Primary Plagioclase in Type I Chondrules in Kainsaz: Evidence for High Solid Densities during Chondrule Formation

Introduction: Primary igneous plagioclase is found in approximately 10 % of type I chondrules in CO chondrites [1]. The presence of plagioclase has potential to help interpret conditions of chondrule formation, if the conditions of plagioclase nuclea-tion and growth are understood. Because primary plagioclase typically contains a significant albite component, the presence of this mineral is significant for the discussion of the behavior of sodium in the chondrule-forming environment [2, 3]. We have carried out a study of plagioclase-bearing type I chondrules from the CO3.2 chondrite Kainsaz, in order to understand the textural and mineralogical occurrence of primary plagioclase and to understand the role of sodium during chondrule formation. Results: Textures. Chondrules included in this study have typical type I porphyritic textures, with varying abundances of olivine and pyroxene phenocrysts. Pyroxene is commonly at the periphery of the chondrules, with olivine poikilitically enclosed within pyroxene. Fe-metal and troilite blebs are dispersed throughout all chondrules. In most plagioclase-bearing type I chondrules, plagioclase is interstitial to olivine and pyroxene, and mesostasis is in low abundance. In two very pyroxene-rich chon-drules, plagioclase is concentrated in the center of the chondrule. Plagioclase grain size ranges from 5-50µm. Sodium content of chondrules. In all chondrules, plagioclase is An-rich, but it contains a significant albite component: compositions range from An 75-An 83. Because there is little mesostasis, the melts from which this plagioclase grew must have contained at least 2.5 wt% Na 2 O, based on the Na 2 O content of plagioclase. We determined bulk silicate compositions of four plagio-clase-bearing type I chondrules using modal recombination analysis. Bulk silicate compositions are within the range of compositions of non-plagioclase-bearing type I chondrules in Kainsaz [4]. The modal abundance of plagioclase in these chondrules is 10-20 vol. %. The bulk silicate Na 2 O content is 0.25-0.5 wt. %. Discussion: The presence of primary igneous plagioclase with a significant Ab component means that Na was present in the chondrule melt. It has been suggested recently that chon-drules that had Na present in the melt during cooling formed in regions of high solid densities [2,3]. Experiments have shown that Na can be preserved in silicate melt under oxidizing conditions , at very fast cooling rates, or in a high solid density environment [e.g. 5]. In the case of plagioclase-bearing type I chon-drules, conditions were clearly not oxidizing, and the porphyritic textures and large plagioclase grains are not likely …