Chondrules in the Murray CM2 meteorite and compositional differences between CM-CO and ordinary chondrite chondrules

Abstract

Bulk compositions were determined by broad-beam electron microprobe analysis for thirteen of the least aqueously altered chondrules in Murray (CM2). These and literature data reveal compositional differences between CM-CO and ordinary chondrite (OC) chondrules: 1. (a) CO chondrules are richer in refractory lithophiles and poorer in Cr, Mn and volatile lithophiles than OC chondrules; much lower refractory lithophile abundances in CM chondrules resulted from aqueous alteration, 2. (b) in CM-CO chondrites, abundances of refractory lithophiles are higher in nonporphyritic than porphyritic chondrules, whereas in H-L-LL3 chondrites the converse is true, 3. (c) Cr ranges are greater and Cr and Mn correlate more strongly in chondrules in CM-CO than in H-L-LL3 chondrites. We find evidence for two important lithophile precursor components of CM-CO chondrite chondrules: 1. (1) pyroxene- and refractory-rich, FeO-poor; 2. (2) olivine-rich, refractory and FeO-poor. The occurrence of a few FeO-rich chondrules attests to a third component similar to matrix: olivine- and FeO-rich, refractories not characterized. The first two components differ from those inferred for OC chondrules, consistent with formation at different locations. The pyroxene- and refractory-rich, FeO-poor lithophile precursor component probably formed by an incomplete evaporation of presolar silicates that brought these materials into the enstatite stability field.