Unravelling chondrule formation processes: Clues from the potassium isotopic composition of chondrules from unequilibrated ordinary chondrites

Abstract

Understanding chondrule formation processes has been a major focus of the cosmochemistry community for many decades. In order to help further this understanding, here we apply high-precision K isotope analyses to chondrule fractions from the four Antarctic unequilibrated ordinary chondrites of QUE 97008 (L3.05), MET 00452 (L(LL)3.05), GRO 95658 (LL3.3), and GRO 95539 (LL3.2). The K isotope ratios of the chondrules fractions from all four of these samples lie within the range of −2.20 ‰ to 0.14 ‰ δ41K, with QUE 97008, MET 00452, GRO 95658, and GRO 95539 showing chondrule fraction δ41K ranges of −1.54 to 0.14 ‰, −0.76 to −0.28 ‰, −2.20 to −1.23 ‰, and −1.30 to −0.84 ‰, respectively. Overall, no strong correlations between K isotope ratio and K concentration are observed among the chondrule fractions for any of the four chondrites. Additionally, unlike what was seen previously for the LL4 Hamlet, no correlation between chondrule mass and K isotope ratio was observed. In conjunction with previous studies, the data here suggest that a combination of secondary parent body processes and nebular processes involved in chondrule formation are the dominant controls on the K isotope systematics of the chondrules from unequilibrated ordinary chondrites. The effects of secondary parent body processing vary significantly from chondrule to chondrule, however, the dominant effect is the migration of K from the K rich matrix to the K poor chondrules. As such, parent body alteration partially overprinted and disturbed the initial chondrule K compositions to various degrees. Nevertheless, even with the effects of parent body processing, the key observation that the vast majority of the chondrule fractions show δ41K values lighter than, or equal to, their respective matrix and bulk compositions is best explained by these chondrules experiencing incomplete condensation in the solar nebula. This aligns with K isotope observations made for the carbonaceous chondrites where the matrix-dominated CI chondrites are enriched in heavier K isotopes and the chondrule-rich carbonaceous chondrites are enriched in lighter K isotopes. The K isotopes of individual chondrules in this study suggest that chondrules from ordinary chondrites were also formed via incomplete condensation from a supersaturated medium, agreeing with the previous conclusion drawn for carbonaceous chondrules. This means both CC and OC chondrules likely experienced incomplete condensation, making this chondrule formation process ubiquitous and widespread throughout both the inner and outer regions of early solar nebula.