CHONDRULES - THEIR DIVERSITY AND THE ROLE OF OPEN-SYSTEM PROCESSES DURING THEIR FORMATION

After classification into compositional groups using their cathodolumi nescence properties, chondrules were chiseled from sections of the lea st metamorphosed ordinary chondrites, Semarkona and Krymka. This techn ique avoided biases due to size and friability and ensured that all cl asses were adequately sampled. The chondrules were then analyzed by in strumental neutron activation analysis and their major phases (olivine , pyroxene, metal, mesostasis) were analyzed by electron microprobe. G roup A1 and A2 chondrules of Semarkona and group A3 chondrules of Krym ka have all the properties expected for chondrules which experienced c onsiderable 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 g roup B1, B2, and A5 chondrules (which have unfractionated bulk silicat e compositions, FeO-rich silicates, little or no metal, little pyroxen e, large sizes). Group A chondrules in Semarkona generally have thicke r metal-sulfide-rich rims than group B chondrules, a situation similar to that of the Murchison CM2 chondrite, except that in Murchison aque ous alteration has destroyed the metal and sulfide. Group A chrondrule s sometimes show compositional zoning in their mesostases and we sugge st that both mesostasis zoning and chondrule rims are the products of recondensation during chondrule formation. Cooling rates differ consid erably with chondrule class. Group A1-3 and A5 chondrules cooled relat ively slowly and maintained a degree of equilibrium between melt and p henocrysts, while group B1 and B2 chondrules cooled rapidly and underw ent considerable supercooling. The chondrule-forming process, whatever it was, was clearly capable of acting with a variety of intensities a nd produced a range of cooling rates. We suggest that the diversity of chondrules cannot be attributed mainly to variations in the propertie s and abundances of precursors, although this can sometimes be a facto r, but can be derived from a fairly similar precursor of solar composi tion. (C) 1996 Academic Press, Inc.