Chondrules: Their Diversity and the Role of Open-System Processes during Their Formation

Abstract

After classification into compositional groups using their cathodoluminescence properties, chondrules were chiseled from sections of the least metamorphosed ordinary chondrites, Semarkona and Krymka. This technique avoided biases due to size and friability and ensured that all classes were adequately sampled. The chondrules were then analyzed by instrumental neutron activation analysis and their major phases (olivine, pyroxene, metal, mesostasis) were analyzed by electron microprobe. Group A1 and A2 chondrules of Semarkona and group A3 chondrules of Krymka have all the properties expected for chondrules which experienced considerable Fe reduction and evaporative loss during their formation (elemental depletions related to volatility, Fe-poor silicates, Ni-poor metal, significant pyroxene, small sizes). This is not the case for group B1, B2, and A5 chondrules (which have unfractionated bulk silicate compositions, FeO-rich silicates, little or no metal, little pyroxene, large sizes). Group A chondrules in Semarkona generally have thicker metal–sulfide-rich rims than group B chondrules, a situation similar to that of the Murchison CM2 chondrite, except that in Murchison aqueous alteration has destroyed the metal and sulfide. Group A chrondrules sometimes show compositional zoning in their mesostases and we suggest that both mesostasis zoning and chondrule rims are the products of recondensation during chondrule formation. Cooling rates differ considerably with chondrule class. Group A1–3 and A5 chondrules cooled relatively slowly and maintained a degree of equilibrium between melt and phenocrysts, while group B1 and B2 chondrules cooled rapidly and underwent considerable supercooling. The chondrule-forming process, whatever it was, was clearly capable of acting with a variety of intensities and produced a range of cooling rates. We suggest that the diversity of chondrules cannot be attributed mainly to variations in the properties and abundances of precursors, although this can sometimes be a factor, but can be derived from a fairly similar precursor of solar composition.