Relationship among O, Mg isotopes and the petrography of two spinel-bearing compound chondrules

The petrological properties, and O and Al-Mg isotopic compositions of two spinel-bearing chondrules from the Allende CV chondrite were investigated using scanning electron microscopy and secondary ion mass spectrometry. A coarse spinel grain in a barred-olivine (BO) chondrule is less enriched in O-16 (Delta O-17 similar to -5%; Delta(17)O = delta(17)O - 0.52 delta(18)O), whereas smaller spinel grains in a plagioclase-rich chondrule member of a compound chondrule are extremely O-16-rich (Delta(17)O similar to -17%) and the spinels have a strongly serrated character. The petrological features and O-16-enrichments of the spinels in the plagioclase-rich chondrule indicate that spinels originating in coarse-grained Ca-Al-rich Inclusions (CAIs) were incorporated into chondrule precursors and survived the chondrule-forming event. The degree of O-16-excesses among minerals within each chondrule is correlated to the crystallization sequences. This evidence suggests that the O isotopic variation among minerals may have resulted from incomplete exchange of O isotopes between O-16-rich chondrule melt and O-16-poor nebular gas. Aqueous alteration also has changed the O-isotope compositions in the mesostasis. The feldspathic mesostasis in the BO chondrule shows a disturbed Mg-Al isochron indicating that the BO chondrule experienced secondary alteration. While plagioclase in the plagioclase-rich chondrule member of the compound chondrule shows slight Mg-26-excesses corresponding to (Al-26/(27) Al)(0) = [4.6+/-4.0(2sigma)] x 10(-6), nepheline formed by secondary alteration shows no detectable excess. The Al-Mg isotopic system of these chondrules was disturbed by aqueous alteration and thermal metamorphism on the Allende parent body. Copyright (C) 2003 Elsevier Ltd.