MEGA-CHONDRULES AND LARGE, IGNEOUS-TEXTURED CLASTS IN JULESBERG (L3) AND OTHER ORDINARY CHONDRITES - VAPOR-FRACTIONATION, SHOCK-MELTING, AND CHONDRULE FORMATION

A petrographic-microprobe study of large, metal-poor, igneous-textured objects in Julesberg (L3) and other ordinary chondrites suggests that they can be classified into two petrographic types: mega chondrules a nd large lithic clasts; and into two chemical types: Na-poor and Na-ri ch. Mega-chondrules show textural evidence of having solidified as fre ely-floating melt droplets, whereas lithic clasts formed by the fragme ntation of larger objects, possibly still larger mega-chondrules. Barr ed-olivine or barred-olivine-pyroxene textures are most common for meg a-chondrules, whereas a variety of textures occur in large lithic clas ts. The two petrographic types cannot be distinguished on the basis of modal, bulk, or phase compositions. Sodium-poor objects are character ized by (1) plagioclase or glass with mainly bytownite composition (An (70-90) typical), (2) subchondritic Na/Al ratios and typically subchon dritic volatile-element (Na, K, and Mn) abundances, and (3) bulk-chemi cal trends that resemble those expected for vapor-fractionation proces ses. Some Na-poor objects may have formed by the melting of precursors that formed as condensates or vaporization residues; others may have formed by the melting of precursors that formed by fractional condensa tion or fractional vaporization. Following vapor-fractionation, Na-poo r objects or their precursors appear to have reequilibrated at lower t emperatures, which raised the bulk Na content of the objects, but not to the levels seen in Na-rich objects. Enrichments of Fe2+, Na, K, and P on the margins of some Na-poor objects suggest that they partly rea cted with volatile-rich surroundings, both before and after brecciatio n. Sodium-rich objects are characterized by (1) plagioclase or,glass w ith oligoclase or albite composition (An(2-25)) (2) roughly chondritic Na/Al ratios and volatile-element (Na, K: Mn) abundances, and (3) bul k-chemical trends similar to those shown by melt-pocket glasses in ord inary chondrites. Sodium-rich melt objects could have formed by the sh ock-melting of chondritic precursors. Feldspathic compositions for som e Na-rich melt objects can be explained by preferential shock-melting of feldspar or feldspathic glass in chondritic target materials. Liter ature data imply that the same two chemical populations of objects, Na -poor and Na-rich, occur among smaller, normal-sized chondrules in Typ e 3 ordinary chondrites, suggesting that the same processes that affec ted large melt objects also affected chondrules. Copyright (C) 1998 El sevier Science Ltd.