Mineralogically zoned chondrules in ordinary chondrites as evidence for open system chondrule behaviour

Chondrules are a major component of chondritic meteorites. Understanding their formation conditions provides fundamental insights about how the early solar system formed and evolved. We studied the textures of 650 chondrules from all three groups (H, L, LL) of ordinary chondrites, in 2-dimensional (2D) sections through the meteorites. About 40% of the chondrules are mineralogically zoned. They consist of an olivine-rich core, which is surrounded by a low-Ca pyroxene-rich rim. Chondrules sectioned through their low-Ca pyroxene rim do not appear as zoned chondrules, hence, considering such sectioning effects, their true fraction might be as high as 50%. Mineralogical zonation is, therefore, a typical chondrule texture in basically all ordinary chondrites, and records a fundamental process during chondrule formation. Chondrules were open systems, and initially olivine-rich chondrules reacted with their surrounding gas to form low-Ca pyroxene rims. Zoned and unzoned chondrules have the same range of bulk compositions, thus ordinary chondrite chondrules were likely affected by two sequential episodes: in the first episode, gaseous SiO was added to all chondrules, thereby forming low-Ca pyroxene rims around all chondrules. In the second episode, only a portion of the chondrules were reheated, thereby remelting and homogenizing their initial pyroxene rims, but retaining their bulk compositions. It is therefore likely that all chondrules in ordinary chondrites were affected by gas-melt interactions during their formation. Open system exchange is consistent with previous studies of chondrule formation and can explain many chondrule textures and bulk chondrule compositional variations in single meteorites. Hence, the open system behaviour recorded in zoned chondrules provides a pivotal constraint on chondrule formation conditions.