ Si isotope and chemical constraints on formation and processing of chondrules in the Allende meteorite

Chondrules in undifferentiated meteorites are former silicate melt droplets of variable texture and composition. Although widely studied, the chondrule formation mechanisms and conditions that explain all properties of chondrules are yet to be identified. To further constrain the processes that affected chondrules in the solar nebula and on the meteorite parent body, we determined in situ Si isotope ratios and major and trace element compositions of minerals in chondrules of variable types and sizes from the Allende CV3 chondrite. The delta Si-30 in chondrule minerals ranges from -1.28 +/- 0.19 to 0.55 +/- 0.20 parts per thousand (2SE). The delta Si-30 in chondrules shows no direct relationship with chondrule sizes or with distance between core and rim. Barred olivine-rich chondrules record the highest delta Si-30, likely because of faster cooling and less interaction with isotopically light nebular gas. Type I non-porphyritic and some porphyritic chondrules show overall higher delta Si-30 compared to type II porphyritic chondrules. Furthermore, Mg-rich olivine and Mg-rich pyroxene have systematically higher delta Si-30 compared to Fe-rich olivine and Fe-rich pyroxene. The variable delta Si-30 of type I chondrule silicates (Mg-rich) compared to type II chondrule silicates (Fe-rich) may be explained by variable interaction of chondrule silicates with the nebular gas in the solar nebula. We envision a combination of equilibrium and kinetic isotope fractionation of Si between nebular gas and Fe-poor silicates (such as forsterite, anorthite, enstatite and mesostasis) and Fe-rich olivine and orthopyroxene. Petrographic evidence suggests that the enrichment of Fe in some highly altered porphyritic chondrules and at chondrule rims was likely caused by hydrothermal alteration on the parent body. Therefore, the correlation of Fe and delta S-30 of the chondrule minerals might serve as an indicator for the extent of further secondary processing of some chondrule minerals. The sum of these observations suggests that the formation and alteration of type II chondrules occurred by oxidation of originally reduced, metal-rich type I chondrules, both in the solar nebula and later on the meteorite parent body. Remaining delta S-30 depleted gas contributed to the isotopic composition of matrix silicates. The evidence favours the formation of chondrules and matrix of the Allende meteorite in nebular settings rather than by asteroid impacts. (C) 2021 The Author(s). Published by Elsevier Ltd.

Automated summary

Chondrules in undifferentiated meteorites are silicate melt droplets with variable texture and composition. Si isotope ratios and element compositions of chondrules in the Allende CV3 chondrite were determined to understand the formation mechanisms and conditions, suggesting that chondrules and matrix likely formed in nebular settings rather than by asteroid impacts.